Ion-molecule reactions in helium nanodroplets doped with C60 and water clusters

Chemistry and physics of dopants embedded in helium droplets

Helium droplets represent a cold inert matrix, free of walls with outstanding properties to grow complexes and clusters at conditions that are perfect to simulate cold and dense regions of the interstellar medium. At sub-Kelvin temperatures, barrierless reactions triggered by radicals or ions have been observed and studied by optical spectroscopy and mass spectrometry. The present review summarizes developments of experimental techniques and methods and recent results they enabled.

London dispersion dominating diamantane packing in helium nanodroplets

Diamantane clusters formed inside superfluid helium nanodroplets were analyzed by time-of-flight mass spectrometry. Distinct cluster sizes were identified as "magic numbers" and the corresponding feasible structures for clusters consisting of up to 19 diamantane molecules were derived from meta-dynamics simulations and subsequent DFT computations. The obtained interaction energies were attributed to London dispersion attraction. Our findings demonstrate that diamantane units readily form assemblies even at low pressures and near-zero Kelvin temperatures, confirming the importance of the intermolecular dispersion effect for condensation of matter.

Signature of the neighbor's quantum nuclear dynamics in the electron transfer mediated decay spectra

We computed fully quantum nuclear dynamics, which accompanies electron transfer mediated decay (ETMD) in weakly bound polyatomic clusters. We considered two HeLi₂ clusters - with Li₂ being either in the singlet electronic ground state or in the triplet first excited state - in which ETMD takes place after ionization of He. The electron transfer from Li₂ to He⁺ leads to the emission of another electron from Li₂ into the continuum. Due to the weak binding of He to Li₂ in the initial states of both clusters, the involved nuclear wavepackets are very extended. This makes both the calculation of their evolution and the interpretation of the results difficult. We showed that despite the highly delocalized nature of the wavepackets the nuclear dynamics in the decaying state is imprinted on the ETMD electron spectra. The analysis of the latter helps understanding the effect which electronic structure and binding strength in the cluster produce on the quantum motion of the nuclei in the decaying state. The results produce a detailed picture of this important charge transfer process in polyatomic systems.

An intense source for cold cluster ions of a specific composition

The demand for nanoscale materials of ultra-high purity and narrow size distribution is addressed. Clusters of Au, C₆₀, H₂O, and serine are produced inside helium nanodroplets using a combination of ionization, mass filtering, collisions with atomic or molecular vapor, and electrostatic extraction, in a specific and novel sequence. The helium droplets are produced in an expansion of cold helium gas through a nozzle into vacuum. The droplets are ionized by electron bombardment and subjected to a mass filter. The ionic and mass-selected helium droplets are then guided through a vacuum chamber filled with atomic or molecular vapor where they collide and "pick up" the vapor. The dopants then agglomerate inside the helium droplets around charge centers to singly charged clusters. Evaporation of the helium droplets is induced by collisions in a helium-filled radio frequency (RF)-hexapole, which liberates the cluster ions from the host droplets. The clusters are analyzed with a time-of-flight mass spectrometer. It is demonstrated that using this sequence, the size distribution of the dopant cluster ions is distinctly narrower compared to ionization after pickup. Likewise, the ion cluster beam is more intense. The mass spectra show, as well, that ion clusters of the dopants can be produced with only few helium atoms attached, which will be important for messenger spectroscopy. All these findings are important for the scientific research of clusters and nanoscale materials in general.

Quantum-classical approach to the reaction dynamics in a superfluid helium nanodroplet. The Ne2 dimer and Ne-Ne adduct formation reaction Ne + Ne-doped nanodroplet

The dynamics of the Ne₂ dimer and Ne-Ne adduct formation in a superfluid helium nanodroplet [(⁴He)_N; T = 0.37 K], Ne + Ne@(⁴He)_N→ Ne₂@(⁴He)_N'/Ne-Ne@(⁴He)_N' + (N-N')⁴He with N = 500, has been investigated using a hybrid approach (quantum and classical mechanics (QM-CM) descriptions for helium and the Ne atoms, respectively) and taking into account the angular momentum of the attacking Ne atom, Ne₍₁₎. Comparison with zero angular momentum QM results of our own shows that the present results are similar to the quantum ones for the initial Ne₍₁₎ velocities (v₀) of 500 and 800 m s^-1 (the former one being the most probable velocity of Ne at 300 K), in all cases leading to the Ne₂ dimer (r_e = 3.09 Å). However, significant differences appear below v₀ = 500 m s^-1, because in the QM-CM dynamics, instead of the dimer, a Ne-Ne adduct is formed (r₀ = 5.45 Å). The formation of this adduct will probably dominate as the contribution to reactivity of angular momenta larger than zero is the leading one and angular momentum strongly acts against the Ne₂ production. Angular momentum adds further difficulties in producing the dimer, since it makes it more difficult to remove the helium density between both Ne atoms to lead, subsequently, to the Ne₂ molecule. Hence, the formation of the neon-neon adduct, Ne-Ne@(⁴He)_N', clearly dominates the reactivity of the system, which results in the formation of a "quantum gel"/"quantum foam", because the two Ne atoms essentially maintain their identity inside the nanodroplet. Large enough Ne₍₁₎ initial angular momentum values can induce the formation of vortex lines by the collapse of superficial excitations (ripplons), but they occur with greater difficulty than in the case of the capture of the Ne atom by a non doped helium nanodroplet, due to the wave interferences induced by the Ne induced by the solvation layers of the Ne atom originally placed inside the nanodroplet. We hope that this work will encourage other researchers to investigate the reaction dynamics in helium nanodroplets, an interesting topic on which there are few studies available.

Solvation of Silver Ions in Noble Gases He, Ne, Ar, Kr, and Xe

We use a novel technique to solvate silver cations in small clusters of noble gases. The technique involves the 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 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 of Ng_nAg_m⁺ 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. On the basis of these data, we identify specific "magic" sizes n of particularly stable ions. There is no evidence, however, 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. Structural models will be tentatively proposed. A sequence of magic numbers n = 12, 32, and 44, indicative of three concentric solvation shells of icosahedral symmetry, is observed for He_nH₂O⁺.

Electron transfer mediated decay in HeLi2 cluster: Potential energy surfaces and decay widths

Electron transfer mediated decay (ETMD) is a process responsible for double ionization of dopants in He droplets. It is initiated by producing He⁺ in the droplet, which is neutralized by ETMD, and has been shown to strongly enhance the dopant's double ionization cross section. The efficiency of ETMD, the spectra of emitted secondary electrons, and the character of the ionic products depend on the nuclear dynamics during the decay. To date, there has been no theoretical investigation of multimode dynamics which accompanies ETMD, which could help to understand such dynamics in a He droplet. In this article, we consider the He-Li₂ cluster where an ab initio examination of multimode dynamics during the electronic decay is feasible. Moreover, this cluster can serve as a minimal model for Li₂ adsorbed on the droplet's surface-a system where ETMD can be observed experimentally. In He droplets, Li₂ can be formed in both the ground X¹Σ_g ⁺ and the first excited a³Σ_u ⁺ states. In this article, we present ab initio potential energy surfaces of the electronic states of the He-Li₂ cluster involved in ETMD, as well as the respective decay widths. We show that the structure of these surfaces and expected nuclear dynamics strongly depend on the electronic state of Li₂. Thus, the overall decay rate and the appearance of the observable electron spectra will be dictated by the electronic structure of the dopant.

Mass spectrometry of aerosol particle analogues in molecular beam experiments

Nanometer-size particles such as ultrafine aerosol particles, ice nanoparticles, water nanodroplets, etc, play an important, however, not yet fully understood role in the atmospheric chemistry and physics. These species are often composed of water with admixture of other atmospherically relevant molecules. To mimic and investigate such particles in laboratory experiments, mixed water clusters with atmospherically relevant molecules can be generated in molecular beams and studied by various mass spectrometric methods. The present review demonstrates that such experiments can provide unprecedented details of reaction mechanisms, and detailed insight into the photon-, electron-, and ion-induced processes relevant to the atmospheric chemistry. After a brief outline of the molecular beam preparation, cluster properties, and ionization methods, we focus on the mixed clusters with various atmospheric molecules, such as hydrated sulfuric acid and nitric acid clusters, N_x O_y and halogen-containing molecules with water. A special attention is paid to their reactivity and solvent effects of water molecules on the observed processes.

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.

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.

Formation of Au and tetrapyridyl porphyrin complexes in superfluid helium

Binary clusters containing a large organic molecule and metal atoms have been formed by the co-addition of 5,10,15,20-tetra(4-pyridyl)porphyrin (H2TPyP) molecules and gold atoms to superfluid helium nanodroplets, and the resulting complexes were then investigated by electron impact mass spectrometry. In addition to the parent ion H2TPyP yields fragments mainly from pyrrole, pyridine and methylpyridine ions because of the stability of their ring structures. When Au is co-added to the droplets the mass spectra are dominated by H2TPyP fragment ions with one or more Au atoms attached. We also show that by switching the order in which Au and H2TPyP are added to the helium droplets, different types of H2TPyP-Au complexes are clearly evident from the mass spectra. This study suggests a new route for the control over the growth of metal-organic compounds inside superfluid helium nanodroplets.

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^–.

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.

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.

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.

Methane adsorption on aggregates of fullerenes: site-selective storage capacities and adsorption energies

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.

Optimal covering of C60 fullerene by rare gases

Putative global energy minima of clusters formed by the adsorption of rare gases on a C(60) fullerene molecule, C(60)X(N) (X=Ne, Ar, Kr, Xe; N ≤ 70), are found using basin-hopping global optimization in an empirical potential energy surface. The association energies per rare gas atom as a function of N present two noticeable minima for Ne and Ar and just one for Kr and Xe. The minimum with the smallest N is the deepest one and corresponds to an optimal packing monolayer structure; the other one gives a monolayer with maximum packing. For Kr and Xe, optimal and maximum packing structures coincide. By using an isotropic average form of the X-C(60) interaction, we have established the relevance of the C(60) surface corrugation on the cluster structures. Quantum effects are relevant for Ne clusters. The adsorption of these rare gases on C(60) follows patterns that differ significantly from the ones found recently for He by means of experimental and theoretical methods.

Communication: Electron impact ionization of binary H2O∕X clusters in helium nanodroplets: an ab initio perspective

In a recent experiment (H(2)O)(n)∕X(m) binary clusters (where X = Ar, N(2), CO, CO(2), and several other molecules) were formed in superfluid helium nanodroplets and investigated by electron impact mass spectrometry [Liu et al., Phys. Chem. Chem. Phys. 13, 13920 (2011)]. The addition of dopant X was found to affect the branching ratio between H(3)O(+)(H(2)O)(n) and (H(2)O)(+)(n+2) formation. Specifically, the addition of CO increased the proportion of protonated water cluster ions, whereas dopants such as Ar, N(2), and CO(2), had the opposite effect. In this work ab initio calculations have been performed on [X(H(2)O)(2)](+) ions, where X = Ar, N(2), CO, and CO(2), to try and explain this distinct behavior. CO is found to be unique in that it forms a HOCO-H(3)O(+) unit in the most stable cationic complexes where the binding between HO and CO is stronger than that between H(3)O(+) and OH. Thus, on purely energetic grounds, loss of HOCO rather than CO should be the preferred fragmentation process. No comparable chemistry occurs when X = Ar, N(2), or CO(2) and so the co-dopant requires less energy to depart than OH. The calculations therefore account for the experimental observations and provide evidence that HOCO formation is induced in helium droplets containing (H(2)O)(n) clusters and co-doped with CO when subject to electron impact ionization.

Ionization of Water Clusters Mediated by Exciton Energy Transfer from Argon Clusters

The exciton energy deposited in an argon cluster (Arn, ⟨n = 20⟩) using VUV radiation is transferred to softly ionize doped water clusters ((H2O)n, n = 1-9), leading to the formation of nonfragmented clusters. Following the initial excitation, electronic energy is channeled to ionize the doped water cluster while evaporating the Ar shell, allowing identification of fragmented and complete water cluster ions. Examination of the photoionization efficiency curve shows that cluster evaporation from excitons located above 12.6 eV is not enough to cool the energized water cluster ion and leads to their dissociation to (H2O)n-2H(+) (protonated) clusters.

Structures and energetics of helium cluster cations: equilibrium geometries revisited through the genetic algorithm approach

Equilibrium geometries and dissociation energies of He(N)(+) clusters have been calculated for N=3-35 using an extended genetic algorithm approach and a semiempirical model of intracluster interactions [P. J. Knowles, J. N. Murrell, and E. J. Hodge, Mol. Phys. 85, 243 (1995)]. A general aufbau principle is formulated for both ionic cores and neutral solvation shells, and the results are thoroughly compared with other theoretical data available for helium cluster cations in literature.

Sequential penning ionization: harvesting energy with ions

We report the observation of the ejection of electrons caused by collisions of excited atoms with ions, rather than neutrals, leading to the production of doubly charged ions. Doping superfluid He droplets with methyl iodide and exposing them to electrons enhances the formation of doubly charged iodine atoms at the threshold for the production of two metastable He atoms. These observations point toward a novel ionization process where doubly charged ions are produced by sequential Penning ionization. In some cases, depending on the neutral target, the process also leads to a subsequent Coulomb explosion of the dopant.

Ion-molecule reactions of ammonia clusters with C60 aggregates embedded in helium droplets

Helium nanodroplets are co-doped with C(60) and ammonia. Mass spectra obtained by electron ionization reveal cations containing ammonia clusters complexed with up to four C(60) units. The high mass resolution of Δm/m≈ 1/6000 makes it possible to separate the contributions of protonated, unprotonated and dehydrogenated ammonia. C(60) aggregates suppress the proton-transfer reaction which usually favors the appearance of protonated ammonia cluster ions. Unprotonated C(x)(NH(3))(n)(+) ions (x = 60, 120, 180) exceed the abundance of the corresponding protonated ions if n < 5; for larger values of n the abundances of C(60)(NH(3))(n)(+) and C(60)(NH)(n-1)NH(4)(+) become about equal. Dehydrogenated C(60)NH(2)(+) ions are relatively abundant; their formation is attributed to a transient doubly charged C(60)-ammonia complex which forms either by an Auger process or by Penning ionization following charge transfer between the primary He(+) ion and C(60). The abundance of C(x)NH(3)(+) and C(x)NH(4)(+) ions (x = 120 or 180) is one to two orders of magnitude weaker than the abundance of ions containing one or two additional ammonia molecules. However, a model involving evaporation of NH(3) or NH(4) from the presumably weakly bound C(x)NH(3)(+) and C(x)NH(4)(+) ions is at odds with the lack of enhancement in the abundance of C(120)(+) and C(180)(+). Mass spectra of C(60) dimers complexed with water complement a previous study of C(60)(H(2)O)(n)(+) recorded at much lower mass resolution.

Ionization of doped helium nanodroplets: complexes of C60 with water clusters

Water clusters are known to undergo an autoprotonation reaction upon ionization by photons or electron impact, resulting in the formation of (H(2)O)(n)H(3)O(+). Ejection of OH cannot be quenched by near-threshold ionization; it is only partly quenched when clusters are complexed with inert gas atoms. Mass spectra recorded by electron ionization of water-doped helium droplets show that the helium matrix also fails to quench OH loss. The situation changes drastically when helium droplets are codoped with C(60). Charged C(60)-water complexes are predominantly unprotonated; C(60)(H(2)O)(4)(+) and (C(60))(2)(H(2)O)(4)(+) appear with enhanced abundance. Another intense ion series is due to C(60)(H(2)O)(n)OH(+); dehydrogenation is proposed to be initiated by charge transfer between the primary He(+) ion and C(60). The resulting electronically excited C(60)(+*) leads to the formation of a doubly charged C(60)-water complex either via emission of an Auger electron from C(60)(+*), or internal Penning ionization of the attached water complex, followed by charge separation within {C(60)(H(2)O)(n)}(2+). This mechanism would also explain previous observations of dehydrogenation reactions in doped helium droplets. Mass-analyzed ion kinetic energy scans reveal spontaneous (unimolecular) dissociation of C(60)(H(2)O)(n)(+). In addition to the loss of single water molecules, a prominent reaction channel yields bare C(60)(+) for sizes n=3, 4, or 6. Ab initio Hartree-Fock calculations for C(60)-water complexes reveal negligible charge transfer within neutral complexes. Cationic complexes are well described as water clusters weakly bound to C(60)(+). For n=3, 4, or 6, fissionlike desorption of the entire water complex from C(60)(H(2)O)(n)(+) energetically competes with the evaporation of a single water molecule.