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.

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.

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.

On the Wavelength-Dependent Photochemistry of the Atmospheric Molecule CF3COCl.

The wavelength control of photochemistry usually results from ultrafast dynamics following the excitation of different electronic states. Here, we investigate the CF3COCl molecule, exhibiting wavelength-dependent photochemistry both via (i) depositing increasing internal energy into a single state and (ii) populating different electronic states. We reveal the mechanism behind the photon-energy dependence by combining nonadiabatic ab initio molecular dynamics techniques with the velocity map imaging experiment. We describe a consecutive mechanism of photodissociation where an immediate release of Cl taking place in an excited electronic state is followed by a slower ground-state dissociation of the CO fragment. The CO release is subject to an activation barrier and is controlled by excess internal energy via the excitation wavelength. Therefore, a selective release of CO along with Cl can be achieved. The mechanism is fully supported by both the measured kinetic energy distributions and anisotropies of the angular distributions. Interestingly, the kinetic energy of the released Cl atom is sensitively modified by accounting for spin-orbit coupling. Given the atmospheric importance of CF3COCl, we discuss the consequences of our findings for atmospheric photochemistry.

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.

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.

Gas phase C6H6 - anion: Electronic stabilization by opening of the benzene ring.

It is well established that an isolated benzene radical anion is not electronically stable. In the present study, we experimentally show that electron attachment to benzene clusters leads to weak albeit unequivocal occurrence of a C6H6 - moiety. We propose here-based on electronic structure calculation-that this moiety actually corresponds to linear structures formed by the opening of the benzene ring via electron attachment. The cluster environment is essential in this process since it quenches the internal energy released upon ring opening, which in the gas phase leads to further dissociation of this anion.

Stabilization of benzene radical anion in ammonia clusters.

We investigate electron attachment to large ammonia clusters doped with a single benzene (Bz) molecule (NH3)N·Bz, N̄ ≈ 320. Negatively charged clusters are probed by mass spectrometry, and the energy-dependent ion yields are derived from mass spectra measured at different electron energies. The ion efficiency curves of pure ammonia clusters exhibit two maxima. At around 6 eV, (NH3)n-1NH2- ions are produced via dissociative electron attachment (DEA) to NH3 molecules. (NH3)n- ions produced at this energy are formed by DEA followed by fragment caging. At low energies around 1.3 eV, only (NH3)n- ions are formed for cluster sizes n ≥ 35 that correspond to solvated electrons in ammonia clusters. The doped (NH3)n·Bz- cluster ions exhibit essentially the same energy dependence. The (NH3)n·Bz- ions are metastable and evaporate NH3 molecule(s), while pure (NH3)n- ions are stable. The lifetime for NH3 molecule evaporation from the Bz-doped clusters was estimated as τ ≈ 18 µs. We interpret the metastability of the doped clusters by the charge localization on a Bz- ion solvated in the ammonia, which is accompanied by an energy release leading to the evaporation of NH3 molecule(s).

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 BzN 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 BzN 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.

Bimolecular reactions on sticky and slippery clusters: Electron-induced reactions of hydrogen peroxide.

Nanoparticles can serve as an efficient reaction environment for bimolecular reactions as the reactants concentrate either inside the nanoparticle or on the surface of the nanoparticle. The reaction rate is then controlled by the rate of formation of the reaction pairs. We demonstrate this concept on the example of electron-induced reactions in hydrogen peroxide. We consider two types of nanoparticle environments: solid argon particles, only weakly interacting with the hydrogen peroxide reactant, and ice particles with a much stronger interaction. The formation of hydrogen peroxide dimers is investigated via classical molecular dynamics (MD) simulations on a microsecond timescale. With a modified force field for hydrogen peroxide, we found out a fast formation and stabilization of the hydrogen peroxide dimer for argon nanoparticles, while the reaction pair was formed reversibly at a much slower rate on the water nanoparticles. We have further investigated the electron-induced reactions using non-adiabatic ab initio MD simulations, identifying the possible reaction products upon the ionization or electron attachment. The major reaction path in all cases corresponded to a proton transfer. The computational findings are supported by mass spectrometry experiments, where large ArM and (H2O)M nanoparticles are generated, and several hydrogen peroxide molecules are embedded on these nanoparticles in a pickup process. Subsequently, the nanoparticles are ionized either positively by 70 eV electrons or negatively by electron attachment at electron energies below 5 eV. The recorded mass spectra demonstrate the efficient coagulation of H2O2 on ArM, while it is quite limited on (H2O)M.

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 ArN, 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 ArN. 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.

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.

Energy partitioning and spin-orbit effects in the photodissociation of higher chloroalkanes.

We investigate the photodissociation dynamics of the C-Cl bond in chloroalkanes CH3Cl, n-C3H7Cl, i-C3H7Cl, n-C5H11Cl, combining velocity map imaging (VMI) experiment and direct ab initio dynamical simulations. The Cl fragment kinetic energy distributions (KEDs) from the VMI experiment exhibit a single peak with maximum close to 0.8 eV, irrespective of the alkyl chain length and C-Cl bond position. In contrary to CH3Cl, where less than 10% of the available energy is deposited into the internal excitation of the CH3 fragment, for all higher chloroalkanes around 40% to 60% of the available energy goes into the alkyl fragment excitation. We apply the classical hard spheres and spectator model to explain the energy partitioning, and compare the classical approach with direct ab initio dynamics simulations. The alkyl chain appears to be a soft, energy absorbing unit. We further investigate the role of the spin-orbit effects on the excitation and dynamics. Combining our experimental data with theory allows us to derive the probability of the direct absorption into the triplet electronic state as well as the probabilities for intersystem crossing. The results indicate an increasing direct absorption into the triplet state with increasing alkyl chain length.

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.

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 ArN and (H2O)N clusters. The selected examples are discussed from the perspective of the atmospheric and interstellar chemistry, and several future directions are proposed.

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.

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.

Different Dynamics of CH3 and Cl Fragments from Photodissociation of CH3Cl in Clusters.

We investigate the photodissociation of CH3Cl at 193.3 nm using the velocity map imaging technique in (CH3Cl)n clusters in comparison with isolated molecules. Our results for the isolated molecules are in excellent agreement with the previous study of Cl fragments, and we extend it by detecting also the CH3(ν = 0) fragments. For the clusters, the Cl (and Cl*) and CH3 fragment images are dominated by intense central isotropic features. The corresponding kinetic energy distributions (KEDs) reveal significant differences in the CH3 and Cl fragment dynamics. While the CH3 fragments exhibit a very narrow near-zero kinetic energy peak, pointing to almost complete caging of CH3 fragments, the Cl (and Cl*) fragments show more structured KEDs extending all the way to the maximum available kinetic energy. The Cl KED spectra have a bimodal character with two broad peaks close to zero and around 0.6 eV. We observe a higher ICH3(ν=0)/ICl signal ratio from the clusters compared to the monomers. This is attributed to an efficient quenching of the higher vibrationally excited ν2 states of the CH3 fragments generated in the photodissociation. Collisional quenching of these excited states in clusters enhances the detected CH3(ν = 0) state. Finally, we determine the [Cl*]/[Cl] branching ratio for the photodissociation pathways in the clusters as ≈0.55 ± 0.15 compared to 0.86 for the isolated molecules, which is also attributed to the collisional quenching of the excited state in the clusters. The clusters and photofragment dynamics are discussed.

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 ArM, 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.