Kinetics of electron attachment to OH and HNO3 and mutual neutralization of Ar+ with NO2(-) and NO3(-) at 300 and 500 K

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.

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.

Kinetics of Cations with C2 Hydrofluorocarbon Radicals

Reactions of the cations Ar⁺, O₂⁺, CO₂⁺, and CF₃⁺ with the C₂ radicals C₂H₅, H₂C₂F₃, C₂F₃, and C₂F₅ were investigated using the variable electron and neutral density attachment mass spectrometry technique in a flowing afterglow-Langmuir probe apparatus at room temperature. Rate coefficients for observed product channels for these 16 reactions are reported as well as rate coefficients and product branching fractions for the 16 reactions of the same cations with each of the stable neutrals used as radical precursors (the species RI, where R is the radical studied). Reactions with the stable neutrals proceed by charge transfer at or near the collisional rate coefficient where energetically allowed; where charge transfer is endothermic, bond-breaking/bond-making chemistry occurs. While also efficient, reactions with the radicals are more likely to occur at a smaller fraction of the collisional rate coefficient, and bond-breaking/bond-making chemistry occurs even in some cases where charge transfer is exothermic. It is noted that unlike radical reactions with neutral species, which occur with rate coefficients that are generally elevated compared to those of stable species, ion-radical reactivity is generally decreased relative to that of reactions with stable species. In particular, long-range charge transfer appears more likely to be frustrated in the ion-radical systems.

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.

Flowing afterglow measurements of the density dependence of gas-phase ion-ion mutual neutralization reactions

We have studied the dependence of several ion-ion mutual neutralization (MN) reactions on helium density in the range from 1.6 × 10(16) to 1.5 × 10(17) cm(-3) at 300 K, using the Variable Electron and Neutral Density Attachment Mass Spectrometry method. The rate coefficients of the reactions Ar(+) + Br2(-), Ar(+) + SF6(-), and Ar(+) + C7F14(-) were found to be independent of gas density over the range studied, in disagreement with earlier observations that similar MN reactions are strongly enhanced at the same gas densities. The cause of the previous enhancement with density is traced to the use of "orbital-motion-limit" theory to infer ion densities from the currents collected by ion-attracting Langmuir probes in a region where it is not applicable.

Electron attachment to CF3 and CF3Br at temperatures up to 890 K: experimental test of the kinetic modeling approach

Thermal rate constants and product branching fractions for electron attachment to CF3Br and the CF3 radical have been measured over the temperature range 300-890 K, the upper limit being restricted by thermal decomposition of CF3Br. Both measurements were made in Flowing Afterglow Langmuir Probe apparatuses; the CF3Br measurement was made using standard techniques, and the CF3 measurement using the Variable Electron and Neutral Density Attachment Mass Spectrometry technique. Attachment to CF3Br proceeds exclusively by the dissociative channel yielding Br(-), with a rate constant increasing from 1.1 × 10(-8) cm(3) s(-1) at 300 K to 5.3 × 10(-8) cm(3) s(-1) at 890 K, somewhat lower than previous data at temperatures up to 777 K. CF3 attachment proceeds through competition between associative attachment yielding CF3 (-) and dissociative attachment yielding F(-). Prior data up to 600 K showed the rate constant monotonically increasing, with the partial rate constant of the dissociative channel following Arrhenius behavior; however, extrapolation of the data using a recently proposed kinetic modeling approach predicted the rate constant to turn over at higher temperatures, despite being only ~5% of the collision rate. The current data agree well with the previous kinetic modeling extrapolation, providing a demonstration of the predictive capabilities of the approach.