Photoelectron spectroscopy of ammonia: Mode-dependent vibrational autoionization

Photoelectron spectroscopy was used to study the mode dependence of vibrati onal autoionization in high-Rydberg states of NH3. Two-color, two-photon re sonant, three-photon excitation via the C' (1)A(1)'(1300) intermediate stat e was used to populate selected autoionizing Rydberg states between the (12 00) and (1300) ionization thresholds of the NH3+ (X) over tilde (2)A(2)" gr ound electronic state, and the ionic vibrational distributions were determi ned from the photoelectron spectra. Excitation of Rydberg states in which t wo different vibrational modes are excited allowed the direct comparison of the autoionization efficiencies for the two modes. Autoionization via the loss of one quantum of vibrational energy from the nontotally symmetric "um brella" mode, nu (2), was found to be the dominant autoionization process. Vibrational branching fractions obtained from the ionic vibrational distrib utions indicate that, for the Rydberg states accessed via the C-' (1)A(1)'( 1300) intermediate state, the nu (2) mode is approximately 25 times more ef ficient at promoting autoionization than the totally symmetric stretching m ode, nu (1). (C) 2000 American Institute of Physics. [S0021-9606(00)01241-1 ].