VIBRONIC STRUCTURE AND PHOTODISSOCIATION DYNAMICS OF THE $\tilde{A}$ STATE OF JET-COOLED AMMONIA

Abstract

Vibrationally mediated photodissociation action spectroscopy of jet-cooled ammonia provides information on the vibronic structure and photodissociaiton dynamics in the $\tilde{A}$ state. 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 and allows assignment of sharp resonances to a progression in the degenerate bending mode $(\nu'{4})$ including umbrella-bending combination states for the first time. We also identify broad, non-Lorentzian shaped resonances as transitions to the symmetric and antisymmetric stretching fundamentals. Our proposed assignment reveals an accidental degeneracy of the umbrella and bending modes with fundamental frequencies of $892 \pm 8 cm^{-1}$ and $906 \pm 30 cm^{-1}$, respectively. The anharmonicities for the bend $x{44} = 9 \pm 6$ and $g_{44}=16 \pm 7 cm^{-1}$ are slightly larger than for the umbrella progression and the near degeneracy leads to a large off diagonal anharmonicity of $x_{24} = 56 \pm 13 cm^{-1}$. Additional state selectivity in infrared-ultraviolet optical double resonance excitation combined with photofragment detection allows rovibronic analysis of the rapidly predissociating levels in the $\tilde{A}$ state of ammonia. The lifetime for $NH_{3} (\tilde{A})$ excited to four quanta of bending motion is as short as $13\pm 4$ fs.