Light absorption and emission by weakly-bound heteronuclear molecular ions in the superlinear crossing transition regime. The example of NeXe+

We develop a semi-analytic theory for describing nonadiabatic bound-bound, free-bound, bound-free, and free-free photoprocesses in heteronuclear ions in the regime of the superlinear potential energy curve crossing. It extends previous semiclassical method for calculating the absorption and emission spectra of strongly and moderately bound diatomic species based on linear curve crossing model, and can be used for molecules and ions with small dissociation energies, D0{less than or equal to}kBT. The use of quasicontinuum approximation for rovibrational levels allows us to give a unified description of the integral contributions of the discrete and continuous spectra of the molecular species with linear and superlinear crossings to the effective cross sections and rate coefficients of the radiative processes in the systems studied. Specific calculations were performed for excimer-like NeXe+ ion (De=37.3 meV). Potential energy curves and dipole transition matrix elements are evaluated using ab initio multi-reference calculations with perturbative description of relativistic effects. In contrast to ArXe+ and KrXe+ ions studied previously, main contributions to the absorption spectra of NeXe+ are due to bound -- bound transitions and photoassociation. The emission spectra at room temperatures are determined predominantly by the bound-bound transitions, while at temperatures above 450 K the most significant contribution to the radiation is made by bound -- free phototransitions. Our calculations are in good agreement with the available experimental data. The results obtained are of interest for chemical physics, spectroscopy of weakly bound molecular systems, physics of radiative processes in gases and plasmas, as well as for kinetics of active media of excilamps and gas lasers based on noble gas mixtures.