Triple-resonance spectroscopy of the higher excited states of NO2. IV. Trends in the mode dependence of vibrational autoionization via asymmetric stretch versus symmetric stretch and bend

Abstract

Multiresonant stepwise excitation of NO2 isolates series of individual rovibronic states converging to vibrationally excited levels of NO+2. Resonances detected by third-photon photoionization of two-photon photoselected intermediate states show characteristic patterns of intensity and linewidth that convey information on the dynamics of vibrational autoionization for relaxation in specific normal modes of the linear NO+2 core. Earlier studies characterized vibrational-to-Rydberg-electronic energy transfer from symmetric stretching (100) and bending (010) excited states [J. Chem. Phys. 93, 2308, 7731 (1990)]. In the present work, a definitive assignment of double-resonant rotational structure confirms two-color selection of 3pσ 2∑+u gateway states that are core excited by one quantum of asymmetric stretch. Ionization-detected optical-absorption scans from the double-resonantly selected N′=2 level of the 3pσ (001) state yield spectra of s, d, and g Rydberg series of vibrationally autoionizing resonances converging to the (001) ionization threshold. Transitions assigned to s and d series range in principal quantum number from 7 to more than 30. As with the spectrum of states built on the (100) core, transitions assigned to g states form a series of sharp resonances that disappears at relatively low n. Regions of the (001) autoionizing spectrum are perturbed by discrete–discrete interactions with underlying series converging to lower thresholds, and comparatively low intensities are found for whole series of states with higher Rydberg orbital angular momentum. Nevertheless, resonant linewidths for (001) can be compared with (100) and (010) for the most penetrating series (ns), and a clear trend manifests itself over complete autoionizing intervals for all three vibrational states of the core. Symmetric stretching states, with the largest dynamic volume change over a cycle of vibration, autoionize fastest. Asymmetric stretch, with a smaller volume change, but comparable N–O bond displacements, is coupled less strongly. Bending, which supports a large dynamic dipole, but little change in the volume of the core charge distribution, displays the slowest relaxation to the NO+2 (000) continuum. These trends in vibrational state dependence are in qualitative accord with a simple long-range picture that relates the strength of Rydberg-continuum coupling, induced by vibrational motion in the core, to the size of the associated dynamic monopole.