Radiative charge transfer in He+ + H2 collisions in the milli- to nano-electron-volt range: A theoretical study within state-to-state and optical potential approaches
Authors: Mrugała Felicja , Kraemer Wolfgang P. 
Published: March 2013

The paper presents a theoretical study of the low-energy dynamics of the radiative charge transfer (RCT) reaction \documentclass[12pt]{minimal}\begin{document}${\rm He}^{+}(^{2\!}S) + {\rm H}_{2}(X\,^{1\!}\Sigma ^{+}_{g}) \rightarrow {\rm He}(^{1\!}S) + {\rm H}_{2}^{+}(X\,^{2}\Sigma ^{+}_{g})+h\nu$\end{document} He +(2S)+H2(XΣg+1)→ He (1S)+H2+(XΣg+2)+hν extending our previous studies on radiative association of \documentclass[12pt]{minimal}\begin{document}${\rm HeH}_{2}^{+}$\end{document} HeH 2+ [F. Mrugała, V. Špirko, and W. P. Kraemer, J. Chem. Phys. 118, 10547 (2003)10.1063/1.1573184; F. Mrugała and W. P. Kraemer, J. Chem. Phys. 122, 224321 (2005)10.1063/1.1924453]. The calculations account for the vibrational and rotational motions of the H2/\documentclass[12pt]{minimal}\begin{document}${\rm H}_2^+$\end{document}H2+ diatomics and for the atom-diatom complex formation in the reactant and the product channels of the RCT reaction. Continuum states of He+ + H2(v = 0, j = 0) in the collision energy range ∼10−7–18.6 meV and all quasi-bound states of the He+ − H2(para; v = 0) complex formed in this range are taken into account. Close-coupling calculations are performed to determine rates of radiative transitions from these states to the continuum and quasi-bound states of the He + \documentclass[12pt]{minimal}\begin{document}${\rm H}^{+}_{2}$\end{document}H2+ system in the energy range extending up to ∼0.16 eV above the opening of the HeH+ + H arrangement channel. From the detailed state-to-state calculated characteristics global functions of the RCT reaction, such as cross-section σ(E), emission intensity I(ν, T), and rate constant k(T) are derived, and are presented together with their counterparts for the radiative association (RA) reaction He+(2S) + \documentclass[12pt]{minimal}\begin{document}${\rm H}_2(X\,^{1\!}\Sigma ^{+}_{g}) \rightarrow$\end{document}H2(XΣg+1)→ \documentclass[12pt]{minimal}\begin{document}${\rm HeH}_{2}^{+}(X\,^{2\!}A^{\prime })\break + h\nu$\end{document} HeH 2+(XA′2)+hν. The rate constant kRCT is approximately 20 times larger than kRA at the considered temperatures, 0.1 μK–50 K. Formation of rotational Feshbach resonances in the reactant channel plays an important role in both reactions. Transitions mediated by these resonances contribute more than 70% to the respective rates. An extension of the one-dimensional optical potential model is developed to allow inclusion of all three vibrational modes in the atom-diatom system. This three-dimensional optical potential model is used to check to which extent the state-to-state RCT rate constant is influenced by the possibility to access ground state continuum levels well above the opening of the HeH++ H arrangement channel. The results indicate that these transitions contribute about 30% to the “true” rate constant kRCT whereas their impact on the populations of the vibration-rotational states of the product \documentclass[12pt]{minimal}\begin{document}${\rm H}_{2}^{+}$\end{document}H2+ ion is only minor. Present theoretical rate constant functions kRCT(T) obtained at different approximation levels are compared to experimental data: 1–1.1 × 10−14 s−1 cm3 at T = 15–35 K and ∼7.5 × 10−15 s−1 cm3 at 40 K [M. M. Schauer, S. R. Jefferts, S. E. Barlow, and G. H. Dunn, J. Chem. Phys. 91, 4593 (1989)10.1063/1.456748]. The most reliable theoretical values of kRCT, obtained by combining results from the state-to-state and the optical potential calculations, are between 2.5 and 3.5 times larger than these experimental numbers. Possible sources for discrepancies are discussed.