Publications

@article{nokey,

title = {CO2 clusters: Forming fragile dications in helium nanodroplets},

author = {Florian Foitzik and Lisa Ganner and Stefan Bergmeister and Fabio Zappa and Paul Scheier and Olof Echt and Elisabeth Gruber},

url = {https://iopscience.iop.org/article/10.1088/1402-4896/ad999a},

doi = {10.1088/1402-4896/ad999a},

year  = {2024},

date = {2024-12-12},

urldate = {2024-12-12},

journal = {Phys. Scr.},

volume = {100},

issue = {1},

pages = {015405},

abstract = {For many atoms and molecules, small multiply charged clusters are unstable with respect to spontaneous charge separation. When bare CO2 clusters are ionized by electrons, the smallest detected dications contain 43 molecules. The same appearance size has been obtained when neutral CO2 clusters embedded in helium nanodroplets are electron ionized. Here we show that electron ionization of singly charged CO2 clusters solvated in helium nanodroplets drastically lowers the appearance size to na = 30. The combination of efficient cooling in superfluid helium, sequential ionization, and gentle collisions of the nanodroplets with helium gas makes it possible to form dications that are barely larger than the Rayleigh limit.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Size Limits and Fission Channels of Doubly Charged Noble Gas Clusters},

author = {Ianessa Stromberg and Stefan Bergmeister and Lisa Ganner and Fabio Zappa and Paul Scheier and Olof Echt and Elisabeth Gruber},

doi = {10.1039/D4CP00658E},

year  = {2024},

date = {2024-03-25},

urldate = {2024-03-25},

journal = {Phys. Chem. Chem. Phys.},

volume = {26},

pages = {13923–13936},

abstract = {Small, highly charged liquid droplets are unstable with respect to spontaneous charge separation when their size drops below the Rayleigh limit or, in other words, their fissility parameter X exceeds the value 1. The absence of small doubly charged atomic cluster ions in mass spectra below an element-specific appearance size na has sometimes been attributed to the onset of barrierless fission at X = 1. However, more realistic models suggest that na marks the size below which the rate of fission surpasses that of competing dissociative channels, and the Rayleigh limit of doubly charged van der Waals clusters has remained unchartered. Here we explore a novel approach to form small dicationic clusters, namely by Penning ionization of singly charged noble gas (Ng) clusters that are embedded in helium nanodroplets; the dications are then gently extracted from the nanodroplets by low-energy collisions with helium gas. We observe Ngn2+ ions that are about 40 % smaller than previously reported for xenon and krypton and about 20 % for argon. These findings suggest that fission barriers have been underestimated in previous theoretical work. Furthermore, we measure the size distributions of fragment ions that are produced by collisional excitation of mass-selected dications. At lowest collision gas pressure, dicationic Kr and Xe clusters that are smaller than previously observed are found to evaporate an atom before they undergo highly symmetric fission. The distribution of fragments resulting from fission of small dicationic Ar clusters is bimodal.},

note = {Open access},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Doubly Charged Dimers and Trimers of Heavy Noble Gases},

author = {Gabriel Schöpfer and Stefan Bergmeister and Milan Ončák and Ianessa Stromberg and Masoomeh Mahmoodi-Darian and Paul Scheier and Olof Echt and Elisabeth Gruber},

doi = {10.1039/D4CP00465E},

year  = {2024},

date = {2024-03-21},

urldate = {2024-03-21},

journal = {Phys. Chem. Chem. Phys.},

volume = {26},

pages = {11482-11490},

abstract = {Many doubly charged heteronuclear dimers are metastable or even thermodynamically stable with respect to charge separation. Homonuclear dicationic dimers, however, are more difficult to form. He22+ was the first noble gas dimer predicted to be metastable and, decades later, observed. Ne22+ is the only other dicationic noble gas dimer that has been detected so far. Here, we present a novel approach to form fragile dicationic species, by post-ionization of singly charged ions that are embedded in helium nanodroplets (HNDs). Bare ions are then extracted by colliding the HNDs with helium gas. We detect homonuclear doubly charged dimers and trimers of krypton and xenon, but not argon. Our multi-reference ab initio calculations confirm the stability of Kr22+, Kr32+, Xe22+, Xe32+, and Ar22+, but put the stability of Ar32+ towards dissociation to Ar+ + Ar2+ into question.},

note = {Open access},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Helium nanodroplets as an efficient tool to investigate hydrogen attachment to alkali cations},

author = {Siegfried Kollotzek and José Campos-Martínez and Massimiliano Bartolomei and Fernando Pirani and Lukas Tiefenthaler and Marta Hernández and Teresa Lazaro and Eva Zunzunegui-Bru and Tomas Gonzalez-Lezana and Javier Hernández-Rojas and José Bretón and Olof Echt and Paul Scheier},

doi = {10.1039/D2CP03841B},

year  = {2022},

date = {2022-11-17},

urldate = {2022-11-17},

journal = {Phys.Chem.Chem.Phys.},

volume = {25},

pages = {462-470},

abstract = {We report a novel method to reversibly attach and detach hydrogen molecules to positively charged sodium clusters formed inside a helium nanodroplet host matrix. It is based on the controlled production of multiply charged helium droplets which, after picking up sodium atoms and exposure to H2 vapor, lead to the formation of Nam+(H2)n clusters, whose population was accurately measured using a time-of-flight mass spectrometer. The mass spectra reveal particularly favorable Na+(H2)n and Na2+(H2)n clusters for specific “magic” numbers of attached hydrogen molecules. The energies and structures of these clusters have been investigated by means of quantum-mechanical calculations employing analytical interaction potentials based on ab initio electronic structure calculations. A good agreement is found between the experimental and the theoretical magic numbers.},

note = {Open access},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Solvation of Large Polycyclic Aromatic Hydrocarbons in Helium: Cationic and Anionic Hexabenzocoronene},

author = {Miriam Kappe and Florent Calvo and Johannes Schöntag and Holger F. Bettinger and Serge Krasnokutski and Martin Kuhn and Elisabeth Gruber and Zappa and Paul Scheier and Olof Echt,},

doi = {10.3390/molecules27196764},

year  = {2022},

date = {2022-10-10},

urldate = {2022-10-10},

journal = {Molecules},

volume = {27},

pages = {6764},

abstract = {The adsorption of helium on charged hexabenzocoronene (Hbc, C42H18), a planar polycyclic aromatic hydrocarbon (PAH) molecule of D6h symmetry, was investigated by a combination of high-resolution mass spectrometry and classical and quantum computational methods. The ion abundance of HenHbc+ complexes versus size n features prominent local anomalies at n = 14, 38, 68, 82, and a weak one at 26, indicating that for these “magic” sizes, the helium evaporation energies are relatively large. Surprisingly, the mass spectra of anionic HenHbc− complexes feature a different set of anomalies, namely at n = 14, 26, 60, and 62, suggesting that the preferred arrangement of the adsorbate atoms depends on the charge of the substrate. The results of our quantum calculations show that the adsorbate layer grows by successive filling of concentric rings that surround the central benzene ring, which is occupied by one helium atom each on either side of the substrate. The helium atoms are fairly localized in filled rings and they approximately preserve the D6h symmetry of the substrate, but helium atoms in partially filled rings are rather delocalized. The first three rings contain six atoms each; they account for magic numbers at n = 14, 26, and 38. The size of the first ring shrinks as atoms are filled into the second ring, and the position of atoms in the second ring changes from hollow sites to bridge sites as atoms are filled into the third ring. Beyond n = 38, however, the arrangement of helium atoms in the first three rings remains essentially frozen. Presumably, another ring is filled at n = 68 for cations and n = 62 for anions. The calculated structures and energies do not account for the difference between charge states, although they agree with the measurements for the cations and show that the first solvation shell of Hbc± is complete at n = 68. Beyond that size, the adsorbate layer becomes three-dimensional, and the circular arrangement of helium changes to hexagonal.

},

note = {Open access},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Adsorption of Helium and Hydrogen on Triphenylene and 1,3,5-Triphenylbenzene},

author = {Stefan Bergmeister and Siegfried Kollotzek and Florent Calvo and Elisabeth Gruber and Fabio Zappa and Paul Scheier and Olof Echt},

doi = {10.3390/molecules27154937},

year  = {2022},

date = {2022-08-03},

urldate = {2022-08-03},

journal = {Molecules},

volume = {27},

pages = {4937},

abstract = {The adsorption of helium or hydrogen on cationic triphenylene (TPL, C18H12), a planar polycyclic aromatic hydrocarbon (PAH) molecule, and of helium on cationic 1,3,5-triphenylbenzene (TPB, C24H18), a propeller-shaped PAH, is studied by a combination of high-resolution mass spectrometry and classical and quantum computational methods. Mass spectra indicate that HenTPL+ complexes are particularly stable if n = 2 or 6, in good agreement with the quantum calculations that show that for these sizes, the helium atoms are strongly localized on either side of the central carbon ring for n = 2 and on either side of the three outer rings for n = 6. Theory suggests that He14TPL+ is also particularly stable, with the helium atoms strongly localized on either side of the central and outer rings plus the vacancies between the outer rings. For HenTPB+, the mass spectra hint at enhanced stability for n = 2, 4 and, possibly, 11. Here, the agreement with theory is less satisfactory, probably because TPB+ is a highly fluxional molecule. In the global energy minimum, the phenyl groups are rotated in the same direction, but when the zero-point harmonic correction is included, a structure with one phenyl group being rotated opposite to the other two becomes lower in energy. The energy barrier between the two isomers is very small, and TPB+ could be in a mixture of symmetric and antisymmetric states, or possibly even vibrationally delocalized.

},

note = {Open access},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Stabilization of phenanthrene anions in helium nanodroplets},

author = {Siegfried Kollotzek and Farhad Izadi and Miriam Meyer and Stefan Bergmeister and Fabio Zappa and Stephan Denifl and Olof Echt and Paul Scheier and Elisabeth Gruber},

doi = {10.1039/d2cp00991a},

year  = {2022},

date = {2022-05-04},

urldate = {2022-05-04},

journal = {Phys.Chem.Chem.Phys.},

volume = {24},

pages = {11662-11667},

abstract = {It has been debated for years if the polycyclic aromatic hydrocarbon phenanthrene exists in its anionic form, or, in other words, if its electron affinity (EA) is positive or negative. In this contribution we confirm that the bare phenanthrene anion Ph− created in a binary collision with an electron at room temperature has a lifetime shorter than microseconds. However, the embedding of neutral phenanthrene molecules in negatively charged helium nanodroplets enables the formation of phenanthrene anions by charge transfer processes and the stabilization of the latter in the ultracold environment. Gentle shrinking of the helium matrix of phenanthrene-doped HNDs by collisions with helium gas makes the bare Ph− visible by high-resolution mass spectrometry. From these and previous measurements we conclude, that the EA of phenanthrene is positive and smaller than 24.55 meV.},

note = {Open access},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Phenanthrene: Establishing lower and upper bounds to the binding energy of a very weakly bound anion},

author = {Elisabeth Gruber and Siegfried Kollotzek and Stefan Bergmeister and Fabio Zappa and Milan Ončák and Paul Scheier and Olof Echt},

doi = {10.1039/d1cp04755h},

year  = {2022},

date = {2022-02-14},

urldate = {2022-02-14},

journal = {Phys.Chem.Chem.Phys.},

volume = {24},

pages = {5138-5143},

abstract = {Quite a few molecules do not form stable anions that survive the time needed for their detection; their electron affinities (EA) are either very small or negative. How does one measure the EA if the anion cannot be observed? Or, at least, can one establish lower and upper bounds to their EA? We propose two approaches that provide lower and upper bounds. We choose the phenanthrene (Ph) molecule whose EA is controversial. Through competition between helium evaporation and electron detachment in HenPh− clusters, formed in helium nanodroplets, we estimate the lower bound of the vertical detachment energy (VDE) of Ph− as about −3 meV. In the second step, Ph is complexed with calcium whose electron affinity is just 24.55 meV. When CaPh− ions are collided with a thermal gas of argon, one observes Ca− product ions but no Ph−, suggesting that the EA of Ph is below that of Ca.},

note = {Open access},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Splashing of large helium nanodroplets upon surface collisions},

author = {Paul Martini and Simon Albertini and Felix Laimer and Miriam Meyer and Michael Gatchell and Olof Echt and Fabio Zappa and Paul Scheier},

doi = {10.1103/PhysRevLett.127.263401},

year  = {2021},

date = {2021-12-23},

journal = {Phys. Rev. Lett},

volume = {127},

pages = {263401},

abstract = {In the present work we observe that helium nanodroplets colliding with surfaces can exhibit splashing in a way that is analogous to classical liquids. We use transmission electron microscopy and mass spectrometry to demonstrate that neutral and ionic dopants embedded in the droplets are efficiently backscattered in such events. High abundances of weakly bound He-tagged ions of both polarities indicate a gentle extraction mechanism of these ions from the droplets upon collision with a solid surface. This backscattering process is observed for dopant particles with masses up to 400 kilodaltons, indicating an unexpected mechanism that effectively lowers deposition rates of nanoparticles formed in helium droplets.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Adsorption of helium on a charged propeller molecule: Hexaphenylbenzene},

author = {Siegfried Kollotzek and Florent Calvo and Serge Krasnokutski and Fabio Zappa and Paul Scheier and Olof Echt},

doi = {10.1140/epjd/s10053-021-00301-6},

year  = {2021},

date = {2021-11-29},

urldate = {2021-11-29},

journal = {Eur. Phys. J. D},

volume = {75},

pages = {299},

abstract = {Physisorption on planar or curved graphitic surfaces or aromatic rings has been investigated by various research groups, but in these studies, the substrate was usually strictly rigid. Here, we report a combined experimental and theoretical study of helium adsorption on cationic hexaphenylbenzene (HPB), a propeller-shaped molecule. The orientation of its propeller blades is known to be sensitive to the environment, with substantial differences between the molecule in the gas phase and in the crystalline solid. Mass spectra of HeHPB, synthesized in helium nanodroplets, indicate enhanced stability for ions containing , or 46 helium atoms. Path-integral molecular dynamics simulations reveal a significant dependence of the dissociation energy on the details of the HPB geometry. Good agreement between the experimental data and calculated dissociation energies is obtained, provided that the symmetry of HPB is reduced from to , such a lower symmetry being suggested from quantum chemical calculations as arising upon electron removal.},

note = {Open access},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Complexes with Atomic Gold ions: Efficient Bisligand Formation},

author = {Felix Duensing and Elisabeth Gruber and Paul Martini and Marcelo Goulart and Michael Gatchell and Bilal Rasul and Olof Echt and Fabio Zappa and Masoomeh Mahmoodi-Darian and Paul Scheier},

doi = {10.3390/molecules26123484},

year  = {2021},

date = {2021-06-08},

urldate = {2021-06-08},

journal = {Molecules},

volume = {26},

pages = {3484},

abstract = {Complexes of atomic gold with a variety of ligands have been formed by passing helium nanodroplets (HNDs) through two pickup cells containing gold vapor and the vapor of another dopant, namely a rare gas, a diatomic molecule (H2, N2, O2, I2, P2), or various polyatomic molecules (H2O, CO2, SF6, C6H6, adamantane, imidazole, dicyclopentadiene, and fullerene). The doped HNDs were irradiated by electrons; ensuing cations were identified in a high-resolution mass spectrometer. Anions were detected for benzene, dicyclopentadiene, and fullerene. For most ligands L, the abundance distribution of AuLn+ versus size n displays a remarkable enhancement at n = 2. The propensity towards bis-ligand formation is attributed to the formation of covalent bonds in Au+L2 which adopt a dumbbell structure, L-Au+-L, as previously found for L = Xe and C60. Another interesting observation is the effect of gold on the degree of ionization-induced intramolecular fragmentation. For most systems gold enhances the fragmentation, i.e., intramolecular fragmentation in AuLn+ is larger than in pure Ln+. Hydrogen, on the other hand, behaves differently, as intramolecular fragmentation in Au(H2)n+ is weaker than in pure (H2)n+ by an order of magnitude.},

note = {Open access},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {SF6+: Stabilizing Transient Ions in Helium Nanodroplets},

author = {Simon Albertini and Stefan Bergmeister and Felix Laimer and Paul Martini and Elisabeth Gruber and Fabio Zappa and Milan Ončák and Paul Scheier and Olof Echt},

doi = {10.1021/acs.jpclett.1c01024},

year  = {2021},

date = {2021-04-22},

urldate = {2021-04-22},

journal = {J. Phys. Chem. Lett.},

volume = {12},

pages = {4112-4117},

abstract = {There are myriad ions that are deemed too short-lived to be experimentally accessible. One of them is SF6+. It has never been observed, although not for lack of trying. We demonstrate that long-lived SF6+ can be formed by doping charged helium nanodroplets (HNDs) with sulfur hexafluoride; excess helium is then gently stripped from the doped HNDs by collisions with helium gas. The ion is identified by high-resolution mass spectrometry (resolution m/Δm = 15000), the close agreement between the expected and observed yield of ions that contain minor sulfur isotopes, and collision-induced dissociation in which mass-selected HenSF6+ ions collide with helium gas. Under optimized conditions, the yield of SF6+ exceeds that of SF5+. The procedure is versatile and suitable for stabilizing many other transient molecular ions.},

note = {Open access},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Electronic Transitions in Rb2+ Dimers Solvated in Helium},

author = {Simon Albertini and Paul Martini and Arne Schiller and Harald Schöbel and Elham Ghavidel and Milan Ončák and Olof Echt and Paul Scheier},

doi = {10.1007/s00214-021-02728-x},

year  = {2021},

date = {2021-03-01},

urldate = {2021-03-01},

journal = {Theor. Chem. Acc.},

volume = {140},

pages = {29},

abstract = {We have measured depletion spectra of the heteronuclear (85Rb87Rb+) dimer cation complexed with up to 10 He atoms. Two absorption bands are observed between 920 and 250 nm. The transition into the repulsive 12Σu+ state of HeRb2+ gives rise to a broad feature at 790 nm (12,650 cm−1); it exhibits a blueshift of 98 cm−1 per added He atom. The transition into the bound 12Πu state of HeRb2+ reveals vibrational structure with a band head at ≤ 15,522 cm−1, a harmonic constant of 26 cm−1, and a spin–orbit splitting of ≤ 183 cm−1. The band experiences an average redshift of − 38 cm−1 per added He atom. Ab initio calculations rationalize the shape of the spectra and spectral shifts with respect to the number of helium atoms attached. For a higher number of solvating helium atoms, symmetric solvation on both ends of the Rb2+ ion is predicted. },

note = {Open access},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {On the Stability of Neon Cluster Ions – Evidence for Isomeric Structures},

author = {Siegfried Kollotzek and Stefan Bergmeister and Lukas Tiefenthaler and Simon Albertini and Elisabeth Gruber and Fabio Zappa and Paul Scheier and Olof Echt},

doi = {10.1016/j.ijms.2021.116528},

year  = {2021},

date = {2021-01-21},

urldate = {2021-01-21},

journal = {Int. J. Mass Spectrom.},

volume = {462},

pages = {116528},

abstract = {We have adopted the newly developed technique of growing cationic clusters in size-to-charge selected helium nanodroplets (HNDs), with subsequent removal of helium in a collision cell, to record high-resolution mass spectra of Nen+. Growth in singly charged HNDs leads to mass spectra that feature the same anomalies in the cluster ion abundance as in previous work, namely maxima at n = 14, 21, 55/56, 75. Several other, weaker but statistically significant anomalies are observed at n = 9, 26, 29, 33, 35, 69, 82, 89. However, when neon clusters are grown in larger HNDs, which are likely to be multiply charged, we observe a different set of magic numbers, at n = 7, 13, 19, 26, 29, 34, 55, 71, 81, plus many other numbers for larger clusters, up to n = 197. A transition from the first to the second set is observed in a limited size range if the collision pressure is increased. The most likely reason for the existence of two different sets of magic numbers appears to be the existence of two distinct structural families.},

note = {Open access},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Proton Transfer at Subkelvin Temperatures},

author = {Lukas Tiefenthaler and Siegfried Kollotzek and Andrew M. Ellis and Paul Scheier and Olof Echt},

doi = {10.1039/d0cp05174h},

year  = {2020},

date = {2020-11-24},

urldate = {2020-11-24},

journal = {Phys. Chem. Chem. Phys.},

volume = {22},

pages = {28165-28172},

abstract = {We demonstrate a novel method to ionize molecules or molecular clusters by proton transfer at temperatures

below 1 K. The method yields nascent ions and largely eliminates secondary reactions, even for notoriously

‘delicate’ molecules. Protonation is achieved inside liquid helium nanodroplets (HNDs) and begins with the

formation of (H2)mH+ ions as the proton donors. In a separate and subsequent step the HNDs are doped with

a proton acceptor molecule, X. Proton transfer occurs between X and the cold proton donor ions inside a

helium droplet, an approach that avoids the large excess energy that is released if HNDs are first doped and

then ionized. Mass spectra, recorded after stripping excess helium and hydrogen in a collision cell, show that

this method offers a new way to determine proton affinities of molecules and clusters by proton-transfer

bracketing, to investigate astrochemically relevant ion–molecule reactions at sub-kelvin temperatures, and to

prepare XH+ ions that are suitable for messenger-tagging action spectroscopy.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Isotope enrichment in neon clusters grown in helium nanodroplets},

author = {Lukas Tiefenthaler and Siegfried Kollotzek and Michael Gatchell and Klavs Hansen and Paul Scheier and Olof Echt},

doi = {10.1063/5.0028056},

year  = {2020},

date = {2020-10-22},

urldate = {2020-10-22},

journal = {Journal of Chemical Physics},

volume = {153},

pages = {164305},

abstract = {Neon cluster ions Nes+ grown in pre-ionized, mass-to-charge selected helium nanodroplets (HNDs) reveal a strong enrichment of the heavy isotope 22Ne that depends on cluster size s and the experimental conditions. For small sizes, the enrichment is much larger than previously reported for bare neon clusters grown in nozzle expansions and subsequently ionized. The enrichment is traced to the massive evaporation of neon atoms in a collision cell that is used to strip helium from the HNDs. We derive a relation between the enrichment of 22Ne in the cluster ion and its corresponding depletion factor F in the vapor phase. The value thus found for F is in excellent agreement with a theoretical expression that relates isotopic fractionation in two-phase equilibria of atomic gases to the Debye temperature. Furthermore, the difference in zero-point energies between the two isotopes computed from F agrees reasonably well with theoretical studies of neon cluster ions that include nuclear quantum effects in the harmonic approximation. Another fitting parameter provides an estimate for the size si of the precursor of the observed Nes+. The value is in satisfactory agreement with the size estimated by modeling the growth of Nes+ and with lower and upper limits deduced from other experimental data. On the other hand, neon clusters grown in neutral HNDs that are subsequently ionized by electron bombardment exhibit no statistically significant isotope enrichment at all. The finding suggests that the extent of ionization-induced dissociation of clusters embedded in HNDs is considerably smaller than that for bare clusters.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Solvation of Ions in Helium [Review]},

author = {Tomás González-Lezana and Olof Echt and Michael Gatchell and Massimiliano Bartolomei and José Campos-Martínez and Paul Scheier},

doi = {10.1080/0144235X.2020.1794585},

year  = {2020},

date = {2020-08-24},

urldate = {2020-08-24},

journal = {International Reviews in Physical Chemistry},

volume = {39},

issue = {4},

pages = {465-516},

abstract = {We review the solvation of atomic, molecular or cluster ions in HNDs. After briefly discussing the properties of snowballs in bulk helium we consider experimental conditions for the efficient synthesis of charged, doped HNDs. We show that the cluster ions observed in conventional mass spectrometers originate from fission of highly charged HNDs. The ionization threshold of HNDs doped with alkalis reveals the minimum cluster size required for full immersion. The abundance distributions of HeNX± ions frequently reveal local anomalies or magic numbers. We demonstrate that the abundance is approximately proportional to the evaporation energy. Observed and calculated magic numbers will be compiled, including data for ions solvated in molecular hydrogen. Alternative methods to forming HeNX+ that do not employ HNDs will be summarized. Electronic excitation spectra of C+ 60 and polycyclic aromatic hydrocarbon ions reveal the properties of the helium adsorption layer in quantitative detail. Next we discuss theoretical efforts to describe the interaction between ions and helium. We close with summarizing the size dependence of physical quantities computed for atomic alkali and alkaline earth cations in helium, such as binding energy, superfluid fraction, structural order, radial density profiles, and the existence of first and higher solvation shells.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{,

title = {Electron Attachment and Electron Ionization of Helium Droplets Containing Clusters of C60 and Formic Acid},

author = {Masoomeh Mahmoodi-Darian and Elias Jabbour Al Maalouf and Samuel Zöttl and Paul Scheier and Olof Echt},

doi = {10.1016/j.ijms.2020.116293},

year  = {2020},

date = {2020-01-16},

urldate = {2020-01-16},

journal = {International Journal of Mass Spectrometry},

volume = {450},

pages = {116293},

abstract = {High-resolution mass spectra of helium droplets doped with C60 and formic acid (FA) are ionized by electrons. Positive ion mass spectra reveal cluster ions [(C60)pFAn]+ together with their hydrogenated and dehydrogenated counterparts. Also observed are ions containing one or more water (W) molecules. The abundance distributions of these ions reveal several interesting features: i) [(C60)pFAn]+ ions are more abundant than hydrogenated [(C60)pFAnH]+ ions even though the opposite is true in the absence of C60 (i.e. if p = 0); ii) although [C60FA]+ is the most abundant ion containing a single C60, multiple C60 suppress the [(C60)pFA]+ signal; iii) an enhanced stability of [(C60)pW1FA5H]+ and [(C60)pW2FA6H]+ mirrors that of [W1FA5H]+ and [W2FA6H]+, respectively. On the other hand, the enhanced stability of [C60FA6H]+ finds no parallel in the stability pattern of [FAnH]+ or FAn+. Negative ion mass spectra indicate a propensity for non-dissociated [(C60)pFAn]- anions if p  1 which contrasts with the dominance of dehydrogenated [FAn-H]- anions.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{,

title = {Solvation of Silver Ions in Noble Gases He, Ne, Ar, Kr, and Xe},

author = {Masoomeh Mahmoodi-Darian and Paul Martini and Lukas Tiefenthaler and Jaroslav Kocisek and Paul Scheier and Olof Echt},

doi = {10.1021/acs.jpca.9b09496},

year  = {2019},

date = {2019-11-14},

urldate = {2019-11-14},

journal = {Journal of Physical Chemistry A},

volume = {123},

issue = {48},

pages = {10426-10436},

abstract = {We use a novel technique to solvate silver cations in small clusters of noble gases. The technique involves formation of large, superfluid helium nanodroplets that are subsequently electron ionized, mass-selected by deflection in an electric field, and doped with silver atoms and noble gases (Ng) in pickup cells. Excess helium is then stripped from the doped nanodroplets by multiple collisions with helium gas at room temperature, producing cluster ions that contain no more than a few dozen noble gas atoms and or just a few (or no) silver atoms. Under gentle stripping conditions helium atoms remain attached to the cluster ions, demonstrating their low vibrational temperature. Under harsher stripping conditions some of the heavier noble gas atoms will be evaporated as well, thus enriching stable clusters NgnAgm+ at the expense of less stable ones. This results in local anomalies in the cluster ion abundance which is measured in a high-resolution time-of-flight mass spectrometer. Based on these data we identify specific “magic” sizes n of particularly stable ions. There is no evidence though for enhanced stability of Ng2Ag+, in contrast to the high stability of Ng2Au+ that derives from the covalent nature of the bond for heavy noble gases. “Magic” sizes are also identified for Ag2+ dimer ions complexed with He or Kr. A sequence of magic numbers n = 12, 32, 44 is observed for HenH2O+. Structural models will be tentatively proposed.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{RN4113,

title = {Atomic Gold Ions Clustered with Noble Gases: Helium, Neon, Argon, Krypton, and Xenon},

author = {Paul Martini and Lorenz Kranabetter and Marcelo Goulart and Bilal Rasul and Michael Gatchell and Paul Scheier and Olof Echt},

doi = {10.1021/acs.jpca.9b06715},

year  = {2019},

date = {2019-01-01},

urldate = {2019-01-01},

journal = {J.Phys.Chem. A},

volume = {123},

pages = {9505-9513},

abstract = {High-resolution mass spectra of helium droplets doped with gold and ionized by electrons reveal HenAu+ cluster ions. Additional doping with heavy noble gases results in NenAu+, ArnAu+, KrnAu+, and XenAu+ cluster ions. The high stability predicted for covalently bonded Ar2Au+, Kr2Au+, and Xe2Au+ is reflected in their relatively high abundance. Surprisingly, the abundance of Ne2Au+, which is predicted to have zero covalent bonding character and no enhanced stability, features a local maximum, too. The predicted size and structure of complete solvation shells surrounding ions with essentially nondirectional bonding depends primarily on the ratio σ* of the ion–ligand versus the ligand–ligand distance. For Au+ solvated in helium and neon, the ratio σ* is slightly below 1, favoring icosahedral packing in agreement with a maximum observed in the corresponding abundance distributions at n = 12. HenAu+ appears to adopt two additional solvation shells of Ih symmetry, containing 20 and 12 atoms, respectively. For ArnAu+, with σ* ≈ 0.67, one would expect a solvation shell of octahedral symmetry, in agreement with an enhanced ion abundance at n = 6. Another anomaly in the ion abundance at Ar9Au+ matches a local maximum in its computed dissociation energy.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{RN4092,

title = {Charged Clusters of C60 and Au or Cu: Evidence for Stable Sizes and Specific Dissociation Channels},

author = {Paul Martini and Marcelo Goulart and Lorenz Kranabetter and Norbert Gitzl and Bilal Rasul and Paul Scheier and Olof Echt},

doi = {10.1021/acs.jpca.9b02768},

year  = {2019},

date = {2019-01-01},

urldate = {2019-01-01},

journal = {J.Phys.Chem. A},

volume = {123},

number = {21},

pages = {4599-4608},

abstract = {We have doped helium nanodroplets with C60 and either gold or copper. Positively or negatively charged (C60)mMn± ions (M = Au or Cu) containing up to ≈10 fullerenes and ≈20 metal atoms are formed by electron ionization. The abundance distributions extracted from high-resolution mass spectra reveal several local anomalies. The sizes of the four most stable (C60)mAun± ions identified in previous calculations for small values of m and n (m ≤ 2 and n ≤ 2, or m = 1 and n = 3) agree with local maxima in the abundance distributions. Our data suggest the existence of several other relatively stable ions including (C60)2Au3± and (C60)3Au4–. Another feature, namely the absence of bare (C60)2±, confirms the prediction that (C60)2M± dissociates by loss of C60± rather than loss of M. The experimental data also reveal the preference for loss of (charged or neutral) C60 over loss of a metal atom from some larger species such as (C60)3M3+. In contrast to these similarities between Au and Cu, the abundance distributions of (C60)3Aun– and (C60)3Cun– are markedly different. In this discussion, we emphasize the similarities and differences between anions and cations, and between gold and copper. Also noteworthy is the observation of dianions (C60)mAun2– for m = 2, 4, and 6.},

note = {Open access},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{RN4091,

title = {Electron attachment and electron ionization of formic acid clusters embedded in helium nanodroplets},

author = {Masoomeh Mahmoodi-Darian and Linnea Lundberg and Samuel Zöttl and Paul Scheier and Olof Echt},

doi = {10.1007/s13361-018-02124-z},

year  = {2019},

date = {2019-01-01},

urldate = {2019-01-01},

journal = {J.Am.Soc.MassSpectrom.},

volume = {30},

number = {5},

pages = {787-795},

abstract = {We report the results of an experimental study of electron ionization of large helium nanodroplets doped with formic acid (FA). Several homologous series of cluster anions are observed, including [FAn-H]−, undissociated FAn−, and these ions complexed with one or more H2O. Some major features resemble those observed upon sputtering of frozen FA films but they differ significantly from results obtained by electron attachment to bare FA clusters in the gas phase. The FAn− and (H2O)[FAn-H]− series show abrupt onsets above n = 2 and 5, respectively. A prominent resonance in the anion yield occurs at 22.5 eV due to the formation of an intermediate He−*. Also observed are homologous series of [FA-H]− or [FA2-H]− complexed with helium. The cation chemistry is dominated by the production of protonated formic acid clusters, [FAnH]+, but various other homologous cluster ion series are observed as well.},

note = {Open access},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{RN4093,

title = {Roadmap on photonic, electronic and atomic collision physics III. Heavy particles: with zero to relativistic speeds},

author = {Friedrich Aumayr and Kiyoshi Ueda and Emma Sokell and Stefan Schippers and Hossein Sadeghpour and Frédéric Merkt and Tom Gallagher and Barry Dunning and Paul Scheier and Olof Echt and Tom Kirchner and Stephan Fritzsche and Andrey Surzhykov and Xinwen Ma and Roberto Rivarola and Omar Fojon and Lokesh Tribedi and Emily Lamour and José R. Crespo López-Urrutia and Yuri A. Litvinov and Vladimir Shabaev and Henrik Cederquist and Henning Zettergren and Marika Schleberger and Richard A. Wilhelm and Toshiyuki Azuma and Philippe Boduch and Henning T. Schmidt and Thomas Stöhlker},

doi = {10.1088/1361-6455/ab26ea},

year  = {2019},

date = {2019-01-01},

urldate = {2019-01-01},

journal = {J. Phys. B: At. Mol. Opt. Phys},

volume = {52},

number = {17},

pages = {171003},

abstract = {We publish three Roadmaps on photonic, electronic and atomic collision physics in order to celebrate the 60th anniversary of the ICPEAC conference. Roadmap III focusses on heavy particles: with zero to relativistic speeds. Modern theoretical and experimental approaches provide detailed insight into the wide range of many-body interactions involving projectiles and targets of varying complexity ranging from simple atoms, through molecules and clusters, complex biomolecules and nanoparticles to surfaces and crystals. These developments have been driven by technological progress and future developments will expand the horizon of the systems that can be studied. This Roadmap aims at looking back along the road, explaining the evolution of the field, and looking forward, collecting nineteen contributions from leading scientists in the field.},

note = {Open access},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{RN4042,

title = {Cold physics and chemistry: Collisions, ionization and reactions inside helium nanodroplets close to zero K [Review]},

author = {A. Mauracher and O. Echt and A. M. Ellis and S. Yang and D. K. Bohme and J. Postler and A. Kaiser and S. Denifl and P. Scheier},

doi = {10.1016/j.physrep.2018.05.001},

year  = {2018},

date = {2018-01-01},

urldate = {2018-01-01},

journal = {Phys.Rep.},

volume = {751},

pages = {1-90},

abstract = {This review provides a comprehensive overview of the applications of helium nanodroplets in the study of ensembles of atoms and molecules, i.e. clusters and complexes. These clusters and complexes must form through collisions inside helium nanodroplets, hence the title of this review. It also provides a particularly detailed overview of the many studies of ions, both positive and negative, that have been carried out in helium nanodroplets.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{RN4071,

title = {Doubly charged coronene clusters-Much smaller than previously observed},

author = {M. Mahmoodi-Darian and S. Raggl and M. Renzler and M. Goulart and S. E. Huber and A. Mauracher and P. Scheier and O. Echt},

doi = {10.1063/1.5028393},

year  = {2018},

date = {2018-01-01},

urldate = {2018-01-01},

journal = {J.Chem.Phys.},

volume = {148},

number = {17},

pages = {174303},

abstract = {The smallest doubly charged coronene cluster ions reported so far, Cor152+, were produced by charge exchange between bare coronene clusters and He2+ [H. A. B. Johansson et al., Phys. Rev. A 84, 043201 (2011)]. These dications are at least five times larger than the estimated Rayleigh limit, i.e., the size at which the activation barrier for charge separation vanishes. Such a large discrepancy is unheard of for doubly charged atomic or molecular clusters. Here we report the mass spectrometric observation of doubly charged coronene trimers, produced by electron ionization of helium nanodroplets doped with coronene. The observation implies that Cor32+ features a non-zero fission barrier too large to overcome under the present experimental conditions. The height of the barriers for the dimer and trimer has been estimated by means of density functional theory calculations. A sizeable barrier for the trimer has been revealed in agreement with the experimental findings. },

note = {Open access},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{RN4072,

title = {Helium nanodroplets doped with copper and water},

author = {Stefan Raggl and Norbert Gitzl and Paul Martini and Paul Scheier and Olof Echt},

doi = {10.1140/epjd/e2018-90150-7},

year  = {2018},

date = {2018-01-01},

urldate = {2018-01-01},

journal = {Eur.Phys.J. D},

volume = {72},

pages = {130},

abstract = {Copper nanoparticles are promising, low-cost candidates for the catalytic splitting of water and production of hydrogen gas. The present gas-phase study, based on the synthesis of copper-water complexes in ultracold helium nanodroplets followed by electron ionization, attempts to find evidence for dissociative water adsorption and H2 formation. Mass spectra show that H2O–Cu complexes containing dozens of copper and water molecules can be formed in the helium droplets. However, ions that would signal the production and escape of H2, such as (H2O)n−2(OH)2Cum+ or the isobaric (H2O)n−1OCum+, could not be detected. We do observe an interesting anomaly though: While the abundance of stoichiometric (H2O)nCum+ ions generally exceeds that of protonated or dehydrogenated ions, the trend is reversed for (H2O)OHCu2+ and (H2O)2OHCu2+; these ions are more abundant than (H2O)2Cu2+ and (H2O)3Cu2+, respectively. Moreover, (H2O)2OHCu2+ is much more abundant than other ions in the (H2O)n−1OHCu2+ series. A byproduct of our experiment is the observation of enhanced stability of He6Cu+, He12Cu+, He24Cu+, and He2Cu2+. },

note = {Open access},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{RN4059,

title = {Cs+ solvated in hydrogen - evidence for several distinct solvation shells},

author = {Lorenz Kranabetter and Marcelo Goulart and Abid Aleem and Thomas Kurzthaler and Martin Kuhn and Eric Barwa and Michael Renzler and Paul Scheier and Olof Echt},

doi = {10.1021/acs.jpcc.6b12057},

year  = {2017},

date = {2017-01-01},

urldate = {2017-01-01},

journal = {J.Phys.Chem. C},

volume = {121},

pages = {10887–10892},

abstract = {Helium nanodroplets are doped with cesium and molecular hydrogen and subsequently ionized by electrons. Mass spectra reveal HxCs+ ions that contain as many as 130 hydrogen atoms. Two features in the spectra are striking: First, the abundance of ions with an odd number of hydrogen atoms is very low; the abundance of HCs+ is only 1% that of H2Cs+. The dominance of even-numbered species is in stark contrast to previous studies of pure or doped hydrogen cluster ions. Second, the abundance of (H2)nCs+ features anomalies at n = 8, 12, 32, 44, and 52. Guided by previous work on ions solvated in hydrogen and helium, we assign the anomalies at n = 12, 32, and 44 to the formation of three concentric, solid-like solvation shells of icosahedral symmetry around Cs+. Preliminary density functional theory calculations for n ≤ 14 are reported as well.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{RN4045,

title = {On enhanced hydrogen adsorption on alkali (cesium) doped C60 and effects of the quantum nature of the H2 molecule on physisorption energies},

author = {Alexander Kaiser and Michael Renzler and Lorenz Kranabetter and Matthias Schwärzler and Rajendra Parajuli and Olof Echt and Paul Scheier},

doi = {10.1016/j.ijhydene.2017.01.069},

year  = {2017},

date = {2017-01-01},

urldate = {2017-01-01},

journal = {Int. J. Hydrogen Energy},

volume = {42},

number = {5},

pages = {3078-3086},

abstract = {Hydrogen storage by physisorption in carbon based materials is hindered by low adsorption energies. In the last decade doping of carbon materials with alkali, earth alkali or other metal atoms was proposed as a means to enhance adsorption energies, and some experiments have shown promising results. We investigate the upper bounds of hydrogen storage capacities of C60Cs clusters grown in ultracold helium nanodroplets by analyzing anomalies in the ion abundance that indicate shell closure of hydrogen adsorption shells. On bare , a commensurate phase with 32H2 molecules was identified in previous experiments. Doping C60 with a single cesium atom leads to an increase in relative ion abundance for the first 10H2 molecules, and the closure of the commensurate phase is shifted from 32 to 42H2 molecules. Density functional theory calculations indicate that thirteen energetically enhanced adsorption sites exist, where six of them fill the groove between Cs and C60 and 7 are located at the cesium atom. We emphasize the large effect of the quantum nature of the hydrogen molecule on the adsorption energies, i.e. the adsorption energies are decreased by around 50% for (H2)C60Cs and up to 80% for (H2)C60 by harmonic zero-point corrections, which represent an upper bound to corrections for dissociation energies (De to D0) by the vibrational ground states. Five normal modes of libration and vibration of H2 physisorbed on the substrate contribute primarily to this large decrease in adsorption energies. A similar effect can be found for H2 physisorbed on benzene and is expected to be found for any other weakly H2-binding substrate.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{RN4060,

title = {Positively and Negatively Charged Cesium and (C60)mCsn Cluster Ions},

author = {Michael Renzler and Lorenz Kranabetter and Marcelo Goulart and Paul Scheier and Olof Echt},

doi = {10.1021/acs.jpcc.6b11928},

year  = {2017},

date = {2017-01-01},

urldate = {2017-01-01},

journal = {J.Phys.Chem. C},

volume = {121},

pages = {10817–10823},

abstract = {We report on the formation and ionization of cesium and C60Cs clusters in superfluid helium nanodroplets. Size distributions of positively and negatively charged (C60)mCsn± ions have been measured for m ≤ 7, n ≤ 12. Reproducible intensity anomalies are observed in high-resolution mass spectra. For both charge states, (C60)mCs3± and (C60)mCs5± are particularly abundant, with little dependence on the value of m. Distributions of bare cesium cluster ions also indicate enhanced stability of Cs3± and Cs5±, in agreement with theoretical predictions. These findings contrast with earlier reports on highly Cs-doped cationic fullerene aggregates which showed enhanced stability of C60Cs6 building blocks attributed to charge transfer. The dependence of the (C60)mCs3– anion yield on electron energy shows a resonance that, surprisingly, oscillates in strength as m increases from 1 to 6.},

note = {Open access},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{RN4063,

title = {Preface to the Special Issue "ISSPIC XVIII: International Symposium on Small Particles and Inorganic Clusters 2016"},

author = {H. Häkkinen and A. Rosen and O. Echt and M. Pettersson},

doi = {10.1021/acs.jpcc.7b01997},

year  = {2017},

date = {2017-01-01},

urldate = {2017-01-01},

journal = {J.Phys.Chem. C},

volume = {121},

pages = {10629–10631},

abstract = {This Special Issue contains 48 invited, peer-reviewed original research articles collected on the occasion of the 18th International Symposium on Small Particles and Inorganic Clusters (ISSPIC XVIII), held August 14–19, 2016, in Jyväskylä, Finland. The conference gathered about 225 participants from 25 countries all over the world. The program consisted of 9 plenary lectures, 24 invited talks, 2 memorial lectures, 38 short “hot topic” talks selected from contributed abstracts, and 107 poster presentations.

The ISSPIC conference series started in 1976 in Lyon, France. For 40 years, it has been one of the premier conferences in the areas of atomic and molecular clusters, nanoparticles, and nanostructures. (1) The conference has provided an interdisciplinary forum for presentations and discussion of fundamental and technological developments in these fields. Researchers in the ISSPIC community have been working on nanoscience long before the term became as popular as it is today. From its start, one central theme in the field has been to try to understand how matter organizes itself from the atomic and molecular dimensions to nanoparticle regime and finally to bulk and how the various physical and chemical properties of a nanometer-size chunk of material are affected by its dimensionality, size, and the environment. ISSPIC XVIII made an effort to enhance the visibility of cluster science to neighboring areas of nanoparticle catalysis, interface chemistry, plasmonics, biological systems, and research on climate change while not forgetting the traditional themes in the field. It is our sincere hope that cluster science continues to be vibrant and will renew itself for the many upcoming ISSPIC conferences, the next one being organized in August 2018 in Hangzhou, China.

Very many great scientists have been influential in the ISSPIC community, starting from the organizer of the very first ISSPIC, the late Jacques Friedel. With sadness but much gratitude, at ISSPIC XVIII we paid tribute to two of the greatest names in cluster science, recently passed away: T. Patrick Martin (1936–2015) and 1996 Nobel Laureate, Sir Harry Kroto (1939–2016). It is hoped that their scientific legacy and example, briefly reviewed below, will motivate new generations of cluster scientists to explore the world of unknown.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{RN4068,

title = {The structure of coronene cluster ions inferred from H2 uptake in the gas phase},

author = {M. Goulart and M. Kuhn and B. Rasul and J. Postler and M. Gatchell and H. Zettergren and P. Scheier and O. Echt},

doi = {10.1039/C7CP04999D},

year  = {2017},

date = {2017-01-01},

urldate = {2017-01-01},

journal = {Phys.Chem.Chem.Phys.},

volume = {19},

pages = {27968 - 27973},

abstract = {Mass spectra of helium nanodroplets doped with H2 and coronene feature anomalies in the ion abundance that reveal anomalies in the energetics of adsorption sites. The coronene monomer ion strongly adsorbs up to n = 38 H2 molecules indicating a commensurate solvation shell that preserves the D6h symmetry of the substrate. No such feature is seen in the abundance of the coronene dimer through tetramer complexed with H2; this observation rules out a vertical columnar structure. Instead we see evidence for a columnar structure in which adjacent coronenes are displaced in parallel, forming terraces that offer additional strong adsorption sites. The experimental value for the number of adsorption sites per terrace, approximately six, barely depends on the number of coronene molecules. The displacement estimated from this number exceeds the value reported in several theoretical studies of the bare, neutral coronene dimer.},

note = {Open access},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{RN4040,

title = {Adsorption of sodium and cesium on aggregates of C60},

author = {M. Harnisch and M. Daxner and P. Scheier and O. Echt},

doi = {10.1140/epjd/e2016-70438-4},

year  = {2016},

date = {2016-01-01},

urldate = {2016-01-01},

journal = {Eur.Phys.J. D},

volume = {70},

pages = {192},

abstract = {We explore the formation of C60 sodium and C60 cesium complexes in superfluid helium nanodroplets. Anomalies in mass spectra of these doped droplets reveal anomalies in the stability of ions. (C60) m Cs+ n ions ( m ≤ 6) are particularly abundant if they contain n = 6m + 1 cesium atoms; (C60) m Cs2+ n dications ( m ≤ 3 or 5) are abundant if n = 6m + 2. These findings are consistent with the notion that alkali metal atoms (A) transfer their valence electrons into the three-fold degenerate lowest unoccupied orbital of C60, resulting in particularly stable C60A6 building blocks. However, (C60) 4CsCs2+ n dications display an entirely different pattern; instead of an expected anomaly at n = 6 × 4 + 2 = 26 we observe a strong odd-even alternation starting at n = 6. Also surprising is the effect of adding one H2O or CO2 molecule to (C60) m Cs n mono- or dications; anomalies shift by two units as if the impurity were acting as an acceptor for two valence electrons from the alkali metal atoms. },

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{RN4035,

title = {Fission of Multiply Charged Alkali Clusters in Helium Droplets - Approaching the Rayleigh Limit},

author = {Michael Renzler and Martina Harnisch and Matthias Daxner and Lorenz Kranabetter and Martin Kuhn and Paul Scheier and Olof Echt},

doi = {10.1039/C6CP00764C},

year  = {2016},

date = {2016-01-01},

urldate = {2016-01-01},

journal = {Phys.Chem.Chem.Phys.},

volume = {18},

pages = {10623-10629},

abstract = {Electron ionization of helium droplets doped with sodium, potassium or cesium results in doubly and, for cesium, triply charged cluster ions. The smallest observable doubly charged clusters are Na92+, K112+, and Cs92+; they are a factor two to three smaller than reported previously. The size of sodium and potassium dications approaches the Rayleigh limit nRay for which the fission barrier is calculated to vanish, i.e. their fissilities are close to 1. Cesium dications are even smaller than nRay, implying that their fissilities have been significantly overestimated. Triply charged cesium clusters as small as Cs193+ are observed; they are a factor 2.6 smaller than previously reported. Mechanisms that may be responsible for enhanced formation of clusters with high fissilities are discussed.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{RN4056,

title = {Nitrogen Cluster Anions},

author = {Nikolaus Weinberger and Johannes Postler and Paul Scheier and Olof Echt},

doi = {10.1021/acs.jpcc.6b09211},

year  = {2016},

date = {2016-01-01},

urldate = {2016-01-01},

journal = {J. Phys. Chem. C},

volume = {121},

pages = {10632–10637},

abstract = {Anions are formed by electron attachment to helium nanodroplets doped with N2. The most prominent ion series is due to odd-numbered Nm– with 3 ≤ m < 140. Neither N– nor N2– are observed. An appearance energy of 11.5 ± 0.5 eV is measured for N3–. The yield of Nm– averaged over 5 ≤ m ≤ 21 shows an appearance energy of 9 eV, just above the estimated thermodynamic threshold for formation of N3– from electron attachment to small N2 clusters embedded in helium droplets. These findings support the notion that odd-numbered Nm– cluster ions are best characterized as N2 van der Waals clusters with an azide anion chromophore, but they are at odds with some theoretical reports. N3–(N2)4 and N3–(N2)11 form local maxima in the abundance distribution, suggesting that these ions are particularly stable. The yield of even-numbered Nm– ions (m ≥ 4) is 2 orders of magnitude lower, barely exceeding the background level. Several other cluster anion series are observed that involve impurities.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{RN4027,

title = {Adsorption of Helium on Isolated C60 and C70 Anions},

author = {Martina Harnisch and Nikolaus Weinberger and Stephan Denifl and Paul Scheier and Olof Echt},

doi = {10.1080/00268976.2015.1018357},

year  = {2015},

date = {2015-01-01},

urldate = {2015-01-01},

journal = {Mol. Phys.},

volume = {113},

number = {15-16},

pages = {2191-2196},

abstract = {Adsorption of helium on free, negatively charged fullerenes is studied in this work. Helium nanodroplets have been doped with fullerenes and ionised by electron attachment. For suitable experimental conditions, C−60 and C−70 anions are found to be complexed with a large number of helium atoms. Prominent anomalies in the ion abundances indicate the high stability of the commensurate 1×1 phase in which all hollow adsorption sites are occupied by one atom each. The adsorption energy for an additional helium atom is about 40% less than for atoms in the commensurate layer, similar to our previous findings for fullerene cations and in agreement with theoretical dissociation energies. Similarly, an anomaly in the adsorption energy occurs when 60 helium atoms are attached to C−60 or 65 to C−70. For C60, the anomaly coincides with the one observed for cationic complexes but for C70 it does not. Implications of these features are discussed in light of several theoretical studies of neutral and positively charged helium–fullerene complexes.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{RN4026,

title = {Helium Droplets Doped with Sulfur and C60},

author = {Martina Harnisch and Nikolaus Weinberger and Stephan Denifl and Paul Scheier and Olof Echt},

doi = {10.1021/jp510870x},

year  = {2015},

date = {2015-01-01},

urldate = {2015-01-01},

journal = {J. Phys. Chem. C},

volume = {119},

number = {20},

pages = {10919–10924},

abstract = {Clusters of sulfur are grown by passing superfluid helium nanodroplets through a pickup cell filled with sulfur vapor. In some experiments the droplets are codoped with C60. The doped droplets are collided with energetic electrons and the abundance distributions of positively and negatively charged cluster ions are recorded. We report, specifically, distributions of Sm+, Sm–, and C60Sm– containing up to 41 sulfur atoms. We also observe complexes of sulfur cluster anions with helium; distributions are presented for HenSm– with n ≤ 31 and m ≤ 3. The similarity between anionic and cationic C60Sm± spectra is in striking contrast to the large differences between spectra of Sm+ and Sm–.},

note = {Special Issue: Current Trends in Clusters and Nanoparticles, Open access},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{RN2463,

title = {Measuring the Heat Capacity of Large Isolated Molecules via Gas-Phase Collisions: C60},

author = {R. Deng and O. Echt},

doi = {10.1021/jp512276e},

year  = {2015},

date = {2015-01-01},

urldate = {2015-01-01},

journal = {J. Phys. Chem. C Special Issue: Current Trends in Clusters and Nanoparticles},

volume = {119},

number = {20},

pages = {11233–11237},

abstract = {We present a novel method to measure the heat capacity of isolated molecules or clusters. Neutral molecules emerge from an effusive source at known temperature. They are heated in a single sticking collision with an atomic ion of known kinetic energy; the breakdown curve of the adduct ion is measured as a function of collision energy. The procedure is repeated for different source temperatures. The heat capacity of the neutral molecule equals the change in the breakdown energy divided by the change in source temperature. The method avoids potential sources of systematic errors inherent to other approaches that involve multiple collisions. The accuracy of the method is demonstrated by colliding an effusive beam of C60 with a monoenergetic beam of Na+ which produces endohedral Na@C60+. The value obtained for the heat capacity of C60 at 535 ± 35 °C agrees with theoretical ones within the experimental uncertainty of 11%.},

note = {Special Issue: Current Trends in Clusters and Nanoparticles},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{RN4012,

title = {Doubly Charged CO2 Clusters Formed by Ionization of Doped Helium Nanodroplets},

author = {M. Daxner and S. Denifl and P. Scheier and O. Echt},

doi = {10.1016/j.ijms.2014.01.016},

year  = {2014},

date = {2014-01-01},

urldate = {2014-01-01},

journal = {Int.J.Mass.Spectrom.},

volume = {365–366},

pages = {200–205},

abstract = {Helium nanodroplets are doped with carbon dioxide and ionized by electrons. Doubly charged cluster ions are, for the first time, identified based on their characteristic patterns of isotopologues. Thanks to the high mass resolution, large dynamic range, and a novel method to eliminate contributions from singly charged ions from the mass spectra, we are able to observe doubly charged cluster ions that are smaller than the ones reported in the past. The likely mechanism by which doubly charged ions are formed in doped helium droplets is discussed. },

note = {Open access},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{RN4013,

title = {Ordered Phases of Ethylene Adsorbed on Charged Fullerenes and their Aggregates},

author = {Samuel Zöttl and Alexander Kaiser and Matthias Daxner and Marcelo Goulart and Andreas Mauracher and Michael Probst and Frank Hagelberg and Stephan Denifl and Paul Scheier and Olof Echt},

doi = {DOI 10.1016/j.carbon.2013.12.017},

year  = {2014},

date = {2014-01-01},

urldate = {2014-01-01},

journal = {Carbon},

volume = {69},

pages = {206–220},

abstract = {In spite of extensive investigations of ethylene adsorbed on graphite, bundles of nanotubes, and crystals of fullerenes, little is known about the existence of commensurate phases; they have escaped detection in almost all previous work. Here we present a combined experimental and theoretical study of ethylene adsorbed on free C60 and its aggregates. The ion yield of [Formula: see text] measured by mass spectrometry reveals a propensity to form a structurally ordered phase on monomers, dimers and trimers of C60 in which all sterically accessible hollow sites over carbon rings are occupied. Presumably the enhancement of the corrugation by the curvature of the fullerene surface favors this phase which is akin to a hypothetical 1 × 1 phase on graphite. Experimental data also reveal the number of molecules in groove sites of the C60 dimer through tetramer. The identity of the sites, adsorption energies and orientations of the adsorbed molecules are determined by molecular dynamics calculations based on quantum chemical potentials, as well as density functional theory. The decrease in orientational order with increasing temperature is also explored in the simulations whereas in the experiment it is impossible to vary the temperature. },

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{RN4008,

title = {On the size and structure of helium snowballs formed around charged atoms and clusters of noble gases},

author = {P. Bartl and C. Leidlmair and S. Denifl and P. Scheier and O. Echt},

doi = {10.1021/jp406540p},

year  = {2014},

date = {2014-01-01},

urldate = {2014-01-01},

journal = {J. Phys. Chem. A},

volume = {118},

number = {37},

pages = {8050–8059},

abstract = {Helium nanodroplets doped with argon, krypton, or xenon are ionized by electrons and analyzed in a mass spectrometer. HenNgx+ ions containing up to seven noble gas (Ng) atoms and dozens of helium atoms are identified; the high resolution of the mass spectrometer combined with advanced data analysis make it possible to unscramble contributions from isotopologues that have the same nominal mass but different numbers of helium or Ng atoms, such as the magic He2084Kr2+ and the isobaric, nonmagic He4184Kr+. Anomalies in these ion abundances reveal particularly stable ions; several intriguing patterns emerge. Perhaps most astounding are the results for HenAr+, which show evidence for three distinct, solid-like solvation shells containing 12, 20, and 12 helium atoms. This observation runs counter to the common notion that only the first solvation shell is solid-like but agrees with calculations by Galli et al. for HenNa+ [ J. Phys. Chem. A 2011, 115, 7300] that reveal three shells of icosahedral symmetry. HenArx+ (2 ≤ x ≤ 7) ions appear to be especially stable if they contain a total of n + x = 19 atoms. A sequence of anomalies in the abundance distribution of HenKrx+ suggests that rings of six helium atoms are inserted into the solvation shell each time a krypton atom is added to the ionic core, from Kr+ to Kr3+. Previously reported strong anomalies at He12Kr2+ and He12Kr3+ [Kim, J. H.; et al. J. Chem. Phys. 2006, 124, 214301] are attributed to a contamination. Only minor local anomalies appear in the distributions of HenXex+ (x ≤ 3). The distributions of HenKr+ and HenXe+ show strikingly similar, broad features that are absent from the distribution of HenAr+; differences are tentatively ascribed to the very different fragmentation dynamics of these ions.},

note = {Open access},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{RN4022,

title = {On Subthreshold Ionization of Helium Droplets, Ejection of He+, and the Role of Anions},

author = {Michael Renzler and Matthias Daxner and Nikolaus Weinberger and Stephan Denifl and Paul Scheier and Olof Echt},

doi = {10.1039/C4CP03236E},

year  = {2014},

date = {2014-01-01},

urldate = {2014-01-01},

journal = {Phys.Chem.Chem.Phys.},

volume = {16},

pages = {22466-22470},

abstract = {The mechanism of ionization of helium droplets has been investigated in numerous reports but one observation has not found a satisfactory explanation: How are He+ ions formed and ejected from undoped droplets at electron energies below the ionization threshold of the free atom? Does this path exist at all? A measurement of the ion yields of He+ and He2+ as a function of electron energy, electron emission current, and droplet size reveals that metastable He*- anions play a crucial role in the formation of free He+ at subthreshold energies. The proposed model is testable.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{RN3998,

title = {Adsorption of Hydrogen on Neutral and Charged Fullerene: Experiment and Theory},

author = {A. Kaiser and C. Leidlmair and P. Bartl and S. Zöttl and S. Denifl and A. Mauracher and M. Probst and P. Scheier and O. Echt},

doi = {10.1063/1.4790403},

year  = {2013},

date = {2013-01-01},

urldate = {2013-01-01},

journal = {J.Chem.Phys.},

volume = {138},

number = {7},

pages = {074311},

abstract = {Helium droplets are doped with fullerenes (either C60 or C70) and hydrogen (H2 or D2) and investigated by high-resolution mass spectrometry. In addition to pure helium and hydrogen cluster ions, hydrogen-fullerene complexes are observed upon electron ionization. The composition of the main ion series is (H2)(n)HC(m)(+) where m = 60 or 70. Another series of even-numbered ions, (H2)(n)C(m)(+), is slightly weaker in stark contrast to pure hydrogen cluster ions for which the even-numbered series (H2)(n)(+) is barely detectable. The ion series (H2)(n)HC(m)(+) and (H2)(n)C(m)(+) exhibit abrupt drops in ion abundance at n = 32 for C60 and 37 for C70, indicating formation of an energetically favorable commensurate phase, with each face of the fullerene ion being covered by one adsorbate molecule. However, the first solvation layer is not complete until a total of 49 H2 are adsorbed on C60(+); the corresponding value for C70(+) is 51. Surprisingly, these values do not exhibit a hydrogen-deuterium isotope effect even though the isotope effect for H2/D2 adsorbates on graphite exceeds 6%. We also observe doubly charged fullerene-deuterium clusters; they, too, exhibit abrupt drops in ion abundance at n = 32 and 37 for C60 and C70, respectively. The findings imply that the charge is localized on the fullerene, stabilizing the system against charge separation. Density functional calculations for C60-hydrogen complexes with up to five hydrogen atoms provide insight into the experimental findings and the structure of the ions. The binding energy of physisorbed H2 is 57 meV for H2C60(+) and (H2)2C60(+), and slightly above 70 meV for H2HC60(+) and (H2)2HC60(+). The lone hydrogen in the odd-numbered complexes is covalently bound atop a carbon atom but a large barrier of 1.69 eV impedes chemisorption of the H2 molecules. Calculations for neutral and doubly charged complexes are presented as well. },

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{RN4007,

title = {Adsorption of Polar and Non-Polar Molecules on Isolated Cationic C60, C70, and their Aggregates},

author = {Olof Echt and Alexander Kaiser and Samuel Zöttl and Andreas Mauracher and Stephan Denifl and Paul Scheier},

doi = {10.1002/cplu.201300198},

year  = {2013},

date = {2013-01-01},

urldate = {2013-01-01},

journal = {ChemPlusChem},

volume = {78},

number = {9},

pages = {910-920},

abstract = {Physisorption on graphite, graphene, nanotubes, and other graphitic structures has been the subject of numerous studies, partly driven by interest in the nature of order in two-dimensional systems, their phase transitions, and the use of graphitic scaffolds for reversible storage of hydrogen at high volumetric density and low mass. In contrast, physisorption on individual fullerenes or small aggregates of fullerenes has remained largely unexplored, last but not least, because of technical challenges. A summary of recent progress in identifying specific adsorption sites on positively charged C60, C70, and their aggregates is given in this Minireview. Adsorption energies and storage capacities for helium, hydrogen, methane, oxygen, nitrogen, water, and ammonia are determined. Mass spectrometric data reveal the formation of a commensurate phase in which all hollow sites of C60 or C70 are occupied. This phase is identified for all nonpolar molecules, including oxygen, which does not form a commensurate phase on planar graphite. The polar molecules, on the other hand, do not wet fullerenes and they do not form this commensurate phase. A hierarchy of other distinct adsorption sites are identified for nonpolar molecules, namely, groove sites for fullerene dimers and beyond, and dimple sites for fullerene trimers and beyond. Furthermore, evidence is presented for the preferential adsorption of hydrogen and methane in registered sites on fullerene dimers. The interpretation of experimental data that merely count the number of preferred adsorption sites is aided by molecular dynamics simulations, which utilize interaction potentials derived from ab initio calculations to determine adsorption energies.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{RN3997,

title = {Cationic Complexes of Hydrogen with Helium},

author = {P. Bartl and C. Leidlmair and S. Denifl and P. Scheier and O. Echt},

doi = {10.1002/cphc.201200664},

year  = {2013},

date = {2013-01-01},

urldate = {2013-01-01},

journal = {ChemPhysChem},

volume = {14},

number = {1},

pages = {227-232},

abstract = {High-resolution mass spectra of helium nanodroplets doped with hydrogen or deuterium reveal that copious amounts of helium can be bound to H+, H2+, H3+, and larger hydrogen-cluster ions. All conceivable HenHx+ stoichiometries are identified if their mass is below the limit of ≈120 u set by the resolution of the spectrometer. Anomalies in the ion yields of HenHx+ for x=1, 2, or 3, and n≤30 reveal particularly stable cluster ions. Our results for HenH1+ are consistent with conclusions drawn from previous experimental and theoretical studies which were limited to smaller cluster ions. The HenH3+ series exhibits a pronounced anomaly at n=12 which was outside the reliable range of earlier experiments. Contrary to findings reported for other diatomic dopant molecules, the monomer ion (i.e. H2+) retains helium with much greater efficiency than hydrogen-cluster ions.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{RN4002,

title = {Methane Adsorption on Aggregates of Fullerenes: Site-Selective Storage Capacities and Adsorption Energies},

author = {A. Kaiser and S. Zöttl and P. Bartl and C. Leidlmair and A. Mauracher and M. Probst and S. Denifl and O. Echt and P. Scheier},

doi = {10.1002/cssc.201300133},

year  = {2013},

date = {2013-01-01},

urldate = {2013-01-01},

journal = {ChemSusChem},

volume = {6},

number = {7},

pages = {1235 – 1244},

abstract = {Methane adsorption on positively charged aggregates of C60 is investigated by both mass spectrometry and computer simulations. Calculated adsorption energies of 118-281 meV are in the optimal range for high-density storage of natural gas. Groove sites, dimple sites, and the first complete adsorption shells are identified experimentally and confirmed by molecular dynamics simulations, using a newly developed force field for methane-methane and fullerene-methane interaction. The effects of corrugation and curvature are discussed and compared with data for adsorption on graphite, graphene, and carbon nanotubes. },

note = {Open access},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{RN4006,

title = {On the Stability of Cationic Complexes of Neon with Helium - Solving an Experimental Discrepancy},

author = {P. Bartl and S. Denifl and P. Scheier and O. Echt},

doi = {10.1039/C3CP52550C},

year  = {2013},

date = {2013-01-01},

urldate = {2013-01-01},

journal = {Phys.Chem.Chem.Phys.},

volume = {15},

pages = {16599-16604},

abstract = {Helium nanodroplets are doped with neon and ionized by electrons. The size-dependence of the ion abundance of HenNex+, identified in high-resolution mass spectra, is deduced for complexes containing up to seven neon atoms and dozens of helium atoms. Particularly stable ions are inferred from anomalies in the abundance distributions. Two pronounced anomalies at n = 11 and 13 in the HenNe+ series confirm drift-tube data reported by Kojima et al. [T. M. Kojima et al., Z. Phys. D, 1992, 22, 645]. The discrepancy with previously published spectra of neon-doped helium droplets, which did not reveal any abundance anomalies [T. Ruchti et al., J. Chem. Phys., 1998, 109, 10679–10687; C. A. Brindle et al., J. Chem. Phys., 2005, 123, 064312], is most likely due to limited mass resolution, which precluded unambiguous analysis of contributions from different ions with identical nominal mass. However, calculated dissociation energies of HenNe+ reported so far do not correlate with the present data, possibly because of challenges in correctly treating the linear, asymmetric [He–Ne–He]+ ionic core in HenNe+. Anomalies identified in the distributions of HenNex+ for x > 1, including prominent ones at He12Ne2+ and He14Ne2+, may help to better understand solvation of Ne+ and Nex+ in helium.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{RN3999,

title = {Electron Ionization of Superfluid Helium Nanodroplets Doped with C60 and Small Molecules},

author = {H. Schöbel and C. Leidlmair and P. Bartl and S. Denifl and T. D. Märk and O. Echt and P. Scheier},

doi = {10.1088/1742-6596/388/1/012044},

year  = {2012},

date = {2012-01-01},

urldate = {2012-01-01},

journal = {J. Phys. C Conference Series},

volume = {388},

pages = {012044},

abstract = {Helium droplets were co-doped with C60 and ammonia, water or carbon dioxide and the resulting cluster size distributions of the produced cationic complexes are presented. Mass spectra obtained by electron ionization were recorded with high mass resolution of m/Δm ~ 6000 (FWHM). Series of C60(H2O)n+, C60(NH3)n+ and C60(CO2)n+ complexes for n up to 20 molecules attached to the C60 were observed among other cations. For C60-water cluster ions, the resulting size distribution revealed magic numbers for n=4 and n=7 water molecules attached to the fullerene. Ionic C60-ammonia complexes showed an enhanced stability in the cluster size distribution for n=4 ammonia molecules attached as well. However, no intensity anomalies were observed for larger n. For the nonpolar carbon dioxide, no magic numbers were found in the cluster size distribution of C60(CO2)n+. These findings suggest that single carbon dioxide molecules are homogeneously distributed on the C60; in contrast to ammonia and water, where clusters are attached to the fullerene. In addition, dehydrogenation and protonation reactions of water and ammonia clusters on C60 were observed in the present study.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}