HIGH-RESOLUTION ZERO KINETIC-ENERGY ELECTRON-SPECTROSCOPY OF AMMONIA

Rotationally resolved "zero kinetic energy (ZEKE)" electron spectra of ammonia obtained in a 2 + 1 photon two-color pump-probe experiment are reported. The rovibronic states with J' = 3, K' = 1 (ortho-NH3) and J' = 3, K' = 2 (para-NH3) in the B approximately (1E") v2 = 2 state are used as intermediate resonances. Rotational energy levels of the NH3+ ion in two vibrational states of the X approximately + (2A"2) electronic ground state, upsilon-2+ = 1 and 2, are observed in the ZEKE spectra. The rotational constants are: B+ = 10.19 +/- 0.03 cm-1 and C+ = 5.30 +/- 0.04 cm-1 for upsilon-2+ = 1, and B + = 9.77 +/- 0.04 cm-1 and C+ = 5.39 0.05 cm-1 for upsilon-2+ = 2. The adiabatic ionization energies are determined as 83 062.5 +/- 1 cm-1 for X approximately + upsilon-2 = 1 and 84 002.9 +/- 1 cm-1 for X approximately + upsilon-2, with respect to the lowest (unoccupied) upsilon-2 = 0+, J" = 0, K" = 0, GAMMA-ever = A1' rotational state of the X approximately (1A'1) electronic ground state of NH3. The observed propensity rules for the change in K quantum number ion <-- neutral in the rotational ZEKE transitions are explained from "near symmetry" conservation rules in electronically allowed transitions.