Measurements of radiative charge-transfer reactions of doubly and singly charged rare-gas ions with rare-gas atoms at thermal energies

Radiative charge-transfer coefficients for single-electron transfer between doubly or singly charged noble-gas ions and noble-gas atoms have been determined at thermal energy using a drift-tube mass-spectrometer apparatus. At 300 K, $k({\mathrm{He}}^{2+}+\mathrm{He})=(4.8\mathrm{}\pm 0.5)\times {10}^{-14\mathrm{}}$ ${\mathrm{cm}}^3$/sec, with slightly smaller coefficients for the reactions ${\mathrm{Ne}}^{2+}$ + Ne, ${\mathrm{Ar}}^{2+}$ + Ar, ${\mathrm{Kr}}^{2+}$ + Kr, and ${\mathrm{Xe}}^{2+}$ + Xe. Differences in reactivity are found for the different, low-lying ${}_{}{}^1{}_{}{}^{}S$, ${}_{}{}^1{}_{}{}^{}D$, and ${}_{}{}^3{}_{}{}^{}P$ states of the doubly charged ions. A three-body contribution to the radiative process is noted, with rate coefficients ranging between ${10}^{-31\mathrm{}}$ and ${10}^{-30\mathrm{}}$ ${\mathrm{cm}}^6$/sec at 300 K for the different noble-gas systems. The radiative charge-transfer coefficients for singly ionized ions are somewhat smaller, $k({\mathrm{He}}^{+}+\mathrm{Ne})\sim 1\times {10}^{-15\mathrm{}}$ ${\mathrm{cm}}^3$/sec and $k({\mathrm{Ne}}^{+}+\mathrm{Ar})\sim 6\times {10}^{-15\mathrm{}}$ ${\mathrm{cm}}^3$/sec at 300 K. The measured value of $k({\mathrm{He}}^{2+}+\mathrm{He})$ is in reasonable agreement with the very recent theoretical calculations of Cohen and Bardsley.