Rydberg states near the ionization continuum: Autoionization in ammonia

Previously undetected autoionizing Rydberg levels of ammonia which exist in the energy region between the first ionization continuum and the first excited vibrational level of the NH+3 ion have been measured. Jet cooled ammonia molecules in high lying Rydberg levels were detected by a two-color, three-photon excitation resonantly enhanced multiphoton ionization process, 2+1′. The Rydberg series can be assigned to a continuation of the (a2″)(nsa1′) series to which the Ã, D̃′, and Ẽ″ states belong. A vibronic autoionization mechanism based on the electrostatic and linear terms [V0 and (∂V/∂Q)Q, respectively] in the potential energy expansion has been suggested.