Rate constants for the reactions of metastable O+ ions with N2 and O2 at collision energies 0.04 to 0.2 eV and the mobilities of these ions at 300 K

Kinetics of associative detachment of O- + N2 and dissociative attachment of e- + N2O up to 1300 K: chemistry relevant to modeling of transient luminous events

The rate constants of O- + N2 → N2O + e- from 800 K to 1200 K and the reverse process e- + N2O → O- + N2 from 700 K to 1300 K are measured using a flowing afterglow - Langmuir probe apparatus. The rate constants for O- + N2 are well described by 3 × 10-12 e-0.28 eV kT-1 cm3 s-1. The rate constants for e- + N2O are somewhat larger than previously reported and are well described by 7 × 10-7 e-0.48 eV kT-1 cm3 s-1. The resulting equilibrium constants differ from those calculated using the fundamental thermodynamics by factors of 2-3, likely due to significantly non-thermal product distributions in one or both reactions. The potential surfaces of N2O and N2O- are calculated at the CCSD(T) level. The minimum energy crossing point is identified 0.53 eV above the N2O minimum, similar to the activation energy for the electron attachment to N2O. A barrier between N2O- and O- + N2 is also identified with a transition state at a similar energy of 0.52 eV. The activation energy of O- + N2 is similar to one vibrational quantum of N2. The calculated potential surface supports the notion that vibrational excitation will enhance reaction above the same energy in translation, and vibrational-state specific rate constants are derived from the data. The O- + N2 rate constants are much smaller than literature values measured in a drift tube apparatus, supporting the contention that those values were overestimated due to the presence of vibrationally excited N2. The result impacts the modeling of transient luminous events in the mesosphere.

Gaseous transport properties of the ground and excited Cr, Co and Ni cations in He: Ab initio study of electronic state chromatography

The electronic state chromatography (ESC) effect allows the differentiation of ions in their ground and metastable states by their gaseous mobilities in the limit of low electrostatic fields. It is investigated here by means of accurate transport calculations with ab initio ion-atom potentials for the Cr, Co and Ni cations in He buffer gas near room temperature. The values for the open-shell ions in degenerate states are shown to be well approximated by using the single isotropic interaction potential. Minimalistic implementation of the multireference configuration interaction (MRCI) method is enough to describe the zero-field transport properties of metastable ions in the 3dm-14s configuration, such as Cr+(a6D), Co+(a5F) and Ni+(4F), due to their weak and almost isotropic interaction with He atom and the low sensitivity of the measured mobilities to the potential well region. By contrast, interactions involving the ions in the ground 3dm states, such as Cr+(a6S), Co+(a3F) and Ni+(2D), are strong and anisotropic; the MRCI potentials poorly describe their transport coefficients. Even the coupled cluster with singles, doubles and non-iterative triples [CCSD(T)] approach taking into account vectorial spin-orbit coupling may not be accurate enough, as shown here for Ni+(2D). The sensitivity of ion mobility and the ESC effect to interaction potentials, similarities in ion-He interactions of the studied ions in distinct configurations, accuracy and possible improvements of the ab initio schemes, and control of the ESC effect by macroscopic parameters are discussed. Extensive sets of improved interaction potentials and transport data are generated.

Stepwise formation of H3O(+)(H2O)n in an ion drift tube: Empirical effective temperature of association/dissociation reaction equilibrium in an electric field

We measured equilibrium constants for H3O(+)(H2O)n-1 + H2O↔H3O(+)(H2O)n (n = 4-9) reactions taking place in an ion drift tube with various applied electric fields at gas temperatures of 238-330 K. The zero-field reaction equilibrium constants were determined by extrapolation of those obtained at non-zero electric fields. From the zero-field reaction equilibrium constants, the standard enthalpy and entropy changes, ΔHn,n-1 (0) and ΔSn,n-1 (0), of stepwise association for n = 4-8 were derived and were in reasonable agreement with those measured in previous studies. We also examined the electric field dependence of the reaction equilibrium constants at non-zero electric fields for n = 4-8. An effective temperature for the reaction equilibrium constants at non-zero electric field was empirically obtained using a parameter describing the electric field dependence of the reaction equilibrium constants. Furthermore, the size dependence of the parameter was thought to reflect the evolution of the hydrogen-bond structure of H3O(+)(H2O)n with the cluster size. The reflection of structural information in the electric field dependence of the reaction equilibria is particularly noteworthy.

Reactions of State-Selected Atomic Oxygen Ions O(+)((4)S, (2)D, (2)P) with Methane

An experimental study has been carried out on the reactions of state selected O(+)((4)S, (2)D, (2)P) ions with methane with the aims of characterizing the effects of both the parent ion internal energy and collision energy on the reaction dynamics and determining the fate of oxygen species in complex media, in particular the Titan ionosphere. Absolute cross sections and product velocity distributions have been determined for the reactions of (16)O(+) or (18)O(+) ions with CH4 or CD4 from thermal to 5 eV collision energies by using the guided ion beam (GIB) technique. Dissociative photoionization of O2 with vacuum ultraviolet (VUV) synchrotron radiation delivered by the DESIRS beamline at the SOLEIL storage ring and the threshold photoion photoelectron coincidence (TPEPICO) technique are used for the preparation of purely state-selected O(+)((4)S, (2)D, (2)P) ions. A complete inversion of the product branching ratio between CH4(+) and CH3(+) ions in favor of the latter is observed for excitation of O(+) ions from the (4)S ground state to either the (2)D or the (2)P metastable state. CH4(+) and CH3(+) ions, which are by far the major products for the reaction of ground state and excited states, are strongly backward scattered in the center of mass frame relative to O(+) parent ions. For the reaction of O(+)((4)S), CH3(+) production also rises with increasing collision energy but with much less efficiency than with O(+) excitation. We found that a mechanism of dissociative charge transfer, mediated by an initial charge transfer step, can account very well for all the observations, indicating that CH3(+) production is associated with the formation of H and O atoms (CH3(+) + H + O) rather than with OH formation by an hydride transfer process (CH3(+) + OH). Therefore, as the CH4(+) production by charge transfer is also associated with O atoms, the fate of oxygen species in these reactions is essentially the O production, except for the reaction of O(+)((4)S), which also produces appreciable amounts of H2O(+) ions but only at very low collision energy. The production of O atoms and the nature of the states in which they are formed are discussed for the reactions of O(+) ions with CH4 and N2.

Interaction potential and transport properties of NeO+

The results of both experimental and theoretical studies of the mobility of O(+) in Ne are reported. Errors in the experiments have been carefully assessed, allowing the obtained data to serve as stringent tests of the ab initio potentials. These potentials were calculated using the RCCSD(T) method, employing basis sets of quintuple-zeta quality. Curves were calculated for the lowest (4)Sigma(-) state [arising from O(+)((4)S) interacting with Ne] and for the (2)Pi state [arising from O(+)((2)D) interacting with Ne]. Then, the effects of spin-orbit coupling were incorporated by using the Breit-Pauli operator. The resulting ground state (Omega=32) of NeO(+) gives mobility values in good agreement with experiment at all field strengths. Values of spectroscopic quantities for the ground electronic state of NeO(+) are also presented.

Ion mobility studies of electronically excited States of atomic transition metal cations: development of an ion mobility source for guided ion beam experiments

The design of an ion mobility source developed to couple to a guided ion beam tandem mass spectrometer is presented. In these exploratory studies, metal ions are created continuously by electron ionization of the volatile hexacarbonyls of the three group 6 transition metals. These ions are focused into a linear hexapole ion trap, which collects the ions and then creates high intensity pulses of ions, avoiding excessive ion losses resulting from the low duty cycle of pulsed operation. The ion pulses are injected into a six-ring drift cell filled with helium where ions having different electronic configurations can separate because they have different ion mobilities. Such separation is observed for chromium ions and compares favorably with the pioneering work of Kemper and Bowers (J. Phys. Chem.1991, 95, 5134). The results are then extended to Mo(+) and W(+), which also show efficient configuration separation. The source conditions needed for high intensities and good configuration separation are discussed in detail and suggestions for further improvements are also provided.

Mobility of O+ in He and interaction potential of HeO+

New experimental measurements are reported for the mobility of O(+) ions in He gas at 300 K. The accuracy of these new values is estimated as +/-2.5%, which allows them to serve as a stringent test of a new ab initio potential that we have calculated using the RCCSD(T) method. We employed the aug-cc-pV5Z basis set with counterpoise corrections and took spin-orbit coupling into account. The present experimental values lie below the calculated ones, but the difference becomes statistically significant only at moderate and high values of the ratio of the electric field strength to the gas number density; even there they are only marginally significant.

A simple method for estimating effective ion source residence time

A method is presented that uses the well-understood O2/Ax ion-molecule reaction system to determine the effective ion source residence time of a chemical ionization source. The process consists of: (1) defining the kinetic system in terms of reactions, reaction rates, and ionization cross sections; (2) solving the differential equations that describe the time evolu-tion of the kinetic system, and (3) comparing the calculated results to experimentally measured relative ion intensities. These steps are repeated for a variety of 02/Ar sample ratios and inlet pressures. The method leads to a simple relationship between inlet pressure and effective ion source residence time, independent of the 02/Ar sample ratio.

Spectroscopic imaging of the thermosphere from the Space Shuttle

An important need for studies of the thermospheric chemistry and dynamics is multispectral information on the emissions over a broad wavelength range. An instrument that has been developed to meet this need is the Imaging Spectrometric Observatory which is currently undergoing integration for flight on the Shuttle Spacelab 1 mission (mid-1983). The instrument represents the coincidence of three major factors: the payload capacity of the Shuttle which permits the placing into orbit of a sufficiently large and comprehensive spectrometric system; the recent advances in solid state imaging detector components which have permitted use of very fast multiplexing detectors; and the maturing of knowledge of the thermosphere permitting clear identification of the desired measurements. The instrument consists of an array of five imaging spectrometers, each covering a portion of the 300-12000-A total wavelength range. The five spectrometers operate simultaneously with the spectrum being dispersed in one dimension on the focal plane detectors and spatial information along the slit length being resolved in the other. The detector system in each spectrometer is an intensified-charge coupled device, optimized for the wavelength region in question. The instrument has been designed in a modular fashion to permit variation in instrument parameters on subsequent flights. For the first flight the typical spectral resolution is 3 A (6 A below 1200 A). The sensitivity has been selected to permit coverage of a large dynamic range extending from weak nocturnal signals (approximately 1 R) to the bright earth (approximately10(7) R/A). The instrument has a minicomputer system which is located in the Spacelab module itself allowing payload crew interaction with the observation sequences. In addition, during the actual 7 days in orbit, the telemetry data stream from the instrument will be recorded directly on an instrument ground support minicomputer, permitting real-time and near-real-time evaluation of the data. The objectives on the initial flight are to obtain a survey atlas of the dayglow, nightglow, and twilightglow over the full wavelength range together with obtaining data necessary for the solution of several specific problems.