Single ionization of an asymmetric diatomic system by relativistic charged projectiles

Abstract

We study single ionization of a heteroatomic system by charged projectiles whose velocity $v$ approaches the speed of light $c$. The system is formed by two loosely bound atomic species, $A$ and $B$, with the ionization potential of $A$ being smaller than excitation energy for a dipole-allowed transition in $B$. In such a case, three single ionization channels occur: (i) single-center ionization of atom $A$, (ii) single-center ionization of atom $B$, and (iii) two-center ionization of $A$. While (i) and (ii) are the well known mechanism of direct impact ionization of a single atom, in channel (iii) ionization of $A$ proceeds via impact excitation of $B$ with consequent radiationless transfer of excitation energy---via (long-range) two-center electron-electron correlations---to $A$, leading to its ionization. We show that, close to the resonance energy, the two-center channel (iii) is so enormously strong that its contribution remains dominant even if the range of emission energies $\ensuremath{\sim}1$ eV, which is orders of magnitude broader than its width, is considered. The influence of relativistic effects, caused by a high collision velocity, on the angular distribution of emitted electrons may be quite strong even at $\ensuremath{\gamma}=1/\sqrt{1\ensuremath{-}{v}^{2}/{c}^{2}}\ensuremath{\approx}2$. However, in the energy distribution and the total cross section, these effects become substantial only at $\ensuremath{\gamma}\ensuremath{\gg}1$. Relativistic effects arising due to a large size of the two-atomic system are shown to be very weak even for a $^{7}\mathrm{Li}\text{\ensuremath{-}}\mathrm{He}$ dimer whose mean size is about 28 \AA{}.