Double-resonance spectroscopy of the high Rydberg states of HCO. II. Mode specificity in the dynamics of vibrational autoionization via CO stretch versus bend

Abstract

We report ionization-detected absorption spectra of vibrationally autoionizing high Rydberg states of formyl radical. Steps of uv–visible double resonance with selected rovibrational levels of the 3pπ 2Π Rydberg state of HCO promote Franck–Condon vertical transitions that isolate series converging to (010) (bend) and (001) (CO stretch) excited states of HCO+. Final state energies in these spectra exceed the threshold for production of the cation ground state. Intensities and linewidths of observed resonances convey information on the dynamics of electron ejection driven by the vibronic relaxation of specific normal modes of the linear triatomic core. Many resonances in (010) and (001) autoionization spectra exhibit widths that approach the resolution of our laser. Other resonances in series converging to both limits are noticeably broadened, with linewidths that display an inverse cubic dependence on principal quantum number. Among these broader resonances, those in series that decay by relaxation of CO stretch exhibit a scaled width that is about a factor of two larger than that of the broadest resonances in series that relax via the bending fundamental. This trend mirrors to some degree one which has been seen in similar experiments on NO2. We consider these dynamics in the light of a multichannel quantum defect theory picture for the autoionization of polyatomic molecules.