P. C. Stancil

The Deuterium Chemistry of the Early Universe

The chemistry of deuterium, as well as that of hydrogen and helium, in the postrecombination era of the expanding early universe is presented. A thorough survey of all potentially important gas-phase reactions involving the primordial elements produced in the Big Bang, with a particular emphasis on deuterium, is given. The reaction set, consisting of 144 processes, is used in a nonequilibrium chemistry model to follow the production of primordial molecules in the postrecombination era. It is found that significant deuterium fractionation occurs for HD+, HD, and H2D+, while the abundance of D+ is reduced compared to the proton abundance. Even with the enhanced fractionation of H2D+, its abundance is predicted to be too small to cause any interesting cosmological consequences, such as possible attenuation of spatial anisotropies in the cosmic background radiation field, detections of the epochs of reionization and reheating, or constraints on the primordial deuterium abundance. HD, being the second most abundant primordial molecule after H2, may play a role in subsequent structure formation because of its cooling radiation.

Stimulated Radiative Association of He and H

The enhancement of the rate coefficient for the radiative association of He and H+ to form HeH+ arising from stimulated emission due to a blackbody radiation field is calculated. The effects on the fractional abundance of HeH+ in the early universe, in supernova ejecta, and in planetary nebulae are small. There may occur some enhancement in the abundance of HeH+ in quasar broad-line clouds.

Charge Transfer in Collisions of C+ with H and H+ with C

Charge transfer rate coefficients for collisions of C+ with H and H+ with C are presented for temperatures from 30,000 to 107 K and from 10 to 107 K, respectively. The rate coefficients were calculated from recommended cross sections deduced in a recent theoretical and experimental investigation that took into account previous measurements. Nonadiabatic radial coupling is the dominant mechanism for both reactions above ~50,000 K, but for lower temperatures the reaction of H+ with C proceeds primarily by radiative charge transfer. Implications, due to the magnitude of the rate coefficients, for various astrophysical environments are discussed.

The Radiative Association of H and D

The formation of the deuterated hydrogen molecule HD by the radiative association of H and D is investigated. Spontaneous association and stimulated association in a blackbody radiation field are considered. For a collision temperature of 1000 K, the rate coefficient for spontaneous radiative association is 1.0 × 10-26 cm3 s-1. At the same collision temperature, the rate coefficient is enhanced by stimulated association to 1.8 × 10-26 cm3 s-1 for a radiation temperature of 10,000 K. The role of radiative association in the deuterium chemistry of the early universe is discussed.

Electron capture in collisions of with H and with C

A comprehensive theoretical and experimental study of electron capture in collisions of with H and with C extending over the energy range to is presented. A variety of theoretical approaches were used including those based on quantal molecular-orbital close-coupling (MOCC), multielectron hidden crossings (MEHC), quantal decay and classical trajectory Monte Carlo techniques. Radiative charge transfer cross sections were computed using the optical potential/distorted wave (OPDW) and fully quantal (FQ) approaches. The MOCC, OPDW and FQ calculations incorporated ab initio potentials, nonadiabatic coupling matrix elements and transition moments computed at the configuration-interaction level. Ab initio potential surfaces in the plane of complex internuclear distance were obtained for the MEHC calculations. Merged-beam measurements were performed between and for the collision system. Diagnostics of the beam with a crossed electron beam could find no presence of a metastable component. The current results, in conjunction with previous measurements, are used to deduce a set of recommended cross sections.

The Photodissociation of SiH+ in Interstellar Clouds and Stellar Atmospheres

Using a combination of both theoretical and experimentally derived potential curves and dipole transition moments, photodissociation cross sections have been calculated for the SiH+ molecule. From the v = 0 level of the X1?+ ground state, with a threshold energy of 3.256 eV, dissociation occurs through the A1? state producing Si+ (3p2P) and H atoms. For energies above 9.45 eV, photodissociation through the 21? state yields excited neutral Si (3p21D) and protons. Excited Si+ (3s3p22D) and H atoms are produced above 10.08 eV by dissociation through 31?+ and 31?. The 21?+ and 41? states are found to be negligible dissociation channels. Photodissociation cross sections as a function of energy and photodissociation rates in the interstellar radiation field for both diffuse and dense molecular clouds are presented. Photodissociation cross sections from LTE distributions of vibrational and rotational levels of the X1?+ state to the A1? state are also obtained for temperatures between 1000 and 9000 K. Their possible contribution to a missing opacity source in the near-ultraviolet spectra of red giant stars is discussed.

Ab initio study of charge transfer in low-energy collisions with helium

Charge transfer cross sections for collisions of ground state and excited state ) with atomic helium are presented for energies less than 250 eV . Using a fully quantum mechanical, molecular-orbital, close-coupling approach, the cross sections are calculated in a diabatic representation. Completely ab initio adiabatic potentials and nonadiabatic radial coupling matrix elements obtained with the spin-coupled valence-bond method are utilized. Rate coefficients for temperatures between 100 and 100 000 K and cross sections for collisions with isotopic are presented. Results for the reverse process, , are also given.

Heavy particle atomic collisions in astrophysics: Beyond H and He targets

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.