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.

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.

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.

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.

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 Um·(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.

Photodissociation of aniline N-H bonds in clusters of different nature.

We investigated the solvent effects on the N-H bond photodisociation dynamics of aniline (PhNH2) in clusters using velocity map imaging (VMI). The VMI experiment was accompanied by a time-of-flight mass spectrometry after electron ionization to reveal the cluster nature. The H-fragment images were recorded at 243 nm in various expansion regimes corresponding to different species: isolated molecules; small (PhNH2)N, N ≤ 3, clusters; larger (PhNH2)N, N ≥ 10; small mixed PhNH2·(H2O)N, N ≤ 10, clusters; and individual PhNH2 molecules deposited on large (H2O)N, N̄ = 430. The H-fragment kinetic energy distributions exhibit fast fragments around 0.8 eV (A) assigned previously to a direct dissociation along a repulsive πσ* state potential, and slow statistical fragments peaking near 0.2 eV (B). In the aniline clusters the contribution of fast fragments (A) decreases relatively to (B) with increasing cluster size. A similar effect is observed when aniline is solvated with water molecules. The experimental data are interpreted with ab initio calculations. Cluster structures were calculated with both N-H bonds of an aniline molecule participating in hydrogen bonding, as well as the ones with free N-H bonds. The latter ones yield preferentially the fast fragments as the isolated molecule. For N-H engaged in hydrogen bonding a barrier increased along the N-H coordinate on the dissociative πσ* state potential surface, and also the energy of πσ*/S0 conical intersection increased. Thus the fast dissociation channel was closed stabilizing the molecule in clusters. The population could be funnelled through other conical intersections into the hot ground state which decayed statistically, yielding the slow H-fragments.

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.

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.

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). The H fragment images are recorded by velocity map imaging (VMI) at 243 nm in various expansion regimes ranging from isolated molecules to clusters of different sizes and compositions. The VMI experiment is accompanied by electron ionization mass spectrometry using a reflectron time-of-flight mass spectrometer (RTOFMS). The experimental data are interpreted using ab initio calculations. The direct S-H bond fission results in a peak of fast fragments at Ekin(H) ≈ 1.25 eV with a partly resolved structure corresponding to vibrational levels of the CH3CH2S cofragment. Clusters of different nature ranging from dimers to large (EtSH)N, N ≥ 10, clusters and to ethanethiol clusters embedded in larger argon "snowballs" are investigated. In the clusters a sharp peak of near-zero kinetic energy fragments occurs due to the caging. The dynamics of the fragment caging is pictured theoretically, using multi-reference ab initio theory for the ethanethiol dimer. The larger cluster character is revealed by the simultaneous analysis of the VMI and RTOFMS experiments; none of these tools alone can provide the complete picture.

Clustering and photochemistry of freon CF2Cl2 on argon and ice nanoparticles.

The photochemistry of CF2Cl2 molecules deposited on argon and ice nanoparticles was investigated. The clusters were characterized via electron ionization mass spectrometry, and the photochemistry was revealed by the Cl fragment velocity map imaging after the CF2Cl2 photodissociation at 193 nm. The complex molecular beam experiment was complemented by ab initio calculations. The (CF2Cl2)n clusters were generated in a coexpansion with Ar buffer gas. The photodissociation of molecules in the (CF2Cl2)n clusters yields predominantly Cl fragments with zero kinetic energy: caging. The CF2Cl2 molecules deposited on large argon clusters in a pickup experiment are highly mobile and coagulate to form the (CF2Cl2)n clusters on ArN. The photodissociation of the CF2Cl2 molecules and clusters on ArN leads to the caging of the Cl fragment. On the other hand, the CF2Cl2 molecules adsorbed on the (H2O)N ice nanoparticles do not form clusters, and no Cl fragments are observed from their photodissociation. Since the CF2Cl2 molecule was clearly adsorbed on (H2O)N, the missing Cl signal is interpreted in terms of surface orientation, possibly via the so-called halogen bond and/or embedding of the CF2Cl2 molecule on the disordered surface of the ice nanoparticles.

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.

Caging of Cl atoms from photodissociation of CF2Cl2 in clusters.

We investigate the photodissociation dynamics of freon CF2Cl2 by velocity map imaging at 193 nm. The Cl fragment images are recorded in various expansion regimes corresponding to isolated molecules and molecules in rare gas clusters. The molecular kinetic energy distributions are dominated by a peak at Ekin(Cl) ≈ 0.97 eV corresponding to the direct C-Cl bond dissociation but they also reveal features at lower kinetic energies. Possible mechanisms leading to these slow Cl atoms are discussed. The photodissociation in clusters is investigated in two regimes: (i) small Ar clusters with the CF2Cl2 molecule embedded in approximately one solvation Ar layer; (ii) large Xen, n¯ ≈ 100-500, clusters with embedded CF2Cl2 molecules. In the former clusters we observe caging yielding the Cl fragments with zero kinetic energy and direct exit resulting in fragments with the kinetic energies corresponding to the fragments from isolated molecules. In the latter case (ii) only the caged Cl fragments are observed.

Hydrogen bond dynamics in the excited states: photodissociation of phenol in clusters.

We have investigated the photodynamics of phenol molecules in clusters. Possible reaction pathways following the photoexcitation of hydrogen-bonded phenol clusters have been identified theoretically using ab initio calculations. Experimentally we have studied the phenol molecules and clusters of various size distributions in a molecular beam apparatus. In particular, we have measured the H-fragment kinetic energy distributions after the excitation with 243 nm and 193 nm laser radiation. At 243 nm the KED spectra did not show any significant difference between the photodissociation of isolated molecules and phenol in larger clusters, while at 193 nm the contribution of the fast H-fragments is significantly suppressed in clusters with respect to the bare phenol molecule. We have interpreted the experimental results within the framework of the suggested reaction pathways.

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

Mixed (HNO3)m(H2O)n clusters generated in supersonic expansion of nitric acid vapor are investigated in two different experiments, (1) time-of-flight mass spectrometry after electron ionization and (2) Na doping and photoionization. This combination of complementary methods reveals that only clusters containing at least one acid molecule are generated, that is, the acid molecule serves as the nucleation center in the expansion. The experiments also suggest that at least four water molecules are needed for HNO3 acidic dissociation. The clusters are undoubtedly generated, as proved by electron ionization; however, they are not detected by the Na doping due to a fast charge-transfer reaction between the Na atom and HNO3. This points to limitations of the Na doping recently advocated as a general method for atmospheric aerosol detection. On the other hand, the combination of the two methods introduces a tool for detecting molecules with sizable electron affinity in clusters.

Photochemistry of HI on argon and water nanoparticles: hydronium radical generation in HI·(H2O)n.

Photochemistry of HI molecules on large Ar(n) and (H(2)O)(n), n ∼ 100-500, clusters was investigated after excitation with 243 nm and 193 nm laser radiation. The measured H-fragment kinetic energy distributions pointed to a completely different photodissociation mechanism of HI on water than on argon clusters. Distinct features corresponding to the fragment caging (slow fragments) and direct exit (fast fragments) were observed in the spectra from HI photodissociation on Ar(n) clusters. On the other hand, the fast fragments were entirely missing in the spectrum from HI·(H(2)O)(n) and the slow-fragment part of the spectrum had a different shape from HI·Ar(n). The HI·(H(2)O)(n) spectrum was interpreted in terms of the acidic dissociation of HI on (H(2)O)(n) in the ground state, and hydronium radical H(3)O formation following the UV excitation of the ionically dissociated species into states of a charge-transfer-to-solvent character. The H(3)O generation was proved by experiments with deuterated species DI and D(2)O. The experiment was complemented by ab initio calculations of structures and absorption spectra for small HI·(H(2)O)(n) clusters, n = 0-5, supporting the proposed model.

Photoinduced processes in hydrogen bonded system: photodissociation of imidazole clusters.

The photodissociation of imidazole in hydrogen bonded clusters has been studied at photodissociation wavelengths 243 and 193 nm. Imidazole clusters of different mean cluster sizes n approximately 3 and 6 have been produced in expansions with He and Ar carrier gases, and the mean cluster sizes were determined by mass spectrometric and crossed beam scattering experiments. Simultaneously, the (C(3)N(2)H(4))(n) clusters were studied by ab initio calculations for n up to 4 molecules, confirming the hydrogen bond N-H...N motif in the clusters. The measured H-fragment kinetic energy distribution spectra exhibit a bimodal character similar to the KEDs found for the bare molecule. (1) At 243 nm the fast H-atoms originate from the direct dissociation process on the repulsive pi sigma* state, and the slow component results from the dynamics populating the vibrationally hot ground state via an S(1)/S(0) conical intersection. In the clusters the contribution of the slow component increases with the cluster size. The slow component is also dominant at the shorter wavelength of 193 nm, where the dynamics starts with the excitation of pi pi* state. It is shown that the slow component in our experiment is a product of subsequent two-photon absorption. We have proposed different mechanisms how the observed enhanced internal conversion can be rationalized. The increased stability with respect to the H-fragment dissociation in clusters can be caused either by hydrogen transfer in the N-H...N bond or by closing the pi sigma* dissociation channel as in the case of pyrrole clusters.

Solvent-induced photostability of acetylene molecules in clusters probed by multiphoton dissociation.

We have studied the multiphoton photodissociation of (C(2)H(2))(n) and (C(2)H(2))(n) x Ar(m) clusters in molecular beams. The clusters were prepared in supersonic expansions under various conditions, and the corresponding mean cluster sizes were determined, for which the photodissociation at 193 nm was studied. The measured kinetic energy distributions (KEDs) of the H fragment from acetylene in clusters peak around 0.2 eV, in agreement with the KED from an isolated C(2)H(2) molecule. However, the KEDs from the clusters extend to kinetic energies of over 2 eV, significantly higher than the maximum fragment energies from an isolated molecule of about 1 eV. The photofragment acceleration upon solvation is a rather unusual phenomenon. The analysis based on ab initio calculations suggests the following scenario: (i) At 193 nm, photodissociation of acetylene occurs mostly in the singlet manifold. (ii) The solvent stabilizes the acetylene molecule, preventing it from undergoing hydrogen dissociation and funneling the population into a vibrationally hot ground state. (iii) The excited C(2)H(2) absorbs the next photon and eventually dissociates, yielding the H fragment with a higher kinetic energy corresponding to the first C(2)H(2) excitation. Thus, the H-fragment KED extending to higher energies is a fingerprint of the cage effect and the multiphoton nature of the observed processes. The photon-flux dependence of the KEDs reflects the rate of the vibrational energy flow from the hot ground state of acetylene to the neighboring solvent molecules.

Water photodissociation in free ice nanoparticles at 243 nm and 193 nm.

The photolysis of (H(2)O)(n) nanoparticles of various mean sizes between 85 and 670 has been studied in a molecular beam experiment. At the dissociation wavelength 243 nm (5.10 eV), a two-photon absorption leads to H-atom production. The measured kinetic energy distributions of H-fragments exhibit a peak of slow fragments below 0.4 eV with maximum at approximately 0.05 eV, and a tail of faster fragments extending to 1.5 eV. The dependence on the cluster size suggests that the former fragments originate from the photodissociation of an H(2)O molecule in the cluster interior leading to the H-fragment caging and eventually generation of a hydronium H(3)O molecule. The photolysis of surface molecules yields the faster fragments. At 193 nm (6.42 eV) a single photon process leads to a small signal from molecules directly photolyzed on the cluster surface. The two photon processes at this wavelength may lead to cluster ionization competing with its photodissociation, as suggested by the lack of H-fragment signal increase. The experimental findings are complemented by theoretical calculations.