An intense source for cold cluster ions of a specific composition

Size limits and fission channels of doubly charged noble gas clusters

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.

Gas-Phase Electronic Structure of Phthalocyanine Ions: A Study of Symmetry and Solvation Effects

Research into and applications of phthalocyanines (Pc) are mostly connected to their intriguing electronic properties. Here, messenger-type UV-vis spectroscopy of two metal-free ions from the phthalocyanine family, cationic H2Pc+ and H2PcD+, along with their hydrates is performed. They show that the electronic properties of both ions can be traced to those in the conjugate base, Pc2-, however, they are affected by state splitting due to the reduced symmetry; in the H2Pc+ radical cation, a new band appears due to excitations into the singly-occupied molecular orbital. Quantum chemical spectra modeling reproduces all important features of the measured spectra and provides insight into the nature of electronic transitions. Hydration of the ions has only a mild effect on the electronic spectra, showing the stability of the electronic structure with respect to solvation effects.

High-level ab initio evidence of bipyramidal Cu5 clusters as fluxional Jahn-Teller molecules

Novel highly selective synthesis techniques have enable the production of atomically precise monodisperse metal clusters (AMCs) of subnanometer size. These AMCs exhibit 'molecule-like' structures that have distinct physical and chemical properties, significantly different from those of nanoparticles and bulk material. In this work, we study copper pentamer Cu5 clusters as model AMCs by applying both density functional theory (DFT) and high-level (wave-function-based) ab initio methods, including those which are capable of accounting for the multi-state multi-reference character of the wavefunction at the conical intersection (CI) between different electronic states and augmenting the electronic basis set till achieving well-converged energy values and structures. After assessing the accuracy of a high-level multi-multireference ab initio protocol for the well-known Cu3 case, we apply it to demonstrate that bypiramidal Cu5 clusters are distorted Jahn-Teller (JT) molecules. The method is further used to evaluate the accuracy of single-reference approaches, finding that the coupled cluster singles and doubles and perturbative triples CCSD(T) method delivers the results closer to our ab initio predictions and that dispersion-corrected DFT can outperform the CCSD method. Finally, we discuss how JT effects and, more generally, conical intersections, are intimately connected to the fluxionality of AMCs, giving them a 'floppy' character that ultimately facilitates their interaction with environmental molecules and thus enhances their functioning as catalysts.

Solvation of cationic copper clusters in molecular hydrogen

Multiply charged superfluid helium nanodroplets are utilized to facilitate the growth of cationic copper clusters (Cun+, where n = 1-8) that are subsequently solvated with up to 50 H2 molecules. Production of both pristine and protonated cationic Cu clusters are detected mass spectrometrically. A joint effort between experiment and theory allows us to understand the nature of the interactions determining the bonding between pristine and protonated Cu+ and Cu2+ cations and molecular hydrogen. The analysis reveals that in all investigated cationic clusters, the primary solvation shell predominantly exhibits a covalent bonding character, which gradually decreases in strength, while for the subsequent shells an exclusive non-covalent behaviour is found. Interestingly, the calculated evaporation energies associated with the first solvation shell markedly surpass thermal values, positioning them within the desirable range for hydrogen storage applications. This comprehensive study not only provides insights into the solvation of pristine and protonated cationic Cu clusters but also sheds light on their unique bonding properties.

Superfluid helium droplet-mediated surface-deposition of neutral and charged silver atomic species

Experimental and theoretical work has delivered evidence of the helium nanodroplet-mediated synthesis and soft-landing of metal nanoparticles, nanowires, clusters, and single atoms on solid supports. Recent experimental advances have allowed the formation of charged metal clusters into multiply charged helium nanodroplets. The impact of the charge of immersed metal species in helium nanodroplet-mediated surface deposition is proved by considering silver atoms and cations at zero-temperature graphene as the support. By combining high-level ab initio intermolecular interaction theory with a full quantum description of the superfluid helium nanodroplet motion, evidence is presented that the fundamental mechanism of soft-deposition is preserved in spite of the much stronger interaction of charged species with surfaces, with high-density fluctuations in the helium droplet playing an essential role in braking them. Corroboration is also presented that the soft-landing becomes favored as the helium nanodroplet size increases.

Spectroscopy of helium-tagged molecular ions-Development of a novel experimental setup

In this contribution, we present an efficient and alternative method to the commonly used RF-multipole trap technique to produce He-tagged molecular ions at cryogenic temperatures, which are perfectly suitable for messenger spectroscopy. The seeding of dopant ions in multiply charged helium nanodroplets, in combination with a gentle extraction of the latter from the helium matrix, enables the efficient production of He-tagged ion species. With a quadrupole mass filter, a specific ion of interest is selected, merged with a laser beam, and the photoproducts are measured in a time-of-flight mass-spectrometer. The detection of the photofragment signal from a basically zero background is much more sensitive than the depletion of the same amount of signal from precursor ions, delivering high quality spectra at reduced data acquisition times. Proof-of-principle measurements of bare and He-tagged Ar-cluster ions, as well as of He-tagged C60 ions, are presented.

Observation of Multiple Ordered Solvation Shells in Doped Helium Droplets: The Case of HeNCa2

In this Letter, we report the experimental detection of likely the largest ordered structure of helium atoms surrounding a monatomic impurity observed to date using a recently developed technique. The mass spectrometry investigation of He_NCa²⁺ clusters, formed in multiply charged helium nanodroplets, reveals magic numbers at N = 12, 32, 44, and 74. Classical optimization and path integral Monte Carlo calculations suggest the existence of up to four shells surrounding the calcium dication which are closed with well-ordered Mozartkugel-like structures: He₁₂Ca²⁺ with an icosahedron, the second at He₃₂Ca²⁺ with a dodecahedron, the third at He₄₄Ca²⁺ with a larger icosahedron, and finally for He₇₄Ca²⁺, we find that the outermost He atoms form an icosidodecahedron which contains the other inner shells. We analyze the reasons for the formation of such ordered shells in order to guide the selection of possible candidates to exhibit a similar behavior.

Structure and formation of copper cluster ions in multiply charged He nanodroplets

The structure of cationic and anionic Cu clusters grown in multiply charged superfluid He nanodroplets was investigated using He tagging as a chemical probe. Further, the structure assignment was done based on the magic-numbered ions, representing the most energetically favorable structures. The exact geometry of the cluster and positions of He is verified by calculations. It was found that the structure of the clusters grown in the He droplets is similar to that produced with a laser ablation source and the lowest energy structures predicted by theoretical investigations. The only difference is the structure of the Cu₅⁺, which in our experiments has a twisted-X geometry, rather than a bipyramid or planar half-wheel geometry suggested by previous studies. This might be attributed to the different cluster formation mechanisms, the absence of the Ar-tag and the ultracold environment. It was also found that He tends to bind to partially more electro-negative or positive areas of the anionic or cationic clusters, respectively.

Non-ergodic fragmentation upon collision-induced activation of cysteine-water cluster cations

Cysteine-water cluster cations Cys(H₂O)_3,6⁺ and Cys(H₂O)_3,6H⁺ are assembled in He droplets and probed by tandem mass spectrometry with collision-induced activation. Benchmark experimental data for this biologically important system are complemented with theory to elucidate the details of the collision-induced activation process. Experimental energy thresholds for successive release of water are compared to water dissociation energies from DFT calculations showing that clusters do not only fragment exclusively by sequential emission of single water molecules but also by the release of small water clusters. Release of clustered water is observed also in the ADMP (atom centered density matrix propagation) molecular dynamics model of small Cys(H₂O)₃⁺ and Cys(H₂O)₃H⁺ clusters. For large clusters Cys(H₂O)₆⁺ and Cys(H₂O)₆H⁺ the less computationally demanding statistical Microcanonical Metropolis Monte-Carlo method (M₃C) is used to model the experimental fragmentation patterns. We are able to detail the energy redistribution in clusters upon collision activation. In the present case, about two thirds of the collision energy redistribute via an ergodic process, while the remaining one third is transferred into a non-ergodic channel leading to ejection of a single water molecule from the cluster. In contrast to molecular fragmentation, which can be well described by statistical models, modelling of collision-induced activation of weakly bound clusters requires inclusion of non-ergodic processes.

Relaxation dynamics of 3He and 4He clusters and droplets studied using near infrared and visible fluorescence excitation spectroscopy

The relaxation dynamics of electronically excited ³He and ⁴He clusters and droplets is investigated using time-correlated near-infrared and visible (NIR/VIS) fluorescence excitation spectroscopy. A rich data set spanning a wide range of cluster and droplet sizes is produced. The spectral features broadly follow the vacuum ultraviolet excitation (VUV) spectra. However, when the NIR/VIS spectra are normalised to the VUV fluorescence, regions with distinctly different cluster size and isotope dependence are identified, enabling deeper insight into the relaxation mechanism. Particle density, location of atomic-like states and their principal quantum number, n, are found to play an important role in the relaxation. For states with n = 3 and higher, only energy within the surface region is transferred to excited atoms which are subsequently ejected from the surface and fluoresce in vacuum. For states with n = 2, energy from the entire region within clusters and droplets is transferred to the surface, leading to the ejection of excited atoms and excimers. Here, the energy is transferred by excitation hopping, which competes with radiative and non-radiative decay, making ejection and NIR/VIS fluorescence inefficient in increasingly larger droplets.

Helium nanodroplets as an efficient tool to investigate hydrogen attachment to alkali cations

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.

An integrated instrument of a tandem quadrupole mass spectrometer for cluster reaction and soft-landing deposition

We have developed an integrated instrument system of a multiple-ion laminar flow tube (MIFT) reactor combined with a tandem quadrupole mass spectrometer (TQMS) and soft-landing deposition (SD) apparatus. A customized water-cooling magnetron sputtering (MagS) source is designed, by which we are able to attain a highly efficient preparation of metal clusters of 1-30 atoms with tunable size distributions. Following the MagS source, a laminar flow tube reactor is designed, allowing for sufficient gas-collision reactions of the as-prepared metal clusters, which is advantageous for probing magic clusters and minimizing wall effects when probing the reaction dynamics of such clusters. The customized TQMS analyzer involves a conical octupole, two linear octupoles, a quadruple ion deflector, and a 19 mm quadruple mass analyzer, allowing to decrease the pressure stepwise (from ∼5 to ∼10^-9 Torr), thus ensuring high sensitivity and high resolution of the mass spectrometry analysis. In addition, we have designed a dual SD apparatus for the mass-selected deposition of clusters and their reaction products. For the whole system, abbreviated as MagS-MIFT-TQMS-SD, we have performed a detailed ions-fly simulation and quantitatively estimated the ions transfer efficiency under vacuum conditions determined by real experiments. Taking these advantages, well-resolved Pb_n ⁺, Ag_n ⁺, and Nb_n ⁺ clusters have been produced, allowing for meticulous studies of cluster reactions under sufficient gas-phase collisions free of electric field trapping. Also, we have tested the efficiency of the dual SD.

Adsorption of Helium and Hydrogen on Triphenylene and 1,3,5-Triphenylbenzene

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.

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.

Mass Spectra Resulting from Collision Processes

A new database and viewer for mass spectra resulting from collision processes is presented that follows the standards of the Virtual Atomic and Molecular Data Centre (VAMDC). A focus was placed on machine read and write access, as well as ease of use. In a browser-based viewer, mass spectra and all parameters related to a given measurement can be shown. The program additionally enables a direct comparison between two mass spectra, either by plotting them on top of each other or their difference to identify subtle variations in the data.

Stabilization of phenanthrene anions in helium nanodroplets

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.

Phenanthrene anions are stabilized in the ultracold environment of helium nanodroplets. Gentle shrinking of the helium matrix by collisions with helium gas makes the bare phenanthrene anion visible by high-resolution mass spectrometry.

First-principles modelling of the new generation of subnanometric metal clusters: Recent case studies

The very recent development of highly selective techniques making possible the synthesis and experimental characterization of subnanometric (subnanometer-sized) metal clusters (even single atoms) is pushing our understanding far beyond the present knowledge in materials science, driving these clusters as a new generation of quantum materials at the lower bounds of nanotechnology. When the size of the metal cluster is reduced to a small number of atoms, the d-band of the metal splits into a subnanometric d-type molecular orbitals network in which all metal atoms are inter-connected, with the inter-connections having the length of a chemical bond (1-2 Å). These molecular characteristics are at the very core of the high stability and novel properties of the smallest metal clusters, with their integration into colloidal materials interacting with the environment having the potential to further boost their performance in applications such as luminescence, sensing, bioimaging, theranostics, energy conversion, catalysis, and photocatalysis. Through the presentation of very recent case studies, this Feature Article is aimed to illustrate how first-principles modelling, including methods beyond the state-of-the-art and an interplay with cutting-edge experiments, is helping to understand the special properties of these clusters at the most fundamental level. Moreover, it will be discussed how superfluid helium droplets can act both as nano-reactors and carriers to achieve the synthesis and surface deposition of metal clusters. This concept will be illustrated with the quantum simulation of the helium droplet-assisted soft-landing of a single Au atom onto a titanium dioxide (TiO₂) surface. Next, it will be shown how the application of first-principles methods have disclosed the fundamental reasons why subnanometric Cu₅ clusters are resistant to irreversible oxidation, and capable of increasing and extending into the visible region the solar absorption of TiO₂, of augmenting its efficiency for photo-catalysis beyond a factor of four, also considering the decomposition and photo-activation of CO₂ as a prototypical (photo-) catalytic reaction. Finally, I will discuss how the modification of the same material with subnanometric Ag₅ clusters has converted it into a "reporter" of a surface polaron property as well as a novel two-dimensional polaronic material.

Efficient Formation of Size-Selected Clusters upon Pickup of Dopants into Multiply Charged Helium Droplets

Properties of clusters often depend critically on the exact number of atomic or molecular building blocks, however, most methods of cluster formation lead to a broad, size distribution and cluster intensity anomalies that are often designated as magic numbers. Here we present a novel approach of breeding size-selected clusters via pickup of dopants into multiply charged helium nanodroplets. The size and charge state of the initially undoped droplets and the vapor pressure of the dopant in the pickup region, determines the size of the dopant cluster ions that are extracted from the host droplets, via evaporation of the helium matrix in a collision cell filled with room temperature helium or via surface collisions. Size distributions of the selected dopant cluster ions are determined utilizing a high-resolution time of flight mass spectrometer. The comparison of the experimental data, with simulations taking into consideration the pickup probability into a shrinking He droplet due to evaporation during the pickup process, provides a simple explanation for the emergence of size distributions that are narrower than Poisson.

Phenanthrene: establishing lower and upper bounds to the binding energy of a very weakly bound anion

Collisional excitation of CaPh−, formed in helium nanodroplets, produces Ca− but no phenanthrene anions. We conclude that the EA of Ph is below that of Ca, i.e. below 24.55 meV.

Splashing of Large Helium Nanodroplets upon Surface Collisions

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.

Adsorption of Helium on Small Cationic PAHs: Influence of Hydrocarbon Structure on the Microsolvation Pattern

The adsorption of up to ∼100 helium atoms on cations of the planar polycyclic aromatic hydrocarbons (PAHs) anthracene, phenanthrene, fluoranthene, and pyrene was studied by combining helium nanodroplet mass spectrometry with classical and quantum computational methods. Recorded time-of-flight mass spectra reveal a unique set of structural features in the ion abundance as a function of the number of attached helium atoms for each of the investigated PAHs. Path-integral molecular dynamics simulations were used with a polarizable potential to determine the underlying adsorption patterns of helium around the studied PAH cations and in good general agreement with the experimental data. The calculated structures of the helium-PAH complexes indicate that the arrangement of adsorbed helium atoms is highly sensitive toward the structure of the solvated PAH cation. Closures of the first solvation shell around the studied PAH cations are suggested to lie between 29 and 37 adsorbed helium atoms depending on the specific PAH cation. Helium atoms are found to preferentially adsorb on these PAHs following the 3 × 3 commensurate pattern common for graphitic surfaces, in contrast to larger carbonaceous molecules like corannulene, coronene, and fullerenes that exhibit a 1 × 1 commensurate phase.

Size and Velocity Distribution of Negatively Charged Helium Nanodroplets

Precharged helium nanodroplets can be used in doping experiments with the advantage that they are amenable to size selection with electrostatic fields, therefore adding a useful tuning parameter for dopant growth. For all these applications, the knowledge of the size distribution of charged droplets is an essential parameter, which we have so far assumed would be equivalent to that of their neutral precursors. Here, this assumption is experimentally investigated for negatively charged clusters for temperatures between 4 and 9 K at a stagnation pressure of 2 MPa. We observe a dependency of the velocity of the droplets on mass per charge, especially at the lowest temperatures of the investigated range, and values 20% lower than those known from the literature. Below 6 K, a large deviation from the literature is also found for the average droplet sizes. This information has to be taken into consideration in future experiments where large, charged droplets are sought to produce large dopant clusters. Possible origins for this deviation are discussed in the text.

Infrared spectroscopy of carbocations upon electron ionization of ethylene in helium nanodroplets

The electron impact ionization of helium droplets doped with ethylene molecules and clusters yields diverse C_XH_Y ⁺ cations embedded in the droplets. The ionization primarily produces C₂H₂ ⁺, C₂H₃ ⁺, C₂H₄ ⁺, and CH₂ ⁺, whereas larger carbocations are produced upon the reactions of the primary ions with ethylene molecules. The vibrational excitation of the cations leads to the release of bare cations and cations with a few helium atoms attached. The laser excitation spectra of the embedded cations show well resolved vibrational bands with a few wavenumber widths-an order of magnitude less than those previously obtained in solid matrices or molecular beams by tagging techniques. Comparison with the previous studies of free and tagged CH₂ ⁺, CH₃ ⁺, C₂H₂ ⁺, C₂H₃ ⁺, and C₂H₄ ⁺ cations shows that the helium matrix typically introduces a shift in the vibrational frequencies of less than about 20 cm^-1, enabling direct comparisons with the results of quantum chemical calculations for structure determination. This work demonstrates a facile technique for the production and spectroscopic study of diverse carbocations, which act as important intermediates in gas and condensed phases.

Complexes with Atomic Gold Ions: Efficient Bis-Ligand Formation

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 AuL_n⁺ 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 AuL_n⁺ is larger than in pure L_n⁺. 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.

SF6+: Stabilizing Transient Ions in Helium Nanodroplets

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 He_nSF₆⁺ 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.

Bimodal velocity and size distributions of pulsed superfluid helium droplet beams

We report detailed measurements of velocities and sizes of superfluid helium droplets produced from an Even-Lavie pulse valve at stagnation pressures of 20-60 atm and temperatures between 5.7 and 18.0 K. By doping neutral droplets with Rhodamine 6G cations produced from an electrospray ionization source and detecting the positively charged droplets at two different locations along the beam path, we determine the velocities of the different groups of droplets. By subjecting the doped droplet beam to a retardation field, size distributions can then be analyzed. We discover that at stagnation temperatures above 8.0 K, a single group of droplets is observed at both locations, but at 8.0 K and below, two different groups of droplets with different velocities are detectable. The slower group, considered from fragmentation of liquid helium, cannot be deterred by the retardation voltage at 9 kV, implying an exceedingly large size. The faster group, considered from condensation of gaseous helium, has a bimodal distribution when the stagnation temperatures are below 12.3 K at 20 and 40 atm, or 16.1 K at 60 atm. We also report similar size measurements using low energy electrons for impact ionization, and this latter method can be used for facile in situ characterization of pulsed droplet beams. The mechanism of the bimodal size distribution of the condensation group and the reason for the coexistence of both the condensation and fragmentation groups remain elusive.

Multiply Charged Helium Droplet Anions

The detection of multiply charged helium droplet anions is reported for the first time. By ionizing droplets of superfluid helium with low energy electrons (up to 25 eV), it was possible to produce droplets containing up to five negative charges, which remain intact on the timescale of the experiment. The appearance sizes for different charge states are determined and are found to be orders of magnitude larger than for the equivalent cationic droplets, starting at 4 million He atoms for dianions. Droplets with He*⁻ as charge carriers show signs of being metastable, but this effect is quenched by the pickup of water molecules.

Pickup and reactions of molecules on clusters relevant for atmospheric and interstellar processes

In this perspective, we review experiments with molecules picked up on large clusters in molecular beams with the focus on the processes in atmospheric and interstellar chemistry. First, we concentrate on the pickup itself, and we discuss the pickup cross sections. We measure the uptake of different atmospheric molecules on mixed nitric acid-water clusters and determine the accommodation coefficients relevant for aerosol formation in the Earth's atmosphere. Then the coagulation of the adsorbed molecules on the clusters is investigated. In the second part of this perspective, we review examples of different processes triggered by UV-photons or electrons in the clusters with embedded molecules. We start with the photodissociation of hydrogen halides and Freon CF₂Cl₂ on ice nanoparticles in connection with the polar stratospheric ozone depletion. Next, we mention reactions following the excitation and ionization of the molecules adsorbed on clusters. The first ionization-triggered reaction observed between two different molecules picked up on the cluster was the proton transfer between methanol and formic acid deposited on large argon clusters. Finally, negative ion reactions after slow electron attachment are illustrated by two examples: mixed nitric acid-water clusters, and hydrogen peroxide deposited on large Ar_N and (H₂O)_N clusters. The selected examples are discussed from the perspective of the atmospheric and interstellar chemistry, and several future directions are proposed.

Isotope enrichment in neon clusters grown in helium nanodroplets

Neon cluster ions Ne_s ⁺ 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 s_i of the precursor of the observed Ne_s ⁺. The value is in satisfactory agreement with the size estimated by modeling the growth of Ne_s ⁺ 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.

Dissociation of Valine Cluster Cations

Independently of the preparation method, for cluster cations of aliphatic amino acids, the protonated form M_nH⁺ is always the dominant species. This is a surprising fact considering that in the gas phase, they dissociate primarily by the loss of 45 Da, i.e., the loss of the carboxylic group. In the present study, we explore the dissociation dynamics of small valine cluster cations M_n⁺ and their protonated counterparts M_nH⁺ via collision-induced dissociation experiments and ab initio calculations with the aim to elucidate the formation of M_nH⁺-type cations from amino acid clusters. For the first time, we report the preparation of valine cluster cations M_n⁺ in laboratory conditions, using a technique of cluster ion assembly inside He droplets. We show that the M_n⁺ cations cooled down to He droplet temperature can dissociate to form both M_n-1H⁺ and [M_n–COOH]⁺ ions. With increasing internal energy, the M_n-1H⁺ formation channel becomes dominant. M_n-1H⁺ ions then fragment nearly exclusively by monomer loss, describing the high abundance of protonated clusters in the mass spectra of amino acid clusters.