Vibrational spectroscopy and photodissociation of jet-cooled ammonia

Vibrationally mediated photodissociation action spectroscopy provides rotation-vibration spectra of jet-cooled ammonia in the 2.3 mum and 3.0 mum regions by detecting the emission of electronically excited NH2((A) over tilde (2)A(1)) produced by the photodissociation of the vibrationally excited molecules. Vibrational excitation changes the relative photofragmentation yield of NH2((A) over tilde (2)A(1)) markedly. Isoenergetic photolysis of ammonia molecules with one quantum of antisymmetric N-H stretching excitation (nu(3)) or two quanta of bend (2nu(4)) yields three times more excited state NH2((A) over tilde (2)A(1)) than photolysis of NH3 with a quantum of symmetric N-H stretch excitation (nu(1)). By contrast, the relative yield is insensitive to initial vibrational excitation of the combination bands nu(1)+nu(2) and nu(2)+nu(3) that contain the umbrella (inversion) motion nu(2). The vibrational mode dependence of the NH2((A) over tilde (2)A(1)) photofragment yield arises from either enhanced Franck-Condon factors for electronic excitation or from an increased probability for the competing nonadiabatic dissociation to form the ground state NH2((X) over tilde B-2(1)) product. (C) 2002 American Institute of Physics.