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In this computational study, we describe a self-consistent trajectory simulation approach to capture the effect of neutral gas pressure on ion–ion mutual neutralization (MN) reactions. The electron transfer probability estimated using Landau–Zener (LZ) transition state theory is incorporated into classical trajectory simulations to elicit predictions of MN cross sections in vacuum and rate constants at finite neutral gas pressures. Electronic structure calculations with multireference configuration interaction and large correlation consistent basis sets are used to derive inputs to the LZ theory. The key advance of our trajectory simulation approach is the inclusion of the effect of ion-neutral interactions on MN using a Langevin representation of the effect of background gas on ion transport. For H+ − H− and Li+ − H(D)−, our approach quantitatively agrees with measured speed-dependent cross sections for up to ∼105 m/s. For the ion pair Ne+ − Cl−, our predictions of the MN rate constant at ∼1 Torr are a factor of ∼2 to 3 higher than the experimentally measured value. Similarly, for Xe+ − F− in the pressure range of ∼20 000–80 000 Pa, our predictions of the MN rate constant are ∼20% lower but are in excellent qualitative agreement with experimental data. The paradigm of using trajectory simulations to self-consistently capture the effect of gas pressure on MN reactions advanced here provides avenues for the inclusion of additional nonclassical effects in future work. [ABSTRACT FROM AUTHOR]

Recently we developed a theoretical model of ion-atom collisions, which was made on the basis of a time-dependent density functional theory description of the electron dynamics and a classical treatment of the heavy particle motion. Taking advantage of the real-space grid method, we introduce a 'coordinate space translation' technique to allow one to focus on a certain space of interest such as the region around the projectile or the target. Benchmark calculations are given for collisions between proton and oxygen over a wide range of impact energy. To extract the probability of charge transfer, the formulation of Lüdde and Dreizler [J. Phys. B 16, 3973 (1983)] has been generalized to ensemble-averaging application in the particular case of O(3P). Charge transfer total cross sections are calculated, showing fairly good agreements between experimental data and present theoretical results. [ABSTRACT FROM AUTHOR]

The diatomic molecules SiPb and GePb were for the first time identified by producing high temperature vapors of the constituent pure elements in a “double-oven-like” molecular-effusion assembly. The partial pressures of the atomic, heteronuclear, and homonuclear gaseous species observed in the vapor, namely, Si, Ge, Pb, SiPb, GePb, Pb2, Gen, and Sin (n=2–3), were mass-spectrometrically measured in the overall temperature ranges 1753–1961 K (Ge–Pb) and 1992–2314 K (Si–Pb). The dissociation energies of the new species were determined by second- and third-law analyses of both the direct dissociation reactions and isomolecular exchange reactions involving homonuclear molecules. The selected values of the dissociation energies at 0 K (D0°) are 165.1±7.3 and 141.6±6.9 kJ/mol, respectively, for SiPb and GePb, and the corresponding enthalpies of formation (ΔfH0°) are 476.4±7.3 and 419.3±6.9 kJ/mol. The ionization efficiency curves of the two species were measured, giving the following values for the first ionization energies: 7.0±0.2 eV (SiPb) and 7.1±0.2 eV (GePb). A computational study of the species SiPb and GePb was also carried out at the CCSD(T) level of theory using the relativistic electron core potential approach. Molecular parameters, adiabatic ionization energies, adiabatic electron affinities, and dissociation energies of the title species were calculated, as well as the enthalpy changes of the exchange reactions involving the other Pb-containing diatomics of group 14. Finally, a comparison between the experimental and theoretical results is presented, and from a semiempirical correlation the unknown dissociation energies of the SiSn and PbC molecules are predicted as 234±7 and 185±11 kJ/mol, respectively. [ABSTRACT FROM AUTHOR]

Close-coupling calculations of single (dissociative and nondissociative) and double electron capture cross sections in He[SUP2+] + H[SUB2] collisions in the range of impact energies 0.5-25 keV/amu are presented and compared with experimental data. The calculations have been carried out at the Franck-Condon level and employing both ab initio expansions, in terms of three-center electronic functions, and the independent particle model approximation. We discuss the mechanisms of the processes and the validity limits of these treatments. [ABSTRACT FROM AUTHOR]

A new theoretical result of the total charge transfer cross section in collisions of O[sup +] ion in the ground state with H[sub 2] molecule is reported. The collision energy of the incident O[sup +]([sup 4]S) ion ranges from 1 eV/u to 10 keV/u, covering the available experimental data. States of [sup 4]A[sub 2] and [sup 4]B[sub 1] symmetry are included in the calculation. The state-resolved cross sections are obtained by using both the semiclassical and full quantum molecular orbital close coupling method. Based on our results, we propose a possible explanation for the large discrepancies in recent measurements among various experimental groups. [ABSTRACT FROM AUTHOR]

The experimental electron-impact total ionization cross sections (TICSs, ICSs) of CF[sub 4], CHF[sub 3], CH[sub 2]F[sub 2], and CH[sub 3]F fluoromethanes reported so far and a new set of data obtained with a linear double focusing time-of-flight mass spectrometer have been compared with the ab initio and (semi)empirical based ICS available methods. TICSs computational methods include: two approximations of the binary-encounter dipole (BED) referred to hereafter as Kim (Kim-BEB) and Khare (Khare-BEB) methods, the Deutsch and Ma¨rk (DM) formalism, also requiring atomic and molecular ab initio information, the modified additivity rule (MAR), and the Harland and Vallance (HV) methods, both based on semiempirical or empirical correlations. The molecular ab initio information required by the Kim, Khare, and DM methods has been computed at a variety of quantum chemistry levels, with and without electron correlation and a comprehensive series of basis sets. The general conclusions are summarized as follows: the Kim method yields TICS in excellent agreement with the experimental method; the Khare method provides TICS very close to that of Kim at low electron-impact energies (<100 eV), but their Mott and Bethe contributions are noticeably different; in the Kim and Khare approximations the electron correlation methods improve the fittings to the experimental profiles in contrast with the large basis sets, that leads to poorer results; the DM formalism yields TICS profiles with shapes similar to the experimental and the BEB methods, but consistently lower and with the profiles maxima shifted towards lower incident electron energies; the MAR method supplies very good ICS profiles, between those of BEB and DM methods; finally, the empirical HV method provides rather poor fittings concomitant with the simplicity and the few empirical parameters used. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Studies the photodynamics in superfluid helium. Femtosecond laser-induced ionization; Charge recombination; Distinct excitation regimes; Laser-induced fluorescence spectroscopy below breakdown; Intensity of the fluorescence spectrum below breakdown.

Describes a method for studying collision dynamics in slow ion/atom-atom collisions. Generalization of the single-electron, two-center hidden crossings method to multielectron systems; Analytic properties of energy surfaces and wave functions of the adiabatic electronic Hamiltonian; Use of the complex version of the unrestricted Hartree-Fock method.

Investigates the capture of lithium atoms by a beam of large cold helium clusters and subsequent ion production by electron impact. How lithium atoms are efficiently picked up by helium droplets; Mass spectrum revealing the presence of lithium atom and dimer ions; How capture by helium-cluster beams can be used for efficient soft ionization.

Describes the measurement of the vibrational populations for hydrogen molecules issuing from a cell containing a mixture of atomic and molecular hydrogen in the presence of an incandescent tungsten filament. Two forms of the gold films; Evidence that the vibrational populations differed from those observed previously with other metals.

Discusses the measurement of the absolute total cross sections for the state-selected reactions of ion-molecule reactions in the center-of-mass collision energy. Formation of excited ions in abundance by solar vacuum ultraviolet photoionization; Importance of excited atomic ions for modeling the ion chemistry of planetary ionospheres.

Absolute partial cross sections for the production of CO+2, CO+, CO2+2, O+, C+, O2+, and C2+ from electron-impact ionization of CO2 are reported for electron energies from threshold to 1000 eV. The product ions are mass analyzed using a time-of-flight mass spectrometer and detected with a position-sensitive detector whose output provides clear evidence that all product ions are completely collected. The overall uncertainty in the absolute cross section values is ±3.5% for singly charged parent ions and is slightly greater for fragment and doubly charged ions. For the fragment ion cross sections, all but one of the previous measurements are observed to be significantly lower than the present results. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

General analytical expressions for cross sections for direct ionization in atom–atom collisions are evaluated using the classical impulse approximation. The approach is also applied to ion–atom and molecule–molecule interactions. The overall accuracy of the obtained cross sections in a broad range of energy is better, when compared with existing measurements for many collision systems, than accuracy of other analytical predictions available in literature. [ABSTRACT FROM AUTHOR]

We present and develop the multiconfigurational particle–particle propagator (MCP2P) method for directly determining vertical double ionization potentials (DIPs) and double electron affinities. The MCP2P method gives highly accurate DIPs. In the first calculations with this method (obtained using a moderate-sized MCP2P complete active space) the average absolute deviation from Δfull configuration interaction within the same basis set for the DIPs to the lowest (in energy) five 1,3S Be2+ states is 0.04 eV. [ABSTRACT FROM AUTHOR]

Direct detection of homonuclear diatomic dications using mass spectrometry has the intrinsic inability to distinguish between fragments with the same mass-to-charge ratio, as is the case of the oxygen molecule. In this work, absolute cross sections for the double ionization of the homoisotopic 16O2 molecule by electron impact, in the 30-400 eV energy range, is reported for the first time, and show significant discrepancies with previous results, obtained with the heteroisotopic 16O17O. The measurements suggest that O2++ is mainly produced through post-collisional Auger-like deexcitation. [ABSTRACT FROM AUTHOR]