next previous
Up: Charge transfer in collisions O+ H+


1 Introduction

Electron capture by oxygen ions due to collisions with neutral hydrogen,

 \begin{displaymath}{\rm O}^+(^4{\rm S}) + {\rm H} \to {\rm O}(^3{\rm P_J}) + {\rm H}^+,
\end{displaymath} (1)

and the reverse process,

 \begin{displaymath}{\rm H}^+ + {\rm O}(^3{\rm P_J}) \to {\rm H} + {\rm O}^+(^4{\rm S}),
\end{displaymath} (2)

are known to play important roles in a variety of astrophysical, atmospheric, and laboratory plasmas because of the relatively large abundances of the collision partners and the accidental near degeneracy of the hydrogen and oxygen ionization potentials. As processes (1) and (2) are quasi-resonant, their cross sections are large, from the thermal to intermediate energy regimes. Both reactions have received considerable experimental and theoretical attention, but all the previous studies have determined the cross sections for collision energies greater than $\sim 1$ eV/u except for the quantal molecular-orbital close-coupling (MOCC) calculation of Chambaud et al. ([1980]), which covered the energy range $\sim 0.02$ to 0.45 eV/u and the semiclassical calculations of Rapp & Ortenburger ([1960]). For process (1), the lowest experimental point is at 1.6 eV/u (Fite et al. 1962). Rutherford & Vroom ([1974]) obtained the lowest experimental energy of 2 eV/u for reaction (2).

For most astrophysical applications, the important quantity is the rate coefficient. Rate coefficients for reactions (1) and (2) have been calculated by Field & Steigman ([1971]) for T = 10 - 10000 K and by Chambaud et al. ([1980]) for T = 10 - 1000 K. Kimura et al. ([1997]) computed rate coefficients for reaction (2) for T = 10000 - 200000 K. The results of Chambaud et al. ([1980]), which are confirmed by the drift-tube measurements of reaction (2) by Federer et al. ([1984]) for $T \sim 1000$ K, are in significant disagreement with Field & Steigman ([1971]).

In an attempt to resolve these discrepancies, we have combined new theoretical calculations, using four different approaches applicable in different energy regimes, with previous experimental and theoretical results, to deduce accurate charge transfer cross sections and rate coefficients for processes (1) and (2). Brief descriptions of the scattering theories are given in Sect. 2. New results and comparison with previous data are presented in Sect. 3 with the cross sections given over the very large energy range, 0.1 meV/u - 10 MeV/u, the higher energies being of relevance to ion precipitation into the Jovian atmosphere. This and other applications of the results to astrophysical and atmospheric environments are discussed in Sect. 4.


next previous
Up: Charge transfer in collisions O+ H+

Copyright The European Southern Observatory (ESO)