[en] The effects of stimulated photon emission on chemical processes in a radiation field are considered and their influence on the chemistry of the early universe and other astrophysical environments is investigated. Spontaneous and stimulated radiative attachment rate coefficients for H(-), Li(-) and C(-) are presented

[en] Charge transfer is an important process in many astrophysical and atmospheric environments. While numerous experimental and theoretical studies exist for H and He targets, data on other targets, particularly metals and molecules, are sparse. Using a variety of theoretical methods and computational techniques the authors are developing methods to estimate the cross sections for electron capture (charge transfer) in slow collisions of low charge state ions with heavy (Mg, Ca, Fe, Co, Ni and Zn) neutrals. In this ongoing work particular attention is paid to ascertaining the importance of double electron capture

[en] The atomic astrophysics group at ORNL produces, collects, evaluates, and disseminates atomic and molecular data relevant to astrophysics and actively models various astrophysical environments utilizing this information. With the advent of the World Wide Web, these data are also being placed on-line to facilitate their use by end-users. In this brief report, the group's recent activities in data production and in modeling are highlighted. For example, the authors describe recent calculations of elastic and transport cross sections relevant to ionospheric and heliospheric studies, charge transfer between metal ions and metal atoms and novel supernova nebular spectra modeling, ion-molecule collision data relevant to planetary atmospheres and comets, and data for early universe modeling

No abstract available

[en] Partition functions and dissociation equilibrium constants for the molecular ions H₂⁺ and He₂⁺ are presented for temperatures T in the range 1500(500)25,000 K and 2000(1000)75,000 K, respectively. The results are obtained by a direct sum of all electronic ground state rotational-vibrational levels whose energies are determined by solving the radial nuclear Schrodinger equation within the Born-Oppenheimer approximation. (author)

[en] Ab initio CI adiabatic potentials and dipole moment matrix elements for the triatomic dication NH₂²⁺ in the collinear, asymmetric arrangement have been calculated. A diabatic description of the collision process is obtained by a unitary transformation in a representation that diagonalizes the dipole moment matrix. Neglecting H₂ orientation and rotation, cross sections for the charge transfer process N²⁺ + H₂(v=0) → N⁺ + H₂⁺(v') are determined with a close-coupled quantal method. Results will be compared to recent measurements

[en] We report laboratory studies of the role played by multiple-electron capture (MEC) in solar wind induced cometary X-ray emission. Collisions of Ne¹⁰⁺ with He, Ne, Ar, CO, and CO₂ have been investigated by means of the traditional singles X-ray spectroscopy in addition to the triple-coincidence measurements of X-rays, scattered projectile, and target recoil ions for the atomic targets. The coincidence measurements enable the reduction of the singles X-ray spectra into partial spectra originating in single-electron capture (SEC) and MEC collisions. The measurements provide unequivocal evidence for a significant role played by MEC, and strongly suggest that models based solely on SEC are bound to yield erroneous conclusions on the solar wind composition and velocities and on cometary atmospheres. The experimental relative importance of MEC collisions is compared with molecular classical-over-the-barrier model (MCBM), classical trajectory Monte Carlo (CTMC), and multi-channel Landau-Zener (MCLZ), calculations which can qualitatively reproduce the experimental trends

[en] A survey of existing data for collisions of isotopes of hydrogen atoms, ions, and molecules is presented. The survey was limited to atom - diatom ionic collisions and to energies generally less than about 10 keV/u. The processes include particle -rearrangement and charge transfer, including both dissociative and non- dissociative channels, with an emphasis on state-to-state (or state-selected) data, where available. Since the last survey, a small number of investigations for deuterium and tritium ion-diatom systems have been performed, with some involving state-resolved data, which include the initial- state -resolved and state-to-state processes. While some progress has been made since the last survey, the database involving hydrogen isotope collisional processes, both total and state resolved, is far from complete

[en] Quantum close-coupling and coupled-states approximation scattering calculations for rotational energy transfer of rotationally excited NH₃ and ND₃ due to collisions with He are presented. Calculations were performed for collision energies between 10^-5 and 10000 cm^-1 using the NH₃-He potential of Hodges and Wheatley. State-to-state and total quenching cross-sections from some selected initial states are presented. Rate coefficients for ortho-NH₃ for the quenching 10+ -> 00+ transition were obtained on potentials scaled to reproduce measurements of second Virial coefficients with the results showing strong sensitivity to the potential, especially at low temperatures. Significant isotope effects are found in quenching cross-sections in the cold to ultra-cold regime particularly in the region dominated by quasi-bound resonances, ∼0.1 to 10 cm^-1. As ammonia has been translational cooled via Stark deceleration methods, it is a good candidate for experimental study of such effects at cold temperatures. Comparison of rate coefficients with available theoretical results are also presented. (authors)

[en] The physical conditions relating to the emission of x-rays from Jovian and cometary atmospheres and to supernova ejecta are briefly described. Emphasis is placed on elucidating the relevance and importance of atomic collision processes, the availability of data, and the outstanding data needs for modeling these environments. Some preliminary theoretical studies of electron capture for important collisions systems, involving molecular and atomic metal targets, are presented. copyright 1998 American Institute of Physics