Richard H. Salter

Guided‐ion beam measurements of the X++H2O(D2O) (X=Ar,N2) collision systems

Guided‐ion beam cross section and product kinetic energy measurements of charge‐transfer and atom‐abstraction reactions of the Ar++H2O(D2O) and N2++H2O(D2O) collision systems are presented for collision energies ranging between 0.2 and 20 eV c.m. Both charge‐transfer systems exhibit large hyperthermal cross sections exceeding 10 A2 and are characterized by a long‐range interaction, evidenced by the small amount of angular scattering observed in product‐ion time‐of‐flight (TOF) measurements. Weak forward‐scattered signals due to orbiting collisions are detected and are interpreted to stem from a dipole orientation that maximizes the long‐range attractive forces. The charge‐transfer product ion velocity distributions are well described by an osculating complex model. The charge‐transfer systems exhibit weak isotope effects that are related to competition with the atom‐abstraction channels. The atom‐abstraction cross sections represent ∼10% of the total cross section and the observed distinct isotope effects...

Cross sections and product kinetic energy analysis of H2O+-H2O collisions at suprathermal energies

The reaction of H2O+ with H2O is studied using a longitudinal geometry double mass spectrometer in the collision energy range Ec.m.=0.5–25 eV. Cross sections are reported for oxonium ion (H3O+) production and the symmetric charge exchange. Isotopic substitution is used to discern the product branches, including the separation of the two channels for oxonium ion production: (i) proton transfer to the target molecule; and (ii) atom pickup by the primary ion. The largest branching ratio is observed for the charge exchange channel, where no isotope effect is detected in the investigated energy range. Proton transfer exhibits the second largest branching ratio and accounts for more than 90% of the oxonium ion production throughout the measured energy range. The proton transfer cross section is dependent on isotopic substitution, while the atom pickup channel is too weak to make a distinct statement on its isotopic behavior. Product ion energies, determined by time‐of‐flight measurements, are also reported for ...

Cross section and product time‐of‐flight measurements of the reaction of N+2 with H2O and D2O at suprathermal energies

Charge exchange and hydrogen atom pickup cross sections, and product ion time‐of‐flight measurements are reported for N+2 –H2O(D2O) collisions at center‐of‐mass collision energies ranging between 1 and 15 eV. No isotope effect is detected for the charge exchange branch, while a significant isotope effect is observed for the atom pickup reaction. Throughout the measured energy range, the time‐of‐flight measurements show that the H2O+(D2O+) charge exchange product is produced with near‐thermal energy in the laboratory frame, implying little or no momentum transfer. The charge exchange reaction products are therefore formed with internal energy comparable to the exothermicity of the reaction (2.96 eV). The atom pickup ion product velocity distributions and the atom pickup isotope effect are consistent with a spectator stripping mechanism.

Luminescence measurements of Ar++H2O and N+2+H2O suprathermal charge transfer collisions: Product state distributions from H2O+Ã 2A1–X̃ 2B1 analysis

Luminescence measurements of the Ar++H2O and N+2+H2O charge transfer systems are reported at collision energies ranging from 0.6 to 783 eV at a maximum resolution of 0.5 nm (FWHM). Both systems produce H2O+A 2A1–X 2B1 emissions throughout the measured energy range. Approximate A state vibrational populations are determined using known spectroscopic constants. At the highest energies investigated, the A state population resembles a Franck–Condon distribution. At low collision energies, near‐resonant vibrational levels of the A state are preferentially populated indicating dominance of large impact parameter charge transfer collisions. Population of high K vibronic sublevels, corresponding to high rotational excitation about the A rotational axis of H2O+, is observed in the nearest‐resonant vibrational levels. A drop to near‐zero population is observed for off‐resonant levels at low collision energies that is associated with the dominance of the atom pickup channel at small impact parameters.

Dynamics of the O+ H2O charge-transfer reaction and its implications for space-borne measurements

Abstract O + ( 4 S) + H 2 O charge-transfer cross-section and product-ion time-of-flight measurements are presented over a center-of-mass collision energy range of 0.2–15 eV. The results are obtained with a newly constructed guided-ion beam experiment. The measured energy dependence of the charge-transfer crosssection agrees well with previous measurements and ADO model predictions of Bates [Chem. Phys. Lett. 82, 396 (1981) Proc. R. Soc. Lond. A384, 289 (1982) Chem. Phys. Lett. 111, 428 (1984)] at low collision energies, but exhibits significant differences at collision energies above 2 eV. The product kinetic energy analysis shows that most of the product ions are produced with near-thermal kinetic energies. At low collision energies, a significant backscattered channel is observed that is associated with complex formation. This channel exhibits a cross-section of approximately 1.4 A2at a collision energy of 2.65 eV, corresponding to a laboratory ion energy of 5 eV.

Energy partitioning in hyperthermal O+ + H2O charge-transfer collisions

Abstract The energy partitioning in hyperthermal O + + H 2 O charge-transfer collisions is determined from the analysis of guided-ion beam time-of-flight measurements. The experimentally determined H 2 O + laboratory velocity distributions exhibit a main band due to near-thermal ions and a weak channel due to center-of-mass backscattered ions. The velocity distributions are fit to simulated distributions based on an osculating complex model. The charge-transfer products formed via the shortest lived complexes exhibit little translational-to-internal energy transfer, resulting in internal energies of 1.0 ± 0.2 eV. Contributions due to longer-lived complexes are observed and result in increased internal excitation. The internal energy distribution is, however, non-statistical because the lifetime of these complexes is considerably shorter than their rotational period.

Chemiluminescence measurements of the N+2, N++H2O charge transfer systems at suprathermal energies: Direct probe of the dynamics of large cross section charge transfer processes

The chemiluminescence from suprathermal N+2–H2O and N+–H2O collisions is studied using a new experiment. Intense H2O+A 2A1–X 2B1 emission is observed from the N+2 +H2O charge transfer whereas no emissions are detected from the N+–H2O system. OH A 2Σ+–X 2Π emission originating from the N+2+H2O atom pickup channel is also observed.

A laboratory study of charge transfer and hydrogen atom pickup reactions in N+-H2O (D2O) collisions at suprathermal energies

Abstract Charge transfer and hydrogen atom pickup cross sections are reported for N + H 2 O(D 2 O) collisions at center-of-mass energies between 1 and 25 eV. Time-of-flight measurements were performed to determine the charge transfer product ion kinetic energies. The exothermic charge transfer proceeds rapidly at large impact parameters with little momentum transfer, and is independent of the water isotope. Above 2 eV, this reaction is governed by a direct mechanism without a long-lived complex. The endothermic hydrogen atom pickup cross section increases with the collision energy, and is approximately two orders of magnitude smaller than the charge transfer cross section.

Reaction cross section and product ion time‐of‐flight measurements for collisions of N+ and N+2 with CO2 at suprathermal energies

Charge exchange and dissociative ionization cross sections are reported for N+–CO2 and N+2–CO2 collisions at center‐of‐mass energies between 2 and 40 eV. Product ion kinetic energies are measured using time‐of‐flight techniques. Energy resonance considerations based on CO2 ionization Franck–Condon factors predict charge exchange to be slow for both of these reaction pairs. The N+–CO2 charge exchange, however, exhibits a large cross section and proceeds about one order of magnitude faster than the N+2–CO2 charge exchange. The formation of CO+ in N+–CO2 collisions is reported, while no dissociative ionization products are observed in the N+2–CO2 system.

Guided‐ion beam measurements of X++NO (X=Ar, N2) reactions

Cross section and product ion time‐of‐flight measurements are presented for the X++NO (X=Ar, N2) collision systems over a collision energy range of 0.1–20 eV (c.m.). The experiments are carried out in an octopole guided‐ion beam apparatus. Charge transfer is the main channel observed and dissociative charge‐transfer products are observed for collision energies equal to or greater than their respective thermodynamic thresholds. A weak channel leading to ArN+ and/or ArO+ is observed in the X=Ar system. The charge‐transfer cross sections of the two collision systems are similar in magnitude and have approximately an E−1/2 dependence at near‐thermal collision energies. The near‐thermal cross sections are significantly less than the Langevin–Gioumousis–Stevens predictions for a reaction mediated by ion–induced dipole interactions and complex formation. The time‐of‐flight distributions of the Ar+, N+2+NO charge‐transfer products are characteristic of near‐resonant charge‐transfer processes. At all collision ene...