Ionization of Ammonia Nanoices with Adsorbed Methanol Molecules

Large ammonia clusters represent a model system of ices that are omnipresent throughout the space. The interaction of ammonia ices with other hydrogen-boding molecules such as methanol or water and their behavior upon an ionization are thus relevant in the astrochemical context. In this study, ammonia clusters (NH₃) _N with the mean size N̅ ≈ 230 were prepared in molecular beams and passed through a pickup cell in which methanol molecules were adsorbed. At the highest exploited pickup pressures, the average composition of (NH₃) _N(CH₃OH) _M clusters was estimated to be N: M ≈ 210:10. On the other hand, the electron ionization of these clusters yielded about 75% of methanol-containing fragments (NH₃) _n(CH₃OH) _mH⁺ compared to 25% contribution of pure ammonia (NH₃) _nH⁺ ions. On the basis of this substantial disproportion, we propose the following ionization mechanism: The prevailing ammonia is ionized in most cases, resulting in NH₄⁺ core solvated most likely with four ammonia molecules, yielding the well-known "magic number" structure (NH₃)₄NH₄⁺. The methanol molecules exhibit a strong propensity for sticking to the fragment ion. We have also considered mechanisms of intracluster reactions. In most cases, proton transfer between ammonia units take place. The theoretical calculations suggested the proton transfer either from the methyl group or from the hydroxyl group of the ionized methanol molecule to ammonia to be the energetically open channels. However, the experiments with selectively deuterated methanols did not show any evidence for the D⁺ transfer from the CD₃ group. The proton transfer from the hydroxyl group could not be excluded entirely or confirmed unambiguously by the experiment.

Uptake and accommodation of water clusters by adamantane clusters in helium droplets: interplay between magic number clusters

We report an experimental study of water clusters as guests in interactions with clusters of adamantane (Ad) as hosts that occur in doped helium droplets at extremely low temperatures. Separate experiments with pure water as dopant showed ready formation of a distribution of water clusters (H2O)mH+ that peaks at m = 11 and extends beyond m = 100 with local maxima at m = 4, 11, 21, 28 and 30 with (H2O)21H+ being the most anomalous and showing the greatest stability with respect to clusters immediately adjacent in water content. When adamantane is also added as a dopant, extensive hydration is seen in the formation of water/adamantane clusters, (H2O)mAdn+; magic number clusters (H2O)21Adn+ are seen for all the adamantane clusters. Other magic numbers for water clusters attached to adamantane, (H2O)mAdn+, are as for pristine protonated water, with m = 28 and m = 30. The icosahedral shell closure of pure adamantane at n = 13 and 19 appears to be preserved with (H2O)21 replacing one adamantane. (H2O)21Ad12+ and (H2O)21Ad18+ stand out in intensity and demonstrate the interplay of magic number water clusters with magic number adamantane clusters, observed perhaps for the first time in gas-phase cluster chemistry. There was no clear evidence for the formation of clathrate hydrates in which adamantane is trapped within structured water.

Doubly charged coronene clusters-Much smaller than previously observed

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.

Highly Stable [C60AuC60]+/- Dumbbells

Ionic complexes between gold and C₆₀ have been observed for the first time. Cations and anions of the type [Au(C₆₀)₂]^+/- are shown to have particular stability. Calculations suggest that these ions adopt a C₆₀-Au-C₆₀ sandwich-like (dumbbell) structure, which is reminiscent of [XAuX]^+/- ions previously observed for much smaller ligands. The [Au(C₆₀)₂]^+/- ions can be regarded as Au(I) complexes, regardless of whether the net charge is positive or negative, but in both cases, the charge transfer between the Au and C₆₀ is incomplete, most likely because of a covalent contribution to the Au-C₆₀ binding. The C₆₀-Au-C₆₀ dumbbell structure represents a new architecture in fullerene chemistry that might be replicable in synthetic nanostructures.

The adsorption of helium atoms on small cationic gold clusters

Adducts formed between small gold cluster cations and helium atoms are reported for the first time. These binary ions, Aun+Hem, were produced by electron ionization of helium nanodroplets doped with neutral gold clusters and were detected using mass spectrometry. For a given value of n, the distribution of ions as a function of the number of added helium atoms, m, has been recorded. Peaks with anomalously high intensities, corresponding to so-called magic number ions, are identified and interpreted in terms of the geometric structures of the underlying Aun+ ions. These features can be accounted for by planar structures for Aun+ ions with n ≤ 7, with the addition of helium having no significant effect on the structures of the underlying gold cluster ions. According to ion mobility studies and some theoretical predictions, a 3-D structure is expected for Au8+. However, the findings for Au8+ in this work are more consistent with a planar structure.

Isomeric Broadening of C60+ Electronic Excitation in Helium Droplets: Experiments Meet Theory

Helium is considered an almost ideal tagging atom for cold messenger spectroscopy experiments. Although helium is bound very weakly to the ionic molecule of interest, helium tags can lead to shifts and broadenings that we recorded near 963.5 nm in the electronic excitation spectrum of C60+ solvated with up to 100 helium atoms. Dedicated quantum calculations indicate that the inhomogeneous broadening is due to different binding energies of helium to the pentagonal and hexagonal faces of C60+, their dependence on the electronic state, and the numerous isomeric structures that become available for intermediate coverage. Similar isomeric effects can be expected for optical spectra of most larger molecules surrounded by nonabsorbing weakly bound solvent molecules, a situation encountered in many messenger-tagging spectroscopy experiments.

Complexes of gold and imidazole formed in helium nanodroplets

We have studied complexes of gold atoms and imidazole (C₃N₂H₄) produced in helium nanodroplets.

We have studied complexes of gold atoms and imidazole (C₃N₂H₄, abbreviated Im) produced in helium nanodroplets. Following the ionization of the doped droplets we detect a broad range of different Au_mIm_n⁺ complexes, however we find that for specific values of m certain n are “magic” and thus particularly abundant. Our density functional theory calculations indicate that these abundant clusters sizes are partially the result of particularly stable complexes, e.g. AuIm₂⁺, and partially due to a transition in fragmentation patterns from the loss of neutral imidazole molecules for large systems to the loss of neutral gold atoms for smaller systems.

Temperature dependence of dissociative electron attachment to bromo-chlorotoluene isomers: Competition between detachment of Cl- and Br. J Chem Phys 2018;148:074301. [PMID: 29471640 DOI: 10.1063/1.5013606]

Dissociative electron attachment to three isomers of bromo-chlorotoluene was investigated in the electron energy range from 0 to 2 eV for gas temperatures in the range of 392-520 K using a crossed electron-molecular beam apparatus with a temperature regulated effusive molecular beam source. For all three molecules, both Cl^- and Br^- are formed. The ion yields of both halogenides show a pronounced temperature effect. In the case of Cl^- and Br^-, the influence of the gas temperature can be observed at the threshold peak close to 0 eV. The population of molecules that have some of their out-of-plane modes excited varies strongly in the temperature range investigated, indicating that such vibrations might play a role in the energy transfer towards bond breaking. Potential energy curves for the abstraction of Cl^- and Br^- were calculated and extrapolated into the metastable domain. The barriers in the diabatic curves approximated in this way agree well with the ones derived from the temperature dependence observed in the experiments.

The structure of coronene cluster ions inferred from H2 uptake in the gas phase

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 D_6h 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.

Positively and Negatively Charged Cesium and (C60) m Cs n Cluster Ions

We report on the formation and ionization of cesium and C60Cs clusters in superfluid helium nanodroplets. Size distributions of positively and negatively charged (C60) m Cs n± ions have been measured for m ≤ 7, n ≤ 12. Reproducible intensity anomalies are observed in high-resolution mass spectra. For both charge states, (C60) m Cs3± and (C60) m Cs5± 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) m Cs3- anion yield on electron energy shows a resonance that, surprisingly, oscillates in strength as m increases from 1 to 6.

High-Resolution Electron Attachment to the Water Dimer Embedded in Helium Droplets: Direct Observation of the Electronic Conduction Band Formation

For bulk liquid helium the bottom of the conduction band (V₀) is above the vacuum level. In this case the surface of the liquid represents an electronic surface barrier for an electron to be injected into the liquid. Here we study the electronic conduction band for doped helium droplets of different sizes. Utilizing an electron monochromator, the onset of the (H₂O)₂^- ion yield corresponding to V₀ is determined for helium droplets doped with the water dimer. While for larger droplets the onset approaches the well-known bulk value of about 1 eV, the barrier does not continuously decrease with smaller droplet size. A minimum value of V₀ = 0.76 ± 0.10 eV is observed, which corresponds to a droplet size of N_min = 1600 ± 900. For droplet sizes below N_min, a peak at ∼0 eV appears, which is well-known from neat H₂O clusters. Hence, we interpret N_min as the smallest droplet size in which the electronic band structure is formed in liquid helium droplets.

Communication: Dopant-induced solvation of alkalis in liquid helium nanodroplets

Alkali metal atoms and small alkali clusters are classic heliophobes and when in contact with liquid helium they reside in a dimple on the surface. Here we show that alkalis can be induced to submerge into liquid helium when a highly polarizable co-solute, C₆₀, is added to a helium nanodroplet. Evidence is presented that shows that all sodium clusters, and probably single Na atoms, enter the helium droplet in the presence of C₆₀. Even clusters of cesium, an extreme heliophobe, dissolve in liquid helium when C₆₀ is added. The sole exception is atomic Cs, which remains at the surface.

Electron ionization of helium droplets containing C60 and alcohol clusters

Alcoholic chemical reactions at similar conditions as the interstellar medium can be heavily hampered by the presence of C₆₀.

Anionic Hydrogen Cluster Ions as a New Form of Condensed Hydrogen

We report the first experimental observation of negatively charged hydrogen and deuterium cluster ions, H_{n}^{-} and D_{n}^{-}, where n≥5. These anions are formed by an electron addition to liquid helium nanodroplets doped with molecular hydrogen or deuterium. The ions are stable for at least the lifetime of the experiment, which is several tens of microseconds. Only anions with odd values of n are detected, and some specific ions show anomalously high abundances. The sizes of these "magic number" ions suggest an icosahedral framework of H_{2} (D_{2}) molecules in solvent shells around a central H^{-} (D^{-}) ion. The first three shells, which contain a total of 44 H_{2} or D_{2} molecules, appear to be solidlike, but thereafter a more liquidlike arrangement of the H_{2} (D_{2}) molecules is adopted.

Observation of stable HO4(+) and DO4(+) ions from ion-molecule reactions in helium nanodroplets

Ion-molecule reactions between clusters of H2/D2 and O2 in liquid helium nanodroplets were initiated by electron-induced ionization (at 70 eV). Reaction products were detected by mass spectrometry and can be explained by a primary reaction channel involving proton transfer from H3(+) or H3(+)(H2)n clusters and their deuterated equivalents. Very little HO2(+) is seen from the reaction of H3(+) with O2, which is attributed to an efficient secondary reaction between HO2(+) and H2. On the other hand HO4(+) is the most abundant product from the reaction of H3(+) with oxygen dimer, (O2)2. The experimental data suggest that HO4(+) is a particularly stable ion and this is consistent with recent theoretical studies of this ion.

Building Carbon Bridges on and between Fullerenes in Helium Nanodroplets

We report the observation of sequential encounters of fullerenes with C atoms in an extremely cold environment. Experiments were performed with helium droplets at 0.37 K doped with C60 molecules and C atoms derived from a novel, pure source of C atoms. Very high-resolution mass spectra revealed the formation of carbenes of the type C60(C:)n with n up to 6. Bridge-type bonding of the C adatoms to form the known dumbbell C60═C═C60 also was observed. Density functional theory calculations were performed that elucidated the carbene character of the C60(C:)n species and their structures. Mass spectra taken in the presence of water impurities and in separate experiments with added H2 also revealed the formation of the adducts C60C(n)(H2O)n and C60C(n)(H2)n probably by H-OH and H-H bond insertion, respectively, and nonreactivity for the dumbell. So C adatoms that form carbenes C60(C:)n can endow pristine C60 with a higher chemical reactivity.

Fission of multiply charged alkali clusters in helium droplets - approaching the Rayleigh limit

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 Na9(2+), K11(2+), and Cs9(2+); 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 Cs19(3+) 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.

Decomposition of nitroimidazole ions: experiment and theory

Nitroimidazoles are important compounds with chemotherapeutic applications as antibacterial drugs or as radiosensitizers in radiotherapy. Despite their use in biological applications, little is known about the fundamental properties of these compounds. Understanding the ionization reactions of these compounds is crucial in evaluating the radiosensitization potential and in developing new and more effective drugs. Thus, the present study investigates the decomposition of negative and positive ions of 2-nitroimidazole and 4(5)-nitroimidazole using low- and high-energy Collision-Induced Dissociation (CID) and Electron-Induced Dissociation (EID) by two different mass spectrometry techniques and is supported by quantum chemistry calculations. EID of [M+H](+) leads to more extensive fragmentation than CID and involves many radical cleavages including loss of H˙ leading to the formation of the radical cation, M˙(+). The stability (metastable decay) and the fragmentation (high-energy CID) of the radical cation M˙(+) have been probed in a crossed-beam experiment involving primary electron ionization of the neutral nitroimidazole. Thus, fragments in the EID spectra of [M+H](+) that come from further dissociation of radical cation M˙(+) have been highlighted. The loss of NO˙ radical from M˙(+) is associated with a high Kinetic Energy Release (KER) of 0.98 eV. EID of [M-H](-) also leads to additional fragments compared to CID, however, with much lower cross section. Only EID of [M+H](+) leads to a slight difference in the decomposition of 2-nitroimidazole and 4(5)-nitroimidazole.

Experimental evidence for the influence of charge on the adsorption capacity of carbon dioxide on charged fullerenes

The adsorption of CO₂ is sensitive to charge on a capturing model carbonaceous surface, such as C₆₀ fullerenes.

Electron-Induced Chemistry of Cobalt Tricarbonyl Nitrosyl (Co(CO)3NO) in Liquid Helium Nanodroplets

Electron addition to cobalt tricarbonyl nitrosyl (Co(CO₃NO) and its clusters has been explored in helium nanodroplets. Anions were formed by adding electrons with controlled energies, and reaction products were identified by mass spectrometry. Dissociative electron attachment (DEA) to the Co(CO)₃NO monomer gave reaction products similar to those reported in earlier gas phase experiments. However, loss of NO was more prevalent than loss of CO, in marked contrast to the gas phase. Since the Co-N bond is significantly stronger than the Co-C bond, this preference for NO loss must be driven by selective reaction dynamics at low temperature. For [Co(CO)₃NO] _N clusters, the DEA chemistry is similar to that of the monomer, but the anion yields as a function of electron energy show large differences, with the relatively sharp resonances of the monomer being replaced by broad profiles peaking at much higher electron energies. A third experiment involved DEA of Co(CO)₃NO on a C₆₀ molecule in an attempt to simulate the effect of a surface. Once again, broad ion yield curves are seen, but CO loss now becomes the most probable reaction channel. The implication of these findings for understanding focused electron beam induced deposition of cobalt is described.

Reactions in Nitroimidazole and Methylnitroimidazole Triggered by Low-Energy (0-8 eV) Electrons

Low-energy electrons (0-8 eV) effectively decompose 4-nitroimidazole (4NI) and the two methylated isomers 1-methyl-5-nitroimidazole and 1-methyl-4-nitroimidazole via dissociative electron attachment (DEA). The involved unimolecular decompositions range from simple bond cleavages (loss of H(•), formation of NO2(-)) to complex reactions possibly leading to a complete degradation of the target molecule (formation of CN(-), etc.). At energies below 2 eV, the entire rich chemistry induced by DEA is completely quenched by methylation, as demonstrated in a previous communication (Tanzer, K.; Feketeová, L.; Puschnigg, B.; Scheier, P.; Illenberger. E.; Denifl, S. Angew. Chem., Int. Ed. 2014, 53, 12240). The observation that in 4NI neutral radicals and radical anions are formed via DEA at high efficiency already at threshold (0 eV) may have significant implications for the development of nitroimidazole-based radiosensitizers in tumor radiation therapy.

Helium Droplets Doped with Sulfur and C60

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 S m+, S m-, and C60S m- containing up to 41 sulfur atoms. We also observe complexes of sulfur cluster anions with helium; distributions are presented for He n S m- with n ≤ 31 and m ≤ 3. The similarity between anionic and cationic C60S m± spectra is in striking contrast to the large differences between spectra of S m+ and S m-.

On subthreshold ionization of helium droplets, ejection of He(+), and the role of anions

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.

Extracting cluster distributions from mass spectra: IsotopeFit

The availability of high resolution mass spectrometry in the study of atomic and molecular clusters opens up challenges for the interpretation of the data. In complex systems each resolved mass peak may contain contributions from multiple species because of the isotope structure of constituent elements and because a multitude of different types of clusters with different compositions are present. A computational procedure which can help to identify a specific cluster from this complex dataset and quantify its relative abundance would be extremely helpful to many who work in this field. Here some new software designed for this purpose, known as IsotopeFit, is described.

Electron-driven self-assembly of salt nanocrystals in liquid helium

The self-assembly of salt nanocrystals from chemical reactions inside liquid helium is reported for the first time. Reaction is initiated by an electron impacting a helium nanodroplet containing sodium atoms and SF6 molecules, leading to preferential production of energetically favorable structures based on the unit cell of crystalline NaF. These favorable structures are observed as magic number ions (anomalously intense peaks) in mass spectra and are seen in both cationic and anionic channels in mass spectra, for example, (NaF)n Na(+) and (NaF)n F(-) . In the case of anions the self-assembly is not directly initiated by electrons: the dominant process involves resonant electron-induced production of metastable electronically excited He(-) anions, which then initiate anionic chemistry by electron transfer.

Reactions in nitroimidazole triggered by low-energy (0-2 eV) electrons: methylation at N1-H completely blocks reactivity

Low-energy electrons (LEEs) at energies of less than 2 eV effectively decompose 4-nitroimidazole (4NI) by dissociative electron attachment (DEA). The reactions include simple bond cleavages but also complex reactions involving multiple bond cleavages and formation of new molecules. Both simple and complex reactions are associated with pronounced sharp features in the anionic yields, which are interpreted as vibrational Feshbach resonances acting as effective doorways for DEA. The remarkably rich chemistry of 4NI is completely blocked in 1-methyl-4-nitroimidazole (Me4NI), that is, upon methylation of 4NI at the N1 site. These remarkable results have also implications for the development of nitroimidazole based radiosensitizers in tumor radiation therapy.

Formation of dianions in helium nanodroplets

The formation of dianions in helium nanodroplets is reported for the first time. The fullerene cluster dianions (C60)n(2-) and (C70)n(2-) were observed by mass spectrometry for n≥5 when helium droplets containing the appropriate fullerene were subjected to electron impact at approximately 22 eV. A new mechanism for dianion formation is described, which involves a two-electron transfer from the metastable He(-) ion. As well as the prospect of studying other dianions at low temperature using helium nanodroplets, this work opens up the possibility of a wider investigation of the chemistry of He(-), a new electron-donating reagent.

On the size and structure of helium snowballs formed around charged atoms and clusters of noble gases

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 He20(84)Kr2(+) and the isobaric, nonmagic He41(84)Kr(+). 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.

Detection of Negative Charge Carriers in Superfluid Helium Droplets: The Metastable Anions He*- and He2*-

Helium droplets provide the possibility to study phenomena at the very low temperatures at which quantum mechanical effects are more pronounced and fewer quantum states have significant occupation probabilities. Understanding the migration of either positive or negative charges in liquid helium is essential to comprehend charge-induced processes in molecular systems embedded in helium droplets. Here, we report the resonant formation of excited metastable atomic and molecular helium anions in superfluid helium droplets upon electron impact. Although the molecular anion is heliophobic and migrates toward the surface of the helium droplet, the excited metastable atomic helium anion is bound within the helium droplet and exhibits high mobility. The atomic anion is shown to be responsible for the formation of molecular dopant anions upon charge transfer and thus, we clarify the nature of the previously unidentified fast exotic negative charge carrier found in bulk liquid helium.

Electron attachment to CO2 embedded in superfluid He droplets

Electron attachment to CO₂ embedded in superfluid He droplets leads to ionic complexes of the form (CO₂)_n^– and (CO₂)_nO^– and, at much lower intensities, He containing ions of the form He_m(CO₂)_nO^–. At low energies (<5 eV), predominantly the non-decomposed complexes (CO₂)_n^– are formed via two resonance contributions, similar to electron attachment to pristine CO₂ clusters. The significantly different shapes and relative resonance positions, however, indicate particular quenching and mediation processes in CO₂@He. A series of further resonances in the energy range up to 67 eV can be assigned to electronic excitation of He and capture of the inelastically scattered electron generating (CO₂)_n^– and two additional processes where an intermediately formed He* leads to the nonstoichiometric anions (CO₂)_nO^–.

Doubly charged CO2 clusters formed by ionization of doped helium nanodroplets

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.

Ordered phases of ethylene adsorbed on charged fullerenes and their aggregates

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.

Monocarbon cationic cluster yields from N2/CH4 mixtures embedded in He nanodroplets and their calculated binding energies

The formation of monocarbon cluster ions has been investigated by electron ionization mass spectrometry of cold helium nanodroplets doped with nitrogen/methane mixtures. Ion yields for two groups of clusters, CHmN2(+) or CHmN4(+), were determined for mixtures with different molecular ratios of CH4. The possible geometrical structures of these clusters were analyzed using electronic structure computations. Little correlation between the ion yields and the associated binding energies has been observed indicating that in most cases kinetic control is more important than thermodynamic control for forming the clusters.

Decorating (C60) n+, n = 1-3, with CO2 at low temperatures: Sterically enhanced physisorption

Multiple attachment of CO₂ to the monomer, dimer and trimer cations of C₆₀ has been observed in the mass spectra of He nanodroplets sequentially doped with C₆₀ and CO₂ and exposed to electron ionization at 50 eV. Remarkable anomalies were seen in the ion yield for CO₂ coverage for (C₆₀)₂⁺(CO₂)₈ and (C₆₀)₃⁺(CO₂)_1,2. These provide insight into the influence of steric properties on the nature of physisorption. The enhanced stabilities of (C₆₀)₂⁺(CO₂)₈ and (C₆₀)₃⁺(CO₂)_1,2 are attributed to physisorption inside the "groove" of the dimer and the two "dimples" in the trimer cations of C₆₀. Molecular dynamics simulations provide a qualitative assessment of the observed physisorption and a useful visualization of structural aspects.

NCO(-), a key fragment upon dissociative electron attachment and electron transfer to pyrimidine bases: site selectivity for a slow decay process

We report gas phase studies on NCO(-) fragment formation from the nucleobases thymine and uracil and their N-site methylated derivatives upon dissociative electron attachment (DEA) and through electron transfer in potassium collisions. For comparison, the NCO(-) production in metastable decay of the nucleobases after deprotonation in matrix assisted laser desorption/ionization (MALDI) is also reported. We show that the delayed fragmentation of the dehydrogenated closed-shell anion into NCO(-) upon DEA proceeds few microseconds after the electron attachment process, indicating a rather slow unimolecular decomposition. Utilizing partially methylated thymine, we demonstrate that the remarkable site selectivity of the initial hydrogen loss as a function of the electron energy is preserved in the prompt as well as the metastable NCO(-) formation in DEA. Site selectivity in the NCO(-) yield is also pronounced after deprotonation in MALDI, though distinctly different from that observed in DEA. This is discussed in terms of the different electronic states subjected to metastable decay in these experiments. In potassium collisions with 1- and 3-methylthymine and 1- and 3-methyluracil, the dominant fragment is the NCO(-) ion and the branching ratios as a function of the collision energy show evidence of extraordinary site-selectivity in the reactions yielding its formation.

Formation of HCN+ in heterogeneous reactions of N2(+) and N+ with surface hydrocarbons

A significant increase of the ion yield at m/z 27 in collisions of low-energy ions of N₂⁺ and N⁺ with hydrocarbon-covered room-temperature or heated surfaces of tungsten, carbon-fiber composite, and beryllium, not observed in analogous collisions of Ar⁺, is ascribed to the formation of HCN⁺ in heterogeneous reactions between N₂⁺ or N⁺ and surface hydrocarbons. The formation of HCN⁺ in the reaction with N⁺ indicated an exothermic reaction with no activation barrier, likely to occur even at very low collision energies. In the reaction with N₂⁺, the formation of HCN⁺ was observed to a different degree on these room-temperature and heated (150 and 300 °C) surfaces at incident energies above about 50 eV. This finding suggested an activation barrier or reaction endothermicity of the heterogeneous reaction of about 3–3.5 eV. The main process in N₂⁺ or N⁺ interaction with the surfaces is ion neutralization; the probability of forming the reaction product HCN⁺ was very roughly estimated for both N₂⁺ and N⁺ ions to about one in 10⁴ collisions with the surfaces.

Adsorption of hydrogen on neutral and charged fullerene: experiment and theory

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.

On the stability of cationic complexes of neon with helium--solving an experimental discrepancy

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.

Adsorption of Polar and Nonpolar Molecules on Isolated Cationic C60 , C70 , and Their Aggregates

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.