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 B̃ (1E″) v2=2 state are used as intermediate resonances. Rotational energy levels of the NH+3 ion in two vibrational states of the X̃+ (2A″2) electronic ground state, v+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 v+2 =1, and B+ =9.77±0.04 cm−1 and C+ =5.39±0.05 cm−1 for v+2 =2. The adiabatic ionization energies are determined as 83 062.5±1 cm−1 for X̃+ v2=1 and 84 002.9±1 cm−1 for X̃+ v2=2, with respect to the lowest (unoccupied) v2=0+, J″=0, K″=0, Γevr=A1 rotational state of the X̃ (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.