ECAMP 15

Mixtures of noble gases are regularly present in the active media of high-power gas lasers and in a variety of UV and VUV radiation sources, including excimer lamps and microplasma cell arrays. In addition to atoms and atomic ions, the plasmas of these mixtures contain homonuclear and heteronuclear molecular ions of inert gases. An important feature of heteronuclear BA$^{+}$ ions is the presence of the excited states of the charge transfer character, which dissociate to A + B$^{+}$ configuration. In the plasma of rare gas mixtures the radiative transitions between these states and the low-lying electronic states which dissociate to A$^{+}$ + B system often result in the wide intense band in the visible, UV- and VUV-range, depending on the specific properties of electronic terms of BA$^{+}$ ions both in the initial and final electronic states. In recent years, experimental [1] and theoretical [2,3] studies of such phototransitions have intensified noticeably. The respective emission has been observed in spectra of discharge glow in plasma of inert gas mixtures [4,5], as well as in a plasma of rare gas mixtures excited by ionizing pumping [1]. Studies of the radiative processes involving the charge transfer states of rare gas molecular ions are of an interest for applied problems of diagnostics of high-temperature plasma in fusion reactors [1], as well as development of new sources of wide-band radiation in visible, UV and VUV-ranges.
Despite the wide range of experimental studies of the radiative transitions above, the theoretical studies of these processes are relatively sparse. This is due to the fact that most of the inert gas heteronuclear cations BA$^{+}$ are weakly (like HeXe$^{+}$, NeXe$^{+}$, NeAr$^{+}$) or moderately bound (like ArXe$^{+}$, KrXe$^{+}$, ArKr$^{+}$), and the energies of their first vibrational quantum are usually low. That means that for the correct description of the dynamics of radiative and collisional reactions with these ions it is necessary to take into account all states of the rovibrational quasicontinuum even at room temperatures of the gas component of plasmas. As a result, the theoretical studies of such processes are mostly limited to the case of the lightest heteronuclear ion, HeNe$^{+}$ [2], which has dissociation energy of 647 meV and the vibrational quantum of 131 meV.
We carry out the theoretical study of the light absorption and emission processes involving the charge transfer states of the weakly bound (NeXe$^{+}$, NeAr$^{+}$) and moderately bound (ArXe$^{+}$, KrXe$^{+}$) rare gas ions. In order to self-consistently treat the contributions from all states of internuclear motion we use an original semiquantal theoretical approach [3,6] based on the quasicontinuum approximation for the rovibrational states of the molecular ion both in the initial and final channels of processes. The dynamics of the processes are described on the basis of the theory of the non-adiabatic transitions between the effective electronic terms given by a sum of the electronic terms of BA$^{+}$ ion and the photon energy (if photon is present) in the initial and final channels of the reaction. Four different channels of radiative transitions are considered: photodissociation, photoassociation, bound-bound and free-free transitions. Our approach allows one to treat all these processes uniformly and to perform a comparative quantitative analysis of their efficiencies. We have calculated the contribution of all four channels into the absorption and emission spectra in visible, UV and VUV region in the wide range of parameters of plasma of rare gas mixtures.
One of the challenging difficulties in the theoretical study of these processes in weakly bound heteronuclear ions (such as HeXe$^{+}$, NeXe$^{+}$ or NeAr$^{+}$) stems from the fact that potential curves of their final and initial electronic states are almost parallel in the vicinity of equilibrium internuclear distances. Within the frameworks of the standard variants of the theory of non-adiabatic transitions this leads to the presence of the singularities near the maxima of the calculated bands in the emission spectra. Our modified theoretical approach addresses this problem, which allowed us to achieve very good agreement with experimental data on emission spectra of charge transfer photo-processes both for weakly bound (NeXe$^{+}$, NeAr$^{+}$) and moderately bound (ArXe$^{+}$, KrXe$^{+}$) rare gas ions.

References
1. K. Samarkhanov, M. Khasenov, E. Batyrbekov, I. Kenzhina, Y. Sapatayev, and V. Bochkov, Sci. Technol. Nucl. Install. $\mathbf{2020}$, 8891891 (2020).
2. X.J. Liu, Y.Z. Qu, B.J. Xiao, C.H. Liu, Y. Zhou, J.G. Wang, R.J. Buenker, Phys. Rev. A $\mathbf{81}$, 022717 (2010).
3. A.A. Narits, K.S. Kislov, V.S. Lebedev, J. Chem. Phys. $\mathbf{157}$, 204307 (2022).
4. Y. Tanaka, K. Yoshino, D. E. Freeman, J. Chem. Phys. $\mathbf{62}$, 4484, (1975).
5. M. Tsuji, M. Tanaka, Y. Nishimura, Chem. Phys. Lett. $\mathbf{262}$, 349 (1996)
6. A. Narits, K. Kislov, V. Lebedev, Atoms $\mathbf{11}$, 60, (2023).