ECAMP 15

The plasmas of noble gas mixture, routinely used as the active media of the powerful gas lasers, as well as of the sources of UV- and VUV-range radiation, contains, apart from the electrons, atomic ions and neutral atoms, also a fraction of homonuclear and heteronuclear molecular ions. Such heteronuclear ions have moderate to small dissociation energies in the range from 13.1 to 647 meV (for HeXe$^{+}$ and HeNe$^{+}$, respectively). As the IR and UV radiation sources above typically operate at room and elevated gas temperatures and the binding energies of the ions are small, the collisional and radiative processes involving the ions proceed often with the participation of the entire rovibrational spectrum, as well as the continuum of the internuclear motion. The heteronuclear BA$^{+}$ ions feature the excited electronic charge transfer states which are described by AB$^{+}$ configuration and correlate in the dissociation limit with A + B$^{+}$ collision system. It is well-known [1] that the transitions between the states with charge transfer character and the low-lying electronic terms of the heteronuclear rare gas ions result in the intense bands of the radiation in the UV, visible or VUV ranges, depending on the specific ion considered. In plasmas, similar transitions can be induced by the resonant collisions of the ions with the free electrons, BA$^{+} + e\rightarrow $ AB $\rightarrow$ AB$^{+} + e$, where the initial and the final states of the internuclear motion of the ions may be either bound or free. Such reactions have been studied in the literature for the heteronuclear ions of the astrophysical importance, like HeH$+$, LiH$^{+}$, CH$^{+}$, and others. For the plasmas of rare gas mixtures of the role of such processes, on the contrary, is yet to be established.
We study the collisions of heteronuclear molecular and quasimolecular ions with electrons which are accompanied by the charge transfer excitations. Depending on the whether the ion in the initial or the final channels is in a discrete or a continuum state, we distinguish the processes of the electron-impact excitation (bound – bound), dissociative excitation (bound – free), electron-impact association (free – bound) or charge transfer induced by electron impact (free – free). The processes are described on a unified ground using an original semiquantal theoretical approach [2,3] based on the theory of the nonadiabatic transitions in B + A$^{+} + e$ system and the quasicontinuum approximation for the rovibrational states of the molecular ion. The cross sections and rate constants of the processes above were calculated for ArXe$^{+}$, KrXe$^{+}$, NeXe$^{+}$, NeAr$^{+}$, NeKr$^{+}$, and NeAr$^{+}$ ions which have rather different binding energies, under conditions typical of the active media of the plasma-based radiation sources and dielectric-barrier discharges. We show that the processes studied may have high efficiencies at the incident electron energies of a few electron-volts. The rate constants and cross-sections often exceed those of the processes commonly considered in the kinetic models of the rare gas mixture plasmas. The roles of the specific channels of the electron-impact change transfer excitation reaction strongly depend on the dissociation energy of a molecular ion, on the structure of its potential energy curves of the initial and final electronic states, and on the gas and electron temperatures of the plasma. We highlight the features of the reactions studies resulting from the small binding energy of the heteronuclear ions considered. The results obtained clearly indicate that the resonant charge transfer excitation processes in the collisions of heteronuclear molecular cations with electrons should be included in the kinetic models of the radiation sources based on the plasmas of noble gas mixtures.

References
1. Y. Tanaka, K. Yoshino, D. E. Freeman, J. Chem. Phys. $\mathbf{62}$, 4484, (1975).
2. A. Narits, K. Kislov, V. Lebedev, Atoms $\mathbf{11}$, 60, (2023).
3. A. Narits, K. Kislov, V. Lebedev, Atoms $\mathbf{12}$, 67, (2024).