Competition between autoionization and predissociation in the HCl and DCl molecules

Abstract

Experimental photodissociation cross sections for HCl and DCl have been recently measured in the autoionization energy region. These cross sections are obtained from the fluorescence excitation spectra of the fragments in the visible [White et al., J. Chem. Phys. 8 7, 6553 (1987)] or VUV [Frohlich et al. (to be published)] energy region. The peaks observed between 770–900 Å in the photodissociation cross sections do not appear in the same place as that observed in the photoionization cross sections. A theoretical explanation of this fact is proposed. The Rydberg states with a A ² Σ⁺ core are predissociated due to the spin–orbit interaction, by Rydberg states belonging to series converging to the repulsive a ⁴ Π state of HCl⁺ . Using an unified MQDT treatment of autoionization and predissociation processes, the photoionization and photodissociation cross sections are calculated and compared with the experimental results. It is concluded that the states which are predissociated do not have the same symmetry as the states which are autoionized by the continuum of the X ² Π state. The photodissociation cross section in HCl represents only a few percent of the photoionization cross section. It is predicted that for the heavier molecule HBr, the photodissociation cross section would be more than 10% of the photoionization cross section.