Radiative charge transfer in He(+) + H2 collisions in the milli- to nano-electron-volt range: a theoretical study...

The paper presents a theoretical study of the low-energy dynamics of the radiative charge transfer (RCT) reaction He(+)((2)S)+H2(X(1)Σg (+))→He((1)S)+H2 (+)(X(2)Σg (+))+hν extending our previous studies on radiative association of HeH2 (+) [F. Mrugała, V. Špirko, and W. P. Kraemer, J. Chem. Phys. 118, 10547 (2003); F. Mrugała and W. P. Kraemer, ibid. 122, 224321 (2005)]. The calculations account for the vibrational and rotational motions of the H2/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 + 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(+)((2)S) + H2(X(1)Σg (+))→ HeH2 (+)(X(2)A('))+hν. The rate constant k(RCT) is approximately 20 times larger than k(RA) 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 k(RCT) whereas their impact on the populations of the vibration-rotational states of the product H2 (+) ion is only minor. Present theoretical rate constant functions k(RCT)(T) obtained at different approximation levels are compared to experimental data: 1-1.1 × 10(-14) s(-1) cm(3) at T = 15-35 K and ∼7.5 × 10(-15) s(-1) cm(3) at 40 K [M. M. Schauer, S. R. Jefferts, S. E. Barlow, and G. H. Dunn, J. Chem. Phys. 91, 4593 (1989)]. The most reliable theoretical values of k(RCT), 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.