TRIPLE-RESONANCE SPECTROSCOPY OF THE HIGHER EXCITED-STATES OF NO2 .2. VIBRATIONAL-MODE SELECTIVITY IN THE COMPETITION BETWEEN PREDISSOCIATION AND AUTOIONIZATION

Three-color, triple-resonance ionization-detected absorption spectra of the autoionizing (010) s and d states of NO2 are presented and compared with similar spectra of the (100) and (110) manifolds. The (010) states are well described by Hund's case (d), and exhibit evidence of vibronic coupling with other vibrational manifolds. The laser limited linewidths of the (010) resonances show autoionization by υ2 to be slower than autoionization via υ1 by a factor of at least 30. The mode dependence of the autoionization rate can be explained by a model established some time ago for diatomics that connects the Rydberg-electron-cation-core coupling dynamics with the time-dependent core charge distribution associated with each vibrationally excited state. Simple calculations, which focus on the monopolar components of the core-vibrational dynamic multipole potentials, agree qualitatively with the experimental results. A large signal increase at the (010) vertical threshold evidences significant decay through predissociation. Comparison with the (100) and (110) vertical thresholds suggests that, in addition to coupling less effectively to the ionization continuum, excitation in υ2 promotes dissociation. © 1990 American Institute of Physics.