Fermi resonance and mode specificity in the vibrational autoionization of NO2

Ionization-detected absorption spectra of autoionizing Rydberg series converging to the (100), (010), (02(0)0), and (02(2)0) vibrational states of NO2+ have been recorded using three-color triple resonant excitation. Resonances produced by relaxation of the core, excited in one quantum of the bending vibration, (010), are found to be much sharper than those associated with decay by relaxation in symmetric stretch, (100), consistent with earlier findings on mode specificity of vibrational autoionization in this system. In contrast, series excited in the symmetric overtone of the bend, (02(0)0), are found to exhibit dynamics that resemble much more those of series built on one quantum of symmetric stretch, indicating that the bending specificity that characterizes (010) is lost in the overtone. This loss of mode specificity with increasing vibrational energy is ascribed to moderate (100)-(02(0)0) Fermi resonance in the NO2+ core, exemplifying how even small vibrational coupling can affect mode-specific pathways in polyatomic molecules. (C) 1996 American Institute of Physics.