Mass spectrometry of aerosol particle analogues in molecular beam experiments

Electron-induced ligand loss from iron tetracarbonyl methyl acrylate

We probe the separation of ligands from iron tetracarbonyl methyl acrylate (Fe(CO)4(C4H6O2) or Fe(CO)4MA) induced by the interaction with free electrons. The motivation comes from the possible use of this molecule as a nanofabrication precursor and from the corresponding need to understand its elementary reactions fundamental to the electron-induced deposition. We utilize two complementary electron collision setups and support the interpretation of data by quantum chemical calculations. This way, both the dissociative ionization and dissociative electron attachment fragmentation channels are characterized. Considerable differences in the degree of precursor fragmentation in these two channels are observed. Interesting differences also appear when this precursor is compared to structurally similar iron pentacarbonyl. The present findings shed light on the recent electron-induced chemistry of Fe(CO)4MA on a surface under ultrahigh vacuum.

Understanding the Formation and Growth of New Atmospheric Particles at the Molecular Level through Laboratory Molecular Beam Experiments

Atmospheric new particle formation (NPF), which exerts comprehensive implications for climate, air quality and human health, has received extensive attention. From molecule to cluster is the initial and most important stage of the nucleation process of atmospheric new particles. However, due to the complexity of the nucleation process and limitations of experimental characterization techniques, there is still a great uncertainty in understanding the nucleation mechanism at the molecular level. Laboratory-based molecular beam methods can experimentally implement the generation and growth of typical atmospheric gas-phase nucleation precursors to nanoscale clusters, characterize the key physical and chemical properties of clusters such as structure and composition, and obtain a series of their physicochemical parameters, including association rate coefficients, electron binding energy, pickup cross section and pickup probability and so on. These parameters can quantitatively illustrate the physicochemical properties of the cluster, and evaluate the effect of different gas phase nucleation precursors on the formation and growth of atmospheric new particles. We review the present literatures on atmospheric cluster formation and reaction employing the experimental method of laboratory molecular beam. The experimental apparatuses were classified and summarized from three aspects of cluster generation, growth and detection processes. Focus of this review is on the properties of nucleation clusters involving different precursor molecules of water, sulfuric acid, nitric acid and NxOy, respectively. We hope this review will provide a deep insight for effects of cluster physicochemical properties on nucleation, and reveal the formation and growth mechanism of atmospheric new particle at the molecular level.

Hydrated Formic Acid Clusters and their Interaction with Electrons

We investigate ion formation in hydrated formic acid (FA) clusters upon collision with electrons of variable energy, focusing on electron ionization at 70 eV (EI) and low-energy (1.5-15 eV) electron attachment (EA). To uncover details about the composition of neutral clusters, we aim to elucidate the ion formation processes in FAM ⋅ WN clusters initiated by interaction with electrons and determine the extent of cluster fragmentation. EI predominantly produces protonated [FAm+H]+ ions, and in FA-rich clusters, the stable ring structures surrounding H3O+ ions are formed. In contrast, EA leads to a competition between the formation of intact [FAm ⋅ Wn]- and dissociated [FAm ⋅ Wn-H]- fragment ions, influenced by the cluster size, level of hydration, and electron energy. Our findings reveal a predisposition of low-energy EA towards forming [FAm ⋅ Wn]-, while higher electron energies tend to favor the formation of [FAm ⋅ Wn-H]- due to intracluster ion-molecule reactions. The comparison of positive and negative ion spectra suggests that the mass spectra of FA-rich clusters may indicate their actual size and composition. On the other hand, the more weakly bound water evaporation from the clusters depends strongly on the ionization. Thus, for the hydrated clusters, the neutral cluster size can hardly be estimated from the mass spectra.

Dynamics of collisions and uptake of alcohol molecules with hydrated nitric acid clusters

We investigate the collisions of different alcohol molecules with hydrated nitric acid clusters using a molecular beam experiment and molecular dynamics simulations. The uptake cross sections σp for the molecules evaluated from the experiment are in excellent agreement with the simulations. This suggests that (i) the nontrivial assumptions implemented in the evaluation procedure of the experimental data are valid, and (ii) the simulations describe correctly the major processes in the molecule-cluster collisions. We observe that σp decreases with the increasing alkyl chain length of the alcohol, and also with the branching of the molecules that have the same mass but different structures. These systematic trends can be rationalized based on the accessibility of the hydrophilic OH group, which decreases with the increasing chain length and steric hindrance. The observed trends and their interpretation differ significantly from the simple model of hard-sphere collisions. The obtained data shall be beneficial not only for the fundamental understanding of the molecule-cluster collisions, but also in the modelling of atmospheric new-particle formation and aerosol growth.

Electron-triggered processes in halogenated carboxylates: dissociation pathways in CF3COCl and its clusters

Trifluoroacetyl chloride, CF3COCl, is produced in the Earth's atmosphere by photooxidative degradation of hydrochlorofluorocarbons, and represents a potential source of highly reactive halogen radicals. Despite considerable insight into photochemistry of CF3COCl, its reactivity towards electrons has not been addressed so far. We investigate the electron ionization and attachment in isolated CF3COCl molecules and (CF3COCl)N, max. N ≥ 10, clusters using a molecular beam experiment in combination with quantum chemical calculations. The ionization of the molecule at 70 eV electron energy leads to strong fragmentation: weakening of the C-C bond yields the CF3+ and COCl+ ions, while the fission of the C-Cl bond produces the major CF3CO+ fragment ion. The cluster spectra are dominated by Mn·COCl+ and Mn·CF3CO+ ions (M = CF3COCl). The electron attachment at energies between 1.5 and 11 eV also leads to the dissociation of the molecule breaking either the C-Cl bond at low energies below 3 eV yielding mainly Cl- ions, or dissociating the C-C bond at higher energies above 4 eV leading mainly to CF3- ions. In the clusters, the intact Mn- ions are stabilized after electron attachment at low energies with contribution of Mn·Cl- fragment ions. At higher energies, the Mn·Cl- fragments dominate the spectra, and C-C bond dissociation occurs as well yielding Mn·CF3-. Interestingly, Mn·Cl2- ions appear in the spectra at higher energies. We briefly discuss possible atmospheric implications.

Does HNO3 dissociate on gas-phase ice nanoparticles?

We investigated the dissociation of nitric acid on large water clusters (H2O)N, N̄ ≈ 30-500, i.e., ice nanoparticles with diameters of 1-3 nm, in a molecular beam. The (H2O)N clusters were doped with single HNO3 molecules in a pickup cell and probed by mass spectrometry after a low-energy (1.5-15 eV) electron attachment. The negative ion mass spectra provided direct evidence for HNO3 dissociation with the formation of NO3-⋯H3O+ ion pairs, but over half of the observed cluster ions originated from non-dissociated HNO3 molecules. This behavior is in contrast with the complete dissociation of nitric acid on amorphous ice surfaces above 100 K. Thus, the proton transfer is significantly suppressed on nanometer-sized particles compared to macroscopic ice surfaces. This can have considerable implications for heterogeneous processes on atmospheric ice particles.

Electron Energy Loss Processes in Methyl Methacrylate: Excitation and Bond Breaking

Details of electron-induced chemistry of methyl methacrylate (MMA) upon complexation are revealed by combining gas-phase 2D electron energy loss spectroscopy with electron attachment spectroscopy of isolated MMA and its clusters. We show that even though isolated MMA does not form stable parent anions, it efficiently thermalizes the incident electrons via intramolecular vibrational redistribution, leading to autodetachment of slow electrons. This autodetachment channel is reduced in clusters due to intermolecular energy transfer and stabilization of parent molecular anions. Bond breaking via dissociative electron attachment leads to an extensive range of anion products. The dominant OCH₃^- channel is accessible via core-excited resonances with threshold above 5 eV, despite the estimated thermodynamic threshold below 3 eV. This changes in clusters, where M_nOCH₃^- anions are observed in a lower-lying resonance due to neutral dissociation of the ¹(n, π*) state and electron self-scavenging. The present findings have implications for electron-induced chemistry in lithography with poly(methyl methacrylate).

Bridging Gaps between Clusters in Molecular-Beam Experiments and Aerosol Nanoclusters

Clusters in molecular beam experiments can mimic aerosol nanoclusters and provide molecular-level details for various processes relevant to atmospheric aerosol research. Aerosol nanoclusters, particles of sizes below 10 nm, are difficult to investigate in ambient atmosphere and thus represent a gap in our understanding of the new particle formation process. Recent field measurements and laboratory experiments are closing this gap; however, experiments with clusters in molecular beams are rarely involved. Yet, they can offer an unprecedented detailed insight into the processes including particles in this size range. In this Perspective, we discuss several up-to-date molecular beam experiments with clusters and demonstrate that the investigated clusters approach aerosol nanoclusters in terms of their complexity and chemistry. We examine remaining gaps between atmospheric aerosols and clusters in molecular beams and speculate about future experiments bridging these gaps.

Analysis of natural organic matter via fourier transform ion cyclotron resonance mass spectrometry: an overview of recent non-petroleum applications

Among the different techniques for mass analysis, ultra-high-resolution Fourier transform ion cyclotron resonance (FTICR) is the method of choice for highly complex samples, as it offers unrivaled mass accuracy and resolving power, combined with a high degree of flexibility in hybrid instruments as well as for ion activation techniques. FTICR instruments are readily embraced by the biological and biomedical research communities and applied over a wide range of applications for the analysis of biomolecules such as carbohydrates, lipids, nucleic acids, and proteins. In the field of natural organic matter (NOM) analysis, petroleum-related studies currently dominate FTICR-MS applications. Recently, however, there is a growing interest in developing high-performance MS methods for the characterization of NOM samples from natural aquatic and terrestrial environments. Here, we present an overview of FTICR-MS techniques for complex, non-petroleum NOM samples, including data analysis and novel tandem mass spectrometry (MS/MS) methods for structural classifications. © 2020 The Authors. Mass Spectrometry Reviews published by John Wiley & Sons Ltd.

Ion spectroscopy in methane activation

This review is devoted to ion spectroscopy studies of complexes relevant for the understanding of methane activation with metal ions and clusters. Methane activation starts with the formation of a complex with a metal ion. The degree of the interaction between an intact methane molecule and the ion can be monitored by the perturbations of C-H stretch vibrations in the methane molecule. Binding mediated by the electrostatic interaction results in a η³ type coordination of methane. In contrast, binding governed by orbital interactions results in a η² type coordination of methane. We further review the spectroscopic characterization of activation products of metal-methane reactions, such as the metal-carbene and carbyne products resulting from the interaction of selected 5d metals with methane. The focus of recent research in the field has shifted towards the investigation of interactions between methane and metal clusters. We show examples highlighting that metal clusters can be more reactive in methane activation reactions.

Uptake of Hydrogen Bonding Molecules by Benzene Nanoparticles

The uptake of molecules on nanometer-size clusters of polyaromatic hydrocarbons (PAHs) is important for the condensation of water on PAH aerosols in the atmosphere and for ice mantle growth on nanoparticles in the interstellar medium. We generate benzene clusters Bz_N of mean size N̅ ≈ 300 (radius R̅ ≈ 2.2 Å) as a model system for the PAH nanoparticles. Using molecular beams and mass spectrometry detection, we investigate the uptake of water, methanol, and ethanol by these clusters. All picked up molecules are highly mobile on Bz_N and generate clusters within <3 ms. The relative uptakes for the different investigated molecules can be directly compared and quantified. Water molecules exhibit the lowest relative pickup probability that is ∼30% lower than those for methanol and ethanol, which are approximately the same.

Effect of Hydration on Electron Attachment to Methanesulfonic Acid Clusters

We report an experimental and computational study of the electron-induced chemistry of methanesulfonic acid (MSA, MeSO₃H) in clusters. We combine the mass spectra after the 70 eV electron ionization with the negative ion spectra after electron attachment (EA) at low electron energies of 0-15 eV of the MSA molecule, small MSA clusters, and microhydrated MSA clusters to reveal the solvation effects. The MSA/He coexpansion only generates small MSA clusters with up to four molecules, but adding water substantially hydrates the MSA clusters, resulting in clusters composed of 1-2 MSA molecules accompanied by quite a few water molecules. The clustering strongly suppresses the fragmentation of the MSA molecules upon both the positive ionization and EA. The electron-energy-dependent ion yield for different negative ions is measured. For the MSA molecule and pure MSA clusters, EA leads to an H-abstraction yielding MeSO₃^-. It proceeds efficiently at low electron energies below 2 eV with a shoulder at 3-4 eV and a broad, almost 2 orders of magnitude weaker, peak around 8 eV. The hydrated (H₂O)_nMeSO₃^- ions with n ≤ 3 exhibit only a broad peak around 7 eV similar to EA of pure water clusters. Thus, for the small clusters, the electron attachment and hydrogen abstraction from water occur. On the other hand, the larger clusters with n > 4 display a peak below 2 eV, which quickly dominates the spectrum with increasing n. This peak is related to the formation of the H₃O⁺·MeSO₃^- ion pair upon hydration and subsequent dipole-supported electron attachment followed by the hydronium neutralization and H₃O^• radical dissociation. The size-resolved experimental data indicate that the ionic dissociation of MSA starts to occur in the neutral MeSO₃H(H₂O)_N clusters with about four water molecules.

Heterogeneous Reactions of Methane with Cl Radicals on Large ArN Clusters

Heterogeneous chemistry on the surfaces of atmospheric particles has a wide impact on the properties and composition of the Earth's atmosphere. In laboratory studies, clusters can represent proxies to atmospheric aerosols and help to discern the individual steps in reactions on or in aerosols. We investigate the reactivity of Cl and CCl₃ radicals with methane on argon clusters using the pickup method. For radical generation, we built a new pyrolysis source partially adapting the design of radical sources that utilize the supersonic expansion into a heated silicon carbide tube. Large Ar_N, N̅ ≈ 110, clusters were generated in a supersonic expansion, and CH₄ molecules were embedded in the clusters via a pickup process followed by the uptake of the radicals produced in the pyrolysis source. The analysis of the mass spectra recorded under different experimental conditions (i.e., with the pyrolysis ON and OFF and with only one or both reactants) allowed us to identify various products of the radical reactions on Ar_N. We propose a sequence of reactions based on the reaction energetics. It starts with the hydrogen abstraction from CH₄ by a Cl radical resulting in HCl and CH₃ followed by a halogenation step where CCl₄ molecules react with the available CH₃ radicals, yielding CH₃Cl. By analogy, the CH₃Cl enters another hydrogen abstraction by Cl, producing HCl and the CH₂Cl radical, which again undergoes a halogenation step with CCl₄, generating CH₂Cl₂. Further reaction of CH₂Cl₂ with Cl terminates the sequence by the production of HCl and CHCl₂.

Photoionization, Structures and Energetics of Na-doped Formic Acid-Water Clusters

The influence of formic acid on water cluster aggregation has been investigated experimentally by mass spectrometry and tunable UV laser ionization applied to Na-doped clusters formed in the supersonic expansion of water vapors seeded with formic acid (FA) as well as theoretically using high level quantum chemistry methods. The mass spectra of Na-FA(H 2 O) n clusters show an enlarging of mass distribution toward heavier clusters with respect the Na-(H 2 O) n clusters, suggesting similar mass distribution in neutral clusters and an influence of formic acid in water aggregation. DFT and coupled-cluster type (DLPNO-CCSD(T)) calculations have been used to calculate structures and energetics of neutral and ionized Na-FA(H 2 O) n as well as neutral FA(H 2 O) n . Na-doped clusters are characterized by very stable geometries. The theoretical adiabatic ionization potential values match pretty well the measured appearance energies and the calculated first six electronic excited states show Rydberg-type characters, indicating possible autoionization contributions in the mass spectra. Finally, theoretical calculation on neutral FA(H 2 O) n clusters show the possibility of a similarly stable structures in small clusters containing up to n=4-5 water molecules, where FA interacts significantly with waters.This suggests that FA can compete with water molecules in the starting stage of the aggregation process, by forming stable nucleation seed.

Electron attachment to microhydrated 4-nitro- and 4-bromo-thiophenol

We investigate the effect of microhydration on electron attachment to thiophenols with halogen (Br) and nitro (NO₂) functional groups in the para position. We focus on the formation of anions upon the attachment of low-energy electrons with energies below 8 eV to heterogeneous clusters of the thiophenols with water. For nitro-thiophenol (NTP), the primary reaction channel observed is the associative electron attachment, irrespective of the microhydration. On the other hand, bromothiophenol (BTP) fragments significantly upon the electron attachment, producing Br^- and (BTP-H)^- anions. Microhydration suppresses fragmentation of both molecules, however in bromothiophenol, the Br^- channel remains intense and Br(H₂O)_n^- hydrated fragment clusters are observed. The results are supported by the reaction energetics obtained from ab initio calculations. Different dissociation dynamics of NTP and BTP can be related to different products of their plasmon induced reactions on Au nanoparticles. Computational modeling of the simplified BTP(H₂O) system indicates that the electron attachment products reflect the structure of neutral precursor clusters - the anion dissociation dynamics is controlled by the hydration site.

Electron attachment to isolated and microhydrated favipiravir

Electron attachment and its equivalent in complex environments, single-electron reduction, are important in many biological processes. Here, we experimentally study the electron attachment to favipiravir, a well-known antiviral agent. Electron attachment spectroscopy is used to explore the energetics of associative (AEA) and dissociative (DEA) electron attachment to isolated favipiravir. AEA dominates the interaction and the yields of the fragment anions after DEA are an order of magnitude lower than that of the parent anion. DEA primary proceeds via decomposition of the CONH2 functional group, which is supported by reaction threshold calculations using ab initio methods. Mass spectrometry of small favipiravir-water clusters demonstrates that a lot of energy is transferred to the solvent upon electron attachment. The energy gained upon electron attachment, and the high stability of the parent anion were previously suggested as important properties for the action of several electron-affinic radiosensitizers. If any of these mechanisms cause synergism in chemo-radiation therapy, favipiravir could be repurposed as a radiosensitizer.

Core and Valence Level Photoelectron Spectroscopy of Nanosolvated KCl

The solvation of alkali and halide ions in the aqueous environment has been a subject of intense experimental and theoretical research with multidisciplinary interests; yet, a comprehensive molecular-level understanding has still not been obtained. In recent years, electron spectroscopy has been increasingly applied to study the electronic and structural properties of aqueous ions with implications, especially in atmospheric chemistry. In this work, we report core and valence level (Cl 2p, Cl 3p, and K 3p) photoelectron spectra of the common alkali halide, KCl, doped in gas-phase water clusters in the size range of a few hundred water molecules. The results indicate that the electronic structure of these nanosolutions shows a distinct character from that observed at the liquid-vapor interface in liquid microjets and ambient pressure setups. Insights are provided into the unique solvation properties of ions in a nanoaqueous environment, emerging properties of bulk electrolyte solutions with growing cluster size, and sensitivity of the electronic structure to varying solvation configurations.

Temperature evolution in IR action spectroscopy experiments with sodium doped water clusters

The combination of supersonic expansions with IR action spectroscopy techniques is the basis of many successful approaches to study cluster structure and dynamics. The effects of temperature and temperature evolution are important with regard to both the cluster synthesis and the cluster dynamics upon IR excitation. In the past the combination of the sodium doping technique with IR excitation enhanced near threshold photoionization has been successfully applied to study neutral, especially water clusters. In this work we follow an overall examination approach for inspecting the interplay of cluster temperature and cluster structure in the initial cooling process and in the IR excitation induced heating of the clusters. In molecular simulations, we study the temperature dependent photoionization spectra of the sodium doped clusters and the evaporative cooling process by water molecule ejection at the cluster surface. We present a comprehensive analysis that provides constraints for the temperature evolution from the nozzle to cluster detection in the mass spectrometer. We attribute the IR action effect to the strong temperature dependence of sodium solvation in the IR excited clusters and we discuss the effects of geometry changes during the IR multi-photon absorption process with regard to application prospects of the method.

Shining New Light on the Kinetics of Water Uptake by Organic Aerosol Particles

The uptake of water vapor by various organic aerosols is important in a number of applications ranging from medical delivery of pharmaceutical aerosols to cloud formation in the atmosphere. The coefficient that describes the probability that the impinging gas-phase molecule sticks to the surface of interest is called the mass accommodation coefficient, α_M. Despite the importance of this coefficient for the description of water uptake kinetics, accurate values are still lacking for many systems. In this Feature Article, we present various experimental techniques that have been evoked in the literature to study the interfacial transport of water and discuss the corresponding strengths and limitations. This includes our recently developed technique called photothermal single-particle spectroscopy (PSPS). The PSPS technique allows for a retrieval of α_M values from three independent, yet simultaneous measurements operating close to equilibrium, providing a robust assessment of interfacial mass transport. We review the currently available data for α_M for water on various organics and discuss the few studies that address the temperature and relative humidity dependence of α_M for water on organics. The knowledge of the latter, for example, is crucial to assess the water uptake kinetics of organic aerosols in the Earth's atmosphere. Finally, we argue that PSPS might also be a viable method to better restrict the α_M value for water on liquid water.

Water-Assisted Electron-Induced Chemistry of the Nanofabrication Precursor Iron Pentacarbonyl

Focused electron beam deposition often requires the use of purification techniques to increase the metal content of the respective deposit. One of the promising methods is adding H₂O vapor as a reactive agent during the electron irradiation. However, various contrary effects of such addition have been reported depending on the experimental condition. We probe the elementary electron-induced processes that are operative in a heterogeneous system consisting of iron pentacarbonyl as an organometallic precursor and water. We use an electron beam of controlled energy that interacts with free mixed Fe(CO)₅/H₂O clusters. These mimic the heterogeneous system and, at the same time, allow direct mass spectrometric analysis of the reaction products. The anionic decomposition pathways are initiated by dissociative electron attachment (DEA), either to Fe(CO)₅ or to H₂O. The former one proceeds mainly at low electron energies (<3 eV). Comparison of nonhydrated and hydrated conditions reveals that the presence of water actually stabilizes the ligands against dissociation. The latter one proceeds at higher electron energies (>6 eV), where the DEA to H₂O forms OH^- in the first reaction step. This intermediate reacts with Fe(CO)₅, leading to enhanced decomposition, with the desorption of up to three CO ligands. The present results demonstrate that the water action on Fe(CO)₅ decomposition is sensitive to the involved electron energy range and depends on the hydration degree.

Stability of pyruvic acid clusters upon slow electron attachment

Pyruvic acid represents a key molecule in prebiotic chemistry and it has recently been proposed to be synthesized on interstellar ices. In order to probe the stability of pyruvic acid in the interstellar medium with respect to decomposition by slow electrons, we investigate the electron attachment to its homomolecular and heteromolecular clusters. Using mass spectrometry, we follow the changes in the fragmentation pattern and its dependence on the electron energy for various cluster sizes of pure and microhydrated pyruvic acid. The assignment of fragmentation reaction pathways is supported by ab initio calculations. The fragmentation degree dramatically decreases upon clustering. This decrease is even stronger in the heteromolecular clusters of pyruvic acid with water, where the non-dissociative attachment is by far the strongest channel. In the homomolecular clusters, the dissociative channel leading to dehydrogenation is active over a larger electron energy range than in the isolated molecules. To probe the role of the self-scavenging effects, we explore the excited states of pyruvic acid. This has been done both experimentally, by using electron energy loss spectroscopy, and theoretically, by photochemical calculations. Data on both optically-allowed and forbidden states allow for the explanation of processes emerging upon clustering.

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.

How volatile components catalyze vapor nucleation

Gas phase nucleation is a ubiquitous phenomenon in planetary atmospheres and technical processes, yet our understanding of it is far from complete. In particular, the enhancement of nucleation by the addition of a more volatile, weakly interacting gaseous species to a nucleating vapor has escaped molecular-level experimental investigation. Here, we use a specially designed experiment to directly measure the chemical composition and the concentration of nucleating clusters in various binary CO2-containing vapors. Our analysis suggests that CO2 essentially catalyzes nucleation of the low vapor pressure component through the formation of transient, hetero-molecular clusters and thus provides alternative pathways for nucleation to proceed more efficiently. This work opens up new avenues for the quantitative assessment of nucleation mechanisms involving transient species in multicomponent vapors.

Kinetic energy deposited into a nanodroplet, cluster, or molecule in a sticking collision with background gas

In processes when particles such as nanodroplets, clusters, or molecules move through a dilute background gas and undergo capture collisions, it is often important to know how much translational kinetic energy is deposited into the particles by these pick-up events. For sticking collisions with a Maxwell-Boltzmann gas, an exact expression is derived, which is valid for arbitrary relative magnitudes of the particle and thermal gas speeds.

Large cluster ions: soft local probes and tools for organic and bio surfaces

Ionised cluster beams have been produced and employed for thin film deposition and surface processing for half a century. In the last two decades, kiloelectronvolt cluster ions have also proved to be outstanding for surface characterisation by secondary ion mass spectrometry (SIMS), because their sputter and ion yields are enhanced in a non-linear fashion with respect to monoatomic projectiles, with a resulting step change of sensitivity for analysis and imaging. In particular, large gas cluster ion beams, or GCIB, have now become a reference in organic surface and thin film analysis using SIMS and X-ray photoelectron spectroscopy (XPS). The reason is that they induce soft molecular desorption and offer the opportunity to conduct damageless depth-profiling and 3D molecular imaging of the most sensitive organic electronics and biological samples, with a nanoscale depth resolution. In line with these recent developments, the present review focuses on rather weakly-bound, light-element cluster ions, such as noble or other gas clusters, and water or alcohol nanodroplets (excluding clusters made of metals, inorganic salts or ionic liquids) and their interaction with surfaces (essentially, but not exclusively, organic). The scope of this article encompasses three aspects. The first one is the fundamentals of large cluster impacts with surfaces, using the wealth of information provided by molecular dynamics simulations and experimental observations. The second focus is on recent applications of large cluster ion beams in surface characterisation, including mass spectrometric analysis and 2D localisation of large molecules, molecular depth-profiling and 3D molecular imaging. Finally, the perspective explores cutting edge developments, involving (i) new types of clusters with a chemistry designed to enhance performance for mass spectrometry imaging, (ii) the use of cluster fragment ion backscattering to locally retrieve physical surface properties and (iii) the fabrication of new biosurface and thin film architectures, where large cluster ion beams are used as tools to transfer biomolecules in vacuo from a target reservoir to any collector substrate.

Ion and radical chemistry in (H2O2)N clusters

We investigate the ionization induced chemistry of hydrogen peroxide in (H₂O₂)_N clusters generated after the pickup of individual H₂O₂ molecules on large free Ar_M, M[combining macron]≈ 160, nanoparticles in molecular beams. Positive and negative ion mass spectra are recorded after an electron ionization of the clusters at energies 5-70 eV and after a slow electron attachment (below 4 eV), respectively. The spectra demonstrate that (H₂O₂)_N clusters with N≥ 20 are formed on argon nanoparticles. This is the first experimental report on hydrogen peroxide clusters in molecular beams. The major negative cluster ion series (H₂O₂)_nO₂^- indicates O₂^- ion formation. The dissociative electron attachment to H₂O₂ molecules in the gas phase yielded only OH^- and O^- (Nandi et al., Chem. Phys. Lett., 2003, 373, 454). These ions and the series containing them are much less abundant in the clusters. We propose a sequence of ion-molecule and radical reactions to explain the formation of O₂^-, HO₂^- and other ions observed in the negatively charged cluster ion series. Since hydrogen peroxide plays an important role in many areas of chemistry from the Earth's atmosphere to biological tissues, our study opens new horizons for experimental investigations of hydrogen peroxide chemistry in complex environments.

Chemistry of NOx and HNO3 Molecules with Gas-Phase Hydrated O.- and OH- Ions

The gas‐phase reactions of O^.−(H₂O)_n and OH⁻(H₂O)_n, n=20–38, with nitrogen‐containing atmospherically relevant molecules, namely NO_x and HNO₃, are studied by Fourier transform ion cyclotron resonance (FT‐ICR) mass spectrometry and theoretically with the use of DFT calculations. Hydrated O^.− anions oxidize NO^. and NO₂^. to NO₂⁻ and NO₃⁻ through a strongly exothermic reaction with enthalpy of −263±47 kJ mol⁻¹ and −286±42 kJ mol⁻¹, indicating a covalent bond formation. Comparison of the rate coefficients with collision models shows that the reactions are kinetically slow with 3.3 and 6.5 % collision efficiency. Reactions between hydrated OH⁻ anions and nitric oxides were not observed in the present experiment and are most likely thermodynamically hindered. In contrast, both hydrated anions are reactive toward HNO₃ through proton transfer from nitric acid, yielding hydrated NO₃⁻. Although HNO₃ is efficiently picked‐up by the water clusters, forming (HNO₃)_0–2(H₂O)_mNO₃⁻ clusters, the overall kinetics of nitrate formation are slow and correspond to an efficiency below 10 %. Combination of the measured reaction thermochemistry with literature values in thermochemical cycles yields ΔH_f(O⁻(aq.))=48±42 kJ mol⁻¹ and ΔH_f(NO₂⁻(aq.))=−125±63 kJ mol⁻¹.

Photochemistry of Amylene Double Bond in Clusters on Free Argon Nanoparticles

We have investigated reactivity of double bond in 2-methyl-2-butene (also trimethylethylene or amylene) in the excited and ionized states. In a combined experimental and theoretical study, we focused on both the intermolecular and intramolecular reactions. In a molecular beam experiment, we have sequentially picked up several amylene molecules on the surface of argon nanoparticles Ar_M, M̅ ≈ 90, acting as a cold support. Ionization with 70 eV electrons yields mass spectra strongly dominated by amylene cluster ions Am(Am)_n⁺. Interestingly, upon multiphoton ionization with 193 nm (6.4 eV) photons, a new strong fragment series appears in the spectra, nominally corresponding to an addition of two carbon atoms, i.e., (Am)_nC₂⁺. This difference between electron and photoionization suggests a reaction in an excited state of amylene with a neighboring amylene molecule. We used techniques of nonadiabatic molecular dynamics to study the reactivity of amylene molecules both in the excited and in ionized states. Possible reaction pathways are proposed, substantiating the observed differences between the electron ionization and photoionization mass spectra.

Oxidation Enhances Aerosol Nucleation: Measurement of Kinetic Pickup Probability of Organic Molecules on Hydrated Acid Clusters

We investigate the uptake of the most prominent biogenic volatile organic compounds (VOCs)-isoprene, α-pinene, and their selected oxidation products-by hydrated acid clusters in a molecular beam experiment and by DFT calculations. Our experiments provide a unique and direct way of determination of the surface accommodation coefficient (α_S) on the proxies of ultrafine aerosol particles. Since we are able to determine unambiguously the fraction of the clusters to which the molecules stick upon collisions, our α_S is a purely kinetic parameter disentangling the molecule pickup from its evaporation. Oxidation increases the α_S of VOCs by more than an order of magnitude, because oxidized compounds form hydrogen bonds with the clusters, whereas the interactions of the parent VOCs are weaker and nonspecific. This work provides molecular-level insight into the condensation of single molecules into atmospheric particles, which has important implications for aerosol nucleation and growth.

From atoms to aerosols: probing clusters and nanoparticles with synchrotron based mass spectrometry and X-ray spectroscopy

Synchrotron radiation provides insight into spectroscopy and dynamics in clusters and nanoparticles.

Ring Formation and Hydration Effects in Electron Attachment to Misonidazole

We study the reactivity of misonidazole with low-energy electrons in a water environment combining experiment and theoretical modelling. The environment is modelled by sequential hydration of misonidazole clusters in vacuum. The well-defined experimental conditions enable computational modeling of the observed reactions. While the NO 2 - dissociative electron attachment channel is suppressed, as also observed previously for other molecules, the OH - channel remains open. Such behavior is enabled by the high hydration energy of OH - and ring formation in the neutral radical co-fragment. These observations help to understand the mechanism of bio-reductive drug action. Electron-induced formation of covalent bonds is then important not only for biological processes but may find applications also in technology.

Ionization of carboxylic acid clusters in the gas phase and on free ArN and (H2O)N nanoparticles: valeric acid as a model for small carboxylic acids

In ionized valeric acid clusters, not only the expected proton transfer reaction, but also anhydride formation is observed. Could this be a common motif in the ionization chemistry of small carboxylic acid clusters?

Pyruvic acid proton and hydrogen transfer reactions in clusters

We investigate ion chemistry in pyruvic acid (PA) clusters in a molecular beam experiment.

Dissociative electron attachment to HNO3 and its hydrates: energy-selective electron-induced chemistry

The chemistry of mixed nitric acid–water clusters triggered by electron attachment depends on clustering and the electron energy.

Chiral Recognition in Cold Gas-Phase Cluster Ions of Carbohydrates and Tryptophan Probed by Photodissociation

Chiral recognition between tryptophan (Trp) and carbohydrates such as D-glucose (D-Glc), methyl-α-D-glucoside (D-glucoside), D-maltose, and D-cellobiose in cold gas-phase cluster ions was investigated as a model for chemical evolution in interstellar molecular clouds using a tandem mass spectrometer containing a cold ion trap. The photodissociation mass spectra of cold gas-phase clusters that contained Na+, Trp enantiomers, and D-maltose showed that Na+(D-Glc) was formed via the glycosidic bond cleavage of D-maltose from photoexcited homochiral Na+(D-Trp)(D-maltose), while the dissociation did not occur in heterochiral Na+(L-Trp)(D-maltose). The enantiomer-selective dissociation was also observed in the case of D-cellobiose. The enantiomer-selective glycosidic bond cleavage of disaccharides suggested that photoexcited D-Trp could prevent chemical evolution of sugar chains from D-enantiomer of carbohydrates in molecular clouds. The spectra of gas-phase clusters that contained Na+, Trp enantiomers, and D-Glc indicated that enantiomer-selective protonation of L-Trp from D-Glc could induce enantiomeric excess via collision-activated dissociation of the protonated L-Trp. In the case of protonated clusters, photoexcited H+(L-Trp) dissociated via Cα-Cβ bond cleavage in the presence of D-Glc or D-glucoside, where the excited states of H+(L-Trp) contributed to the enantiomer-selective reaction in the clusters. These enantiomer selectivities in cold gas-phase clusters indicated that chirality of a molecule induced enantiomeric excess of other molecules via enantiomer-selective reactions in molecular clouds.

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 of methanol on mixed HNO3/H2O clusters: An absolute pickup cross section

The uptake of atmospheric oxidized organics on acid clusters is relevant for atmospheric new particle formation. We investigate the pickup of methanol (CH₃OH) on mixed nitric acid-water clusters (HNO₃)_M(H₂O)_N by a combination of mass spectrometry and cluster velocity measurements in a molecular beam. The mass spectra of the mixed clusters exhibit (HNO₃)_m(H₂O)_nH⁺ series with m = 0-3 and n = 0-12. In addition, CH₃OH·(HNO₃)_m(H₂O)_nH⁺ series with very similar patterns appear in the spectra after the methanol pickup. The velocity measurements prove that the undoped (HNO₃)_m(H₂O)_nH⁺ mass peaks in the pickup spectra originate from the neutral (HNO₃)_M(H₂O)_N clusters which have not picked up any CH₃OH molecule, i.e., methanol has not evaporated upon the ionization. Thus the fraction of the doped clusters can be determined and the mean pickup cross section can be estimated, yielding σ_s¯≈ 20 Ų. This is compared to the lower estimate of the mean geometrical cross section σ_g¯≈ 60 Ų obtained from the theoretical cluster geometries. Thus the "size" of the cluster corresponding to the methanol pickup is at least 3-times smaller than its geometrical size. We have introduced a method which can yield the absolute pickup cross sections relevant to the generation and growth of atmospheric aerosols, as illustrated in the example of methanol and nitric acid clusters.