15min:

A. BACH, J. M. HUTCHISON, R. J. HOLIDAY AND F. F. CRIM, Department of Chemistry,University of Wisconsin-Madison, Madison, WI 53706.

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Vibrationally mediated photodissociation action spectroscopy of jet-cooled ammonia provides information on the vibronic structure and photodissociaiton dynamics in the 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 ( '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\pm8 cm^-1 and 906\pm30 cm^-1, respectively. The anharmonicities for the bend x₄₄=9\pm6 and g₄₄=16\pm7 cm^-1 are slightly larger than for the umbrella progression and the near degeneracy leads to a large off diagonal anharmonicity of x₂₄=56\pm13 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 state of ammonia. The lifetime for NH₃ ( ) excited to four quanta of bending motion is as short as 13\pm4 fs.