TRIPLE-RESONANCE SPECTROSCOPY OF THE HIGHER EXCITED-STATES OF NO2 .3. /DELTA-NU/-GREATER-THAN-1 AUTOIONIZATION AND VIBRONIC COUPLING

Multiresonant, ionization-detected absorption spectra of autoionizing ns and nd Rydberg states converging to the (200) and (300) vibrational states of the NO2+ core show unexpectedly broad resonances for many states nominally requiring DELTA-v = -2 and -3 vibrational transitions in the core to energetically achieve electron ejection. The resonances observed do not show evidence of significant predissociation, implying that the autoionization process is primarily vibrational. Perturbed line shapes and the presence of many resonances not clearly associated with the assigned s-sigma, d-sigma, and d-pi series indicate extensive vibronic coupling with other Rydberg vibrational manifolds. A particularly broad feature at a total energy of 78 720 cm-1 appears in spectra scanned both from the (200) and (300) levels of the 3p-sigma state. This energy falls just below the threshold for forming NO2+ in the vibrational state (100), and the broad features that appear at this point in the higher vibrational manifolds can be identified as components of a complex resonance reflecting a coupling of the (100), (200), and (300) manifolds with the (000) continuum. We suggest that such vibronic coupling accelerates autoionization throughout the spectrum and that analysis in terms of fixed values of DELTA-v is not possible.