Photoselection and the Appearance of Franck−Condon-Forbidden Thresholds in the ZEKE Spectrum of NO2

Photoionizing transitions that change NO 2 + −core vibrational quantum numbers are examined by means of zero-kinetic-energy (ZEKE) threshold photoelectron spectroscopy. Observable intensity is found in Franck−Condon-off-diagonal transitions from the (000) vibrational ground state of the 3pσ 2 Σ u + Rydberg state of NO 2 to long-lived resonances at thresholds corresponding to (010), (02 0 0), and (100) vibrationally excited states of NO 2 + . Analyses of the rovibrational structure of the originating Rydberg state and the cation establish that NO 2 + −core potential energy surfaces are in each case substantially parallel and thus predict zero Franck−Condon factors for transitions that change vibrational quantum numbers. The appearance of nonvertical transitions is attributed to the mixing of long-lived high-principal-quantum-number discrete Rydberg states that converge to each excited threshold with the optically accessible ground-state free-electron continuum. The vibrationally inelastic interaction that gives rise to this Franck−Condon-forbidden threshold structure mirrors the common one in which discrete Rydberg states converging to a vibrationally excited threshold decay by vibrational autoionization to an underlying relaxed-core continuum. In the present case, continuum states reached in vertical excitation form long-lived resonances by borrowing lifetime from discrete high- n states converging to vibrationally excited thresholds. Apparently, the discrete character conveyed in each case is sufficient to permit evolution in the Stark manifold that stabilizes ZEKE states for detection by pulsed-field ionization. This is the first example of a continuum stabilized by the ZEKE Stark-mixing mechanism. The extent of the effect observed at each threshold is consistent with patterns of electron−core coupling strengths established by mode-dependent trends in vibrational autoionization linewidths.