DOUBLE-RESONANCE SPECTROSCOPY OF THE HIGH RYDBERG STATES OF HCO - II - MODE SPECIFICITY IN THE DYNAMICS OF VIBRATIONAL AUTOIONIZATION VIA CO STRETCH VERSUS BEND

We report ionization-detected absorption spectra of vibrationally auto ionizing high Rydberg states of formyl radical. Steps of uv-visible do uble resonance with selected rovibrational levels of the 3p pi(2) Pi R ydberg state of HCO promote Franck-Condon vertical transitions that is olate series converging to (010) (bend) and (001) (CO stretch) excited states of HCO+. Final state energies in these spectra exceed the thre shold for production of the cation ground state. Intensities and linew idths of observed resonances convey information on the dynamics of ele ctron ejection driven by the vibronic relaxation of specific normal mo des of the linear triatomic core. Many resonances in (010) and (001) a utoionization spectra exhibit widths that approach the resolution of o ur laser. Other resonances in series converging to both limits are not iceably broadened, with linewidths that display an inverse cubic depen dence on principal quantum number. Among these broader resonances, tho se in series that decay by relaxation of CO stretch exhibit a scaled w idth that is about a factor of two larger than that of the broadest re sonances in series that relax via the bending fundamental. This trend mirrors to some degree one which has been seen in similar experiments on NO;?. We consider these dynamics in the light of a multichannel qua ntum defect theory picture for the autoionization of polyatomic molecu les. (C) 1998 American Institute of Physics.