Molecular-orbital treatment of electron capture in an external magnetic field: collisions of ions with H atoms

Abstract

A semiclassical close-coupling equation is formulated for the electron-capture process in ion - atom collisions under the influence of an external magnetic field. The direction of the magnetic field is chosen to be parallel to an initial velocity vector of a projectile. As we deal with the case for the magnetic field strength less than the critical field strength , the second- and higher-order terms for the magnetic field are neglected. Eigenfunctions in zero magnetic field are employed as a basis set for expansion. The equation obtained is the same in structure as that at zero magnetic field except that the relative velocity v is replaced by the scaled velocity . Effects due to the magnetic field are: (i) dynamical couplings, similar to rotational couplings and (ii) the Zeeman splitting of energies. The phase factor due to the gauge transformation is similar in form to the electron-translation factor. We apply this close-coupling equation to a study of electron-capture processes in a singlet system at , for which the zero-field results have been reported earlier (Shimakura et al 1993). Calculated results show that the total electron-capture cross section increases by about 30% at the magnetic field T compared to that at zero magnetic field at collision energy E = 122.5 eV/amu. At low collision energies below 500 eV/amu, the effect of the magnetic field becomes dominant over that of rotational couplings at . Because the magnetic field removes the degeneracy of electronic energies for (and ) states by the Zeeman effect, transition probabilities for and (and also and ) states differ significantly. The transition probabilities oscillate with time even if the collision pair separates considerably, particularly for and states because of the magnetic field.