Nucleation of Mixed Nitric Acid-Water Ice Nanoparticles in Molecular Beams that Starts with a HNO3 Molecule

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.

Why do dipole moments of HCl-water clusters fail to determine acid dissociation?

This paper quantitatively examines why dipole moments of HCl(H2O)n=1-8 cannot serve as the dissociation criterion for acid molecules using the Hirshfeld-I approach. Also, we propose the possible experimental parameter 〈P(HCl)〉, whose statistical average enables the assessment of acid dissociation in mixed clusters. Furthermore, our calculations reveal that a minimum of four water molecules are necessary to dissociate an HCl molecule.

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.

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.

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.

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.

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.

Carbon dioxide and propane nucleation: the emergence of a nucleation barrier

We investigate homogeneous gas-phase nucleation of CO2 and C3H8 in the uniform postnozzle flow of Laval expansions in the temperature range of 31.2 K to 62.9 K and 32.0 K to 42.1 K, respectively. Time-dependent cluster size distributions are recorded with mass spectrometry after single-photon ionization with vacuum ultraviolet light. Net monomer-cluster forward rate constants and experimental nucleation rates J are retrieved from the time-dependent cluster size distributions. The comparison of experimental enhancement factors derived from these net forward rates with calculated enhancement factors provides an indication for the transition from barrier-limited to barrierless nucleation. Our data suggest such a transition for CO2, but not for C3H8. The values of J lie in the range from 9 × 1014 cm-3 s-1 to 6 × 1015 cm-3 s-1. For CO2, the comparison of J with a modeled nucleation rate JQM based on quantum chemical calculations of the free energy barrier also hints at a transition from barrierless condensation to barrier-limited nucleation. Furthermore, we address the influence of the carrier gas pressure on the nucleation rate.

Water nucleation at extreme supersaturation

We report water cluster formation in the uniform postnozzle flow of a Laval nozzle at low temperatures of 87.0 and 47.5 K and high supersaturations of lnS ∼ 41 and 104, respectively. Cluster size distributions were measured after soft single-photon ionization at 13.8 eV with mass spectrometry. Critical cluster sizes were determined from cluster size distributions recorded as a function of increasing supersaturation, resulting in critical sizes of 6-15 and 1, respectively. Comparison with previous data for propane and toluene reveals a systematic trend in the nucleation behavior, i.e., a change from a steplike increase to a gradual increase of the maximum cluster size with increasing supersaturation. Experimental nucleation rates of 5 · 10¹⁵ cm^-3 s^-1 and 2 · 10¹⁵ cm^-3 s^-1 for lnS ∼ 41 and 104, respectively, were retrieved from cluster size distributions recorded as a function of nucleation time. These lie 2-3 orders of magnitude below the gas kinetic collision limit assuming unit sticking probability, but they agree very well with a recent prediction by a master equation model based on ab initio transition state theory. The experimental observations are consistent with barrierless growth at 47.5 K, but they hint at a more complex nucleation behavior for the measurement at 87.0 K.

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.

Mass spectrometry of aerosol particle analogues in molecular beam experiments

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

Uptake 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.

Acid Dissociation in HCl-Water Clusters is Temperature Dependent and Cannot be Detected Based on Dipole Moments

The dissociation of acids in aqueous environments at low temperatures in the presence of a limited amount of water is underlying a wealth of processes from atmospheric to interstellar science. For the paradigmatic case of HCl(H_{2}O)_{n} clusters, our extensive ab initio path integral simulations quantify in terms of free energy differences and barriers that n=4 water molecules are indeed required to dissociate HCl at low temperatures. Increasing the temperature, however, reverses the process and thus counteracts dissociation by fluctuation-driven recombination. The size of the electric dipole moment is shown to not correlate with the acid being in its dissociated or molecular state, thus rendering its measurement as a function of n unable to detect the dissociation transition.

Communication: Charge transfer dominates over proton transfer in the reaction of nitric acid with gas-phase hydrated electrons

The reaction of HNO3 with hydrated electrons (H2O)n- (n = 35-65) in the gas phase was studied using Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry and ab initio molecular dynamics simulations. Kinetic analysis of the experimental data shows that OH-(H2O)m is formed primarily via a reaction of the hydrated electron with HNO3 inside the cluster, while proton transfer is not observed and NO3-(H2O)m is just a secondary product. The reaction enthalpy was determined using nanocalorimetry, revealing a quite exothermic charge transfer with -241 ± 69 kJ mol-1. Ab initio molecular dynamics simulations indicate that proton transfer is an allowed reaction pathway, but the overall thermochemistry favors charge transfer.

Electron-triggered chemistry in HNO3/H2O complexes

Polar stratospheric clouds, which consist mainly of nitric acid containing ice particles, play a pivotal role in stratospheric chemistry. We investigate mixed nitric acid-water clusters (HNO3)m(H2O)n, m ≈ 1-6, n ≈ 1-15, in a laboratory molecular beam experiment using electron attachment and mass spectrometry and interpret our experiments using DFT calculations. The reactions are triggered by the attachment of free electrons (0-14 eV) which leads to subsequent intracluster ion-molecule reactions. In these reactions, the nitrate anion NO3- turns out to play the central role. This contradicts the electron attachment to the gas-phase HNO3 molecule, which leads almost exclusively to NO2-. The nitrate containing clusters are formed through at least three different reaction pathways and represent terminal product ions in the reaction cascade initiated by the electron attachment. Besides, the complex reaction pathways represent a new hitherto unrecognized source of atmospherically important OH and HONO molecules.

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.

Encasing of Na+ ion in dimer-formed acetic acid clusters

Peaks with anomalous abundance found in the mass spectra are associated with ions with enhanced stability. Among the scientific community focused on mass spectrometry, these peaks are called 'magic peaks' and their stability is often because of suggestive symmetric structures. Here, we report findings on ionised Na-acetic acid clusters [Na(+) -(AcA)n ] produced by Na-doping of (AcA)n and UV laser ionisation. Peaks labelled n = 2, 4, 8 are clearly distinguishable in the mass spectra from their anomalous intensity. Ab initio calculations helped elucidate cluster structures and energetic. A plausible interpretation of the magic peaks is given in terms of (AcA)n formed by dimer aggregation. The encasing of Na(+) by twisted dimers is proposed to be the origin of the enhanced cluster stability. A conceivable dimer-formed tube-like closed structure is found for the Na(+) -(AcA)8 .

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.

Proton transfer and isotope-induced reaction in aniline cluster ions

The proton transfer (PT) and other intraclusters reactions occurring after electron ionization of aniline clusters (PhNH2)N are investigated by the time-of-flight mass spectrometry. The mass spectra are recorded for different expansion conditions leading to the generation of different cluster sizes. Several fragment ions are shown to originate from intracluster reactions, namely, [Ph](+), [PhNH3](+) and [Ph-N-Ph](+). Reaction schemes are proposed for these ions starting with the PT process. The mass region beyond the monomer mass is dominated by cluster ions (PhNH2)n(+) accompanied by satellites with ±H and +2H. In experiments with deuterated species, new fragment ions are identified. The aniline isotopomer d5-PhNH2 yields the fragment ions (PhNH2)n⋅(N-Ph-NH2)(+). Analogical series is observed in experiments with d7-PhND2, and additional fragments occur corresponding to (PhND2)n⋅(D2N-ND-Ph-ND-ND2)(+) ions. The possible reaction pathways to these ions and the unusual isotope effects are discussed.

Atmospheric processes on ice nanoparticles in molecular beams

This review summarizes some recent experiments with ice nanoparticles (large water clusters) in molecular beams and outlines their atmospheric relevance: (1) Investigation of mixed water–nitric acid particles by means of the electron ionization and sodium doping combined with photoionization revealed the prominent role of HNO₃ molecule as the condensation nuclei. (2) The uptake of atmospheric molecules by water ice nanoparticles has been studied, and the pickup cross sections for some molecules exceed significantly the geometrical sizes of the ice nanoparticles. (3) Photodissociation of hydrogen halides on water ice particles has been shown to proceed via excitation of acidically dissociated ion pair and subsequent biradical generation and H₃O dissociation. The photodissociation of CF₂Cl₂ molecules in clusters is also mentioned. Possible atmospheric consequences of all these results are briefly discussed.