Jozef Lengyel

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.

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.

Proton Transfer in Hydrogen-Bonded Network of Phenol Molecules: Intracluster Formation of Water

Electron ionization and time-of-flight mass spectrometry was used to investigate the phenol clusters (PhOH)n of different size from single molecule to large clusters: in coexpansion with He, the dimers n = 2 are mostly generated; in Ar, large species of n ≥ 10 also occur. Besides [(PhOH)n](+•) cluster ion series, hydrated phenol cluster ions [(PhOH)n·xH2O](+•) with up to x = 3 water molecules and dehydrated phenol clusters [(PhOH)n-H2O](+•) were observed. The hydrated phenol series exhibits minima and maxima that are interpreted as evidence for proton transfer between the hydrogen bonded cluster ions of cyclic structures. The proton transfer leads to a water generation within the clusters, and subsequent elimination of the diphenyl ether molecule(s) from the cluster yields the hydrated phenol cluster ions. Alternatively, a water molecule release yields a series of dehydrated phenols, among which the diphenyl ether ion [PhOPh](+•) (n = 2) constitutes the maximum.

Photochemistry of Nitrophenol Molecules and Clusters: Intra- vs Intermolecular Hydrogen Bond Dynamics

We investigate both experimentally and theoretically the structure and photodynamics of nitrophenol molecules and clusters, addressing the question how the molecular photodynamics can be controlled by specific inter- and intramolecular interactions. Using quantum chemical calculations, we demonstrate the structural and energetic differences between clusters of 2-nitrophenol and 4-nitrophenol, using phenol as a reference system. The calculated structures are supported by mass spectrometry. The mass spectra of 2-nitrophenol clusters provide an evidence for a stacked structure compared to a strong O-H···O hydrogen bonding for 4-nitrophenol aggregates. We further investigate the photodynamics of nitrophenol molecules and clusters by means of velocity map imaging of the H-fragment generated upon 243 nm photodissociation. The experiments are complemented by ab initio calculations which demonstrate distinct photophysics of phenol, 2-nitrophenol, 4-nitrophenol. The measured H-fragment kinetic energy distributions (KEDs) from 2-nitrophenol molecules are compared to the KEDs from phenol. The comparison points to the intramolecular O-H···O hydrogen bond in 2-nitrophenol, stimulating fast internal conversion into the ground electronic state. This reaction channel is marked by exclusive appearance of slow statistical hydrogen fragments in 2-nitrophenol, which contrasts with fast hydrogen atoms observed for phenol. The photodissociation of 2-nitrophenol clusters yields a fraction of H-fragments with higher kinetic energies than the isolated molecules. These fragments originate from the caging effect in the clusters leading to multiphoton dissociation of molecules excited by the previous photons. We also propose a new ab initio based value for the O-H bond dissociation enthalpy in 2-nitrophenol (4.25 eV), which is in excellent agreement with the maximum measured H-fragment kinetic energy.

Suppression of low-energy dissociative electron attachment in Fe(CO)5 upon clustering

In this work, we probe anion production upon electron interaction with Fe(CO)5 clusters using two complementary cluster-beam setups. We have identified two mechanisms that lead to synthesis of complex anions with mixed Fe/CO composition. These two mechanisms are operative in distinct electron energy ranges. It is shown that the elementary decomposition mechanism that has received perhaps the most attention in recent years (i.e., dissociative electron attachment at energies close to 0 eV) becomes suppressed upon increasing aggregation of iron pentacarbonyl. We attribute this suppression to the electrostatic shielding of a long-range interaction that strongly enhances the dissociative electron attachment in isolated Fe(CO)5.

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.

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.

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, Nx Oy and halogen-containing molecules with water. A special attention is paid to their reactivity and solvent effects of water molecules on the observed processes.