Infrared spectroscopy of vibrationally excited HONO2: Shedding light on the dark states of intramolecular vibrational energy redistribution

Abstract

Infrared predissociation spectroscopy of nitric acid subsequent to vibrational overtone excitation reveals vibrational state mixing of the highly excited levels and probes the character of the coupled dark states. A visible dye laser excites the 4←0 or 5←0 OH stretch transition and an optical parametric oscillator promotes the molecules above the dissociation threshold by a ΔvOH=1 transition from the excited level. Scanning the optical parametric oscillator frequency while monitoring the predissociation products via laser-induced florescence produces an infrared spectrum of the excited molecules. Although the 4νOH vibrational overtone band consists of a single clean rotational contour that falls directly on a Birge–Sponer plot, the infrared transitions from this level indicate that the zeroth-order bright state is extensively mixed. On the average, the zeroth-order bright state is only a minor component of the eigenstates at this energy. The largest collective contribution is from zeroth-order states that have zero quanta of OH stretch.