Imaging of hydrogen halides photochemistry on argon and ice nanoparticles

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.

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.

Mass spectrometry of aerosol particle analogues in molecular beam experiments

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

Clustering of Uracil Molecules on Ice Nanoparticles

We generate a molecular beam of ice nanoparticles (H₂O)_N, N̅ ≈ 130-220, which picks up several individual gas phase uracil (U) or 5-bromouracil (BrU) molecules. The mass spectra of the doped nanoparticles prove that the uracil and bromouracil molecules coagulate to clusters on the ice nanoparticles. Calculations of U and BrU monomers and dimers on the ice nanoparticles provide theoretical support for the cluster formation. The (U)_mH⁺ and (BrU)_mH⁺ intensity dependencies on m extracted from the mass spectra suggest a smaller tendency of BrU to coagulate compared to U, which is substantiated by a lower mobility of bromouracil on the ice surface. The hydrated U_m·(H₂O)_nH⁺ series are also reported and discussed. On the basis of comparison with the previous experiments, we suggest that the observed propensity for aggregation on ice nanoparticles is a more general trend for biomolecules forming strong hydrogen bonds. This, together with their mobility, leads to their coagulation on ice nanoparticles which is an important aspect for astrochemistry.

Biomolecule Analogues 2-Hydroxypyridine and 2-Pyridone Base Pairing on Ice Nanoparticles

Ice nanoparticles (H2O)N, N ≈ 450 generated in a molecular beam experiment pick up individual gas phase molecules of 2-hydroxypyridine and 2-pyridone (HP) evaporated in a pickup cell at temperatures between 298 and 343 K. The mass spectra of the doped nanoparticles show evidence for generation of clusters of adsorbed molecules (HP)n up to n = 8. The clusters are ionized either by 70 eV electrons or by two photons at 315 nm (3.94 eV). The two ionization methods yield different spectra, and their comparison provides an insight into the neutral cluster composition, ionization and intracluster ion-molecule reactions, and cluster fragmentation. Quite a few molecules were reported not to coagulate on ice nanoparticles previously. The (HP)n cluster generation on ice nanoparticles represents the first evidence for coagulating of molecules and cluster formation on free ice nanoparticles. For comparison, we investigate the coagulation of HP molecules picked up on large clusters ArN, N ≈ 205, and also (HP)n clusters generated in supersonic expansions with Ar buffer gas. This comparison points to a propensity for the (HP)2 dimer generation on ice nanoparticles. This shows the feasibility of base pairing for model of biological molecules on free ice nanoparticles. This result is important for hypotheses of the biomolecule synthesis on ice grains in the space. We support our findings by theoretical calculations that show, among others, the HP dimer structures on water clusters.

Lack of Aggregation of Molecules on Ice Nanoparticles

Multiple molecules adsorbed on the surface of nanosized ice particles can either remain isolated or form aggregates, depending on their mobility. Such (non)aggregation may subsequently drive the outcome of chemical reactions that play an important role in atmospheric chemistry or astrochemistry. We present a molecular beam experiment in which the controlled number of guest molecules is deposited on the water and argon nanoparticles in a pickup chamber and their aggregation is studied mass spectrometrically. The studied molecules (HCl, CH3Cl, CH3CH2CH2Cl, C6H5Cl, CH4, and C6H6) form large aggregates on argon nanoparticles. On the other hand, no aggregation is observed on ice nanoparticles. Molecular simulations confirm the experimental results; they reveal a high degree of aggregation on the argon nanoparticles and show that the molecules remain mostly isolated on the water ice surface. This finding will influence the efficiency of ice grain-mediated synthesis (e.g., in outer space) and is also important for the cluster science community because it shows some limitations of pickup experiments on water clusters.

Reactivity of Hydrated Electron in Finite Size System: Sodium Pickup on Mixed N2O-Water Nanoparticles

We investigate the reactivity of hydrated electron generated by alkali metal deposition on small water particles with nitrous oxide dopant by means of mass spectrometry and ab initio molecular dynamics simulations. The mixed nitrous oxide/water clusters were generated in a molecular beam and doped with Na atoms in a pickup experiment, and investigated by mass spectrometry using two different ionization schemes: an electron ionization (EI), and UV photoionization after the Na doping (NaPI). The NaPI is a soft-ionization nondestructive method, especially for water clusters provided that a hydrated electron es– is formed in the cluster. The missing signal for the doped clusters indicates that the hydrated electron is not present in the N2O containing clusters. The simulations reveal that the hydrated electron is formed, but it immediately reacts with N2O, forming first N2O– radical anion, later O–, and finally an OH• and OH– pair.

Photodissociation of aniline N–H bonds in clusters of different nature

The solvent effects on the photodissociation of aniline in cluster environments have been investigated by H-photofragment velocity map imaging at 243 nm, mass spectrometry after electron ionization, and ab initio calculations.

Photodissociation dynamics of ethanethiol in clusters: complementary information from velocity map imaging, mass spectrometry and calculations

We investigate the solvent effects on photodissociation dynamics of the S–H bond in ethanethiol CH₃CH₂SH (EtSH).