PHOTODISSOCIATION AND PHOTOIONIZATION AND THEIR BRANCHING RATIO IN BISBENZENE CHROMIUM

Bisbenzene chromium (BBC) with an unusually low ionization potential, 5.45 eV is a model molecule to study phenomena near the ionization potential. A beam of BBC molecules cooled by a supersonic expansion was used to measure the absorption spectrum and the excitation spectra for ionization and Cr atom production. The uv spectrum is mainly a Rydberg series terminating at the ionization potential together with progressions in the symmetric benzene-Cr stretch. In the range from 320 to 282 nm, the region of a non-Rydberg absorption band, the excitation spectrum for Cr atom production is quite different from the absorption spectrum. In contrast, in the range 282-227 nm the excitation spectrum for Cr atom production is practically identical with the absorption spectrum. This identity suggests a close to 100% yield of Cr atoms. Near the ionization energy of BBC the yield of Cr atoms decreases while the ionization yield increases until a quite steady branching ratio of three Cr+ ions to one Cr atom is reached. The rates of dissociation of excited BBC into Cr atoms are 0.70 +/- 0.02 X 10(6) s-1 and 4.2 +/- 0.1 X 10(6) S-1 at 279.8 and 248 nm, respectively.