The electronic structure and photodissociation of HCl

Continuous absorption is possible from the ground state of HCl to the repulsive states that also arise from the ground state asymptote. Under conditions where vibrational excitation of the HCl is possible, continuum absorption can occur from an onset near 345 to 100 nm in the ultraviolet. Since the two states X 1Σ+ and A 1Π, that are dipole coupled, correlate to the same asymptote, the transition moment varies rapidly with the internuclear distance. Using all-electron ab initio calculations based on multiconfiguration self-consistent field (MC-SCF) and first-order configuration interaction (FOCI), the energy curves and wave functions have been obtained for the X 1Σ+, a 3Π,A 1Π, b 3Σ+ states, and the first excited states of each symmetry. The electronic structure of the states are analyzed and the energy curves are compared favorably to experiment. Using the calculated A–X transition moment, the A–X absorption cross section has been obtained as a function of the ground state vibrational level. The possible impact of this process on the operation of the XeCl UV laser which uses HCl as a fuel would not be significant since the cross section at 310 nm never exceeds a few times 10−19 cm2.