Theoretical study of the ArH+ photodissociation

The multireference Spin - Orbit ( SO) Configuration Interaction (CI) method in its Lambda - S Contracted SO - CI (LSC-SO-CI) version is employed to calculate potential energy curves for the ground and low-lying excited states of the ArH+ cation. For the first time, electric dipole moments are also computed in the approach, including SO coupling for transitions to the states responsible for the first absorption continuum (A-band) of ArH+. On this basis, the partial and total absorption spectra in this energy range are obtained. It is shown that absorption in the A-band is dominated by the parallel A(1)Sigma(+) --> X-1 Sigma(+) transition. In the low-energy part of the band (<95 x 10(3) cm(-1)) the absorption is caused by the perpendicular B-1 Pi <-- X-1 Sigma(+) excitation, but transitions to the b(3)Pi(0+,1) states are also not negligible. The branching ratio Gamma for the final photodissociation products is calculated and it is shown to increase smoothly from 0 in the red tail of the band to 1 at E >= 10(5) cm(-1). The latter value corresponds to the exclusive formation of the spin-excited Ar+(P-2(1/2)) ions, and thus leads to the inverse population of the Ar+(P-2(1/2) - P-2(3/2)) ion states.