Dynamical behavior of free electrons in the recombination process in liquid argon, krypton, and xenon

The time dependence of the luminescence due to the recombination process of electrons and ions in liquid argon, krypton, and xenon excited by ²⁰⁷Bi internal-conversion electrons has been studied by considering the difference curve of two decay curves, the one in the absence of electric field and the one with such a high electric field that all of the observed decay characteristics can be attributed to self-trapped exciton luminescence. The time dependence is well explained by using the kinetic equation for electrons and the recombination process, without diffusion. This provides good evidence for the existence of a strong Coulomb interaction of thermalized electrons and ions. Two excited molecular states ¹Σᵤ⁺ and ³Σᵤ⁺ are formed through the recombination process, as in the case of self-trapped exciton luminescence. The recombination cross sections, in units of 10^-16 cm², are 7000±{}2000 for liquid argon, 120±{}30 for liquid krypton, and 10±{}2 for liquid xenon. The considerably small values for liquid krypton and xenon compared with that for liquid argon are explained by the unfavorable crossing of the R₂⁺ and repulsive R₂** potential curves in the liquid, based on the theory of dissociative recombination R₂⁺+eR₂**→R**+R in the gaseous state.