Dissociation lifetimes and level mixing in overtone-excited HN3(X̃ 1A′)

Abstract

Vibrational overtone photodissociation is used to examine the spectroscopy and vibrational predissociation lifetimes of HN₃ in its ground electronic state. Direct overtone pumping of the N–H stretching levels 5ν_NH and 6ν_NH prepares molecules in selected states (v,J,K) near 15 100 and 17 700 cm⁻¹ of vibrational energy; spin‐forbidden NH(X ³ Σ⁻) dissociation fragments are detected by laser‐induced fluorescence. Photodissociation spectra of beam‐cooled HN₃ display mixing of individual rotational levels of the nν_NH vibrations with several background states, with derived coupling matrix elements in the range 0.01–0.1 cm⁻¹. Vibrational predissociation lifetimes of mixed components of 5ν_NH are state specific, with variations of a factor of 2 for only 0.1 cm⁻¹ energy differences. Average lifetimes for low J, K are 210 ns for 5ν_NH and 0.95 ns for 6ν_NH. The ratio of decay rates for the two overtone levels, k(6ν_NH)/k(5ν_NH)=220, is much greater than predicted by statistical theory, which gives a ratio of 4.