Vibronic structure and photodissociation dynamics of the à state of jet-cooled ammonia

Vibrationally mediated photodissociation action spectroscopy provides vibronic spectra of the à state of jet-cooled ammonia by detecting the H-atoms produced by the photodissociation of vibrationally excited molecules. Initial vibrational excitation to selected rotation-inversion levels in the N–H stretching fundamental changes the Franck–Condon factors for the subsequent electronic transition markedly. Analysis of the vibronic structure in the à state reveals a progression in both the umbrella and the bending modes and provides fundamental frequencies for the symmetric and antisymmetric stretching motions. Additional state selectivity in infrared–ultraviolet optical double resonance excitation combined with photofragment detection allows rovibronic analysis of the rapidly predissociating levels in the à state of ammonia. The lifetime for NH3(Ã) excited to four quanta of bending motion is as short as 13±4 fs.