Triple-resonance spectroscopy of the higher excited states of NO2 : Rovibronic interactions, autoionization, and script-uncoupling in the (100) manifold

Ionization-detected absorption spectra of the (100) vibrationally autoionizing states of NO₂ have been recorded from double-resonantly prepared N'=1 and N'=7 rotational levels of the 3pσ ²Σ⁺_u (100) Rydberg intermediate state. Photoselection associated with three-color triple-resonant absorption spectroscopy resolves single rotational lines in discrete electronic states that lie above the adiabatic ionization threshold at total energies as high as 78 800 cm^-1. Most features observed can be assigned to sσ, dσ, and dπ series converging to the (100) vertical threshold. Identified transitions extend over an interval of principal quantum numbers ranging from 9 to more than 40. A fourth short progression of sharp transitions is assigned as a segment of a g series. The complete spectrum of (100) transitions is modulated in intensity by a sequence of dips that form a series of states, recognized as pσ and pπ, converging to the (110) threshold. Evidence is cited for a mirror-image effect in autoionizing rates, where less-dissociative symmetric-stretch excited states autoionize much more efficiently than above-threshold bending excited states. The spectra span a range from the Hund's case-b limit to that of case-d coupling. Though extensively obscured by broadening and shifting due to coupling with the ionization continuum and the discrete states of the (110) manifold, resonances in the (100) series show rotational structure consistent with the spectrum of accessible transitions and the variation expected in rotational spacing as Rydberg angular momentum systematically uncouples from the molecular axis. Autoionizing resonances differ significantly in width and line shape, evidencing a systematic variation in the strength with which discrete states are coupled with underlying continua. Fano line shapes indicate a component of direct, virbrationally off-diagonal photoexcitation to the (000) continuum, which interferes with discrete state absorption.