Zero kinetic energy (ZEKE) photoelectron spectroscopy of ammonia by nonresonant two-photon ionization from the neutral ground state

Rotationally resolved zero kinetic energy-pulsed field ionization (ZEKE-PFI) photoelectron spectra of NH3 were obtained by nonresonant two-photon ionization from the X̃(1A1′) electronic ground state. The ZEKE spectra were recorded up to an internal ion energy of 9500 cm−1 by pulsed field ionization of long-lived Rydberg states of very high principal quantum numbers. Rovibronic bands of the NH3+ cation with v2+=1–9 (umbrella mode), v4+=1, and for the first time the mode ν1 in the combination band v+=1121 of the X̃+(2A2″) electronic ground state are observed in the ZEKE spectra. Rotational constants and band origins are determined in the analysis. The adiabatic ionization energy obtained by a recent ZEKE measurement is confirmed at 82 159±1 cm−1.