Vibrational autoionization in Rydberg states of ammonia

Rotationally resolved photoelectron spectroscopy is used to elucidate the m echanism for vibrational autoionization via the nontotally symmetric umbrel la vibration of ammonia (NH3). This mechanism depends strongly on the chara cter of the autoionizing level, and the data demonstrate that both the elec tronic and rotational degrees of freedom can compensate for the change in v ibrational symmetry that occurs for odd changes in the quantum number of th e umbrella vibration. This result has implications for models of the mode d ependence of vibrational autoionization in polyatomic molecules.