THE PHOTOIONIZATION OF AMMONIA REVISITED - THE VIBRATIONAL AUTOIONIZATION OF NH3 AND ITS 3 ISOTOPOMERS IN THE 10-12 EV PHOTON ENERGY-RANGE

The photoionization efficiency curves of NH3 and its three isotopomers have been investigated in the photon energy range of the first ionized state. From the analysis of the corresponding vibrational structure, wavenumbers (omega-e) and anharmonicity constants (omega-e-chi-e) are deduced. The detailed investigation of the abundant autoionization structure tends to show the adiabatic ionization energy to be 10.072 +/- 0.010 eV for NH3, NH2D and NHD2 and 10.083 +/- 0.010 eV for ND3. All autoionization features were classified in vibrational progressions (v2 bending mode) belonging to nsa1 (or nd) and npe (n = 5, 6, 7) Rydberg series. Vibrational autoionization occurs through DELTA-upsilon transitions up to -9. A qualitative analysis of the intensity distribution of these series strongly supports that transitions involving odd DELTA-upsilon values are favoured. This observation can be understood by applying group theoretical considerations to the theory of vibrational autoionization.