Two-photon spectroscopy of autoionizing states of ammonia

Two-photon ionization spectra of jet-cooled NH3 have been obtained in the region between the adiabatic ionization potential and the NH3+ (X) over tilde (2)A(2)(")(0200) ionization threshold. Fairly intense np and nf Rydberg series are observed converging to selected rotational levels of the (X) over tilde (2)A(2)(")(0100) and (0200) states. Rydberg series converging to higher vibrational thresholds and requiring vibrational autoionization by Deltav(i)less than or equal to -2 are also observed. Photoelectron spectroscopy reveals that many of the molecules in the latter series do not autoionize, but instead are ionized by the absorption of an additional photon. The new two-photon spectra are compared with previously reported double resonance spectra in the same energy region, providing insight into questions of the spectroscopic assignments. (C) 2001 American Institute of Physics.