The spectroscopy of high Rydberg states of ammonia

This paper extends our knowledge of the higher excited states of the ammonia molecule by presenting detailed measurements of the 2 + 1 resonance enhanced multiphoton ionization (REMPI) spectrum of both NH3 and ND3 obtained following excitation in the wavelength range 298-242 nm, i.e., at energies up to the first ionization energy. Complementary analyses of the wavelength resolved REMPI spectrum and the accompanying REMPI-photoelectron spectra leads to the identification of ten new Rydberg origins of NH3 (four for ND3) with principal quantum numbers n8 and, in most cases, of the accompanying out-of-plane bending vibrational progression. Symmetry assignments for the various newly identified excited states are offered, based on band contour simulation and/or quantum defect considerations. Dominant amongst these are the 1A (5sa1a) state: 0 = 74 118(2) cm - 1 [NH3], 0 = 74 258(2) cm - 1 [ND3], the 1E (5pe1a) state: 0 = 76 220(50) cm - 1 [NH3], 0 = 76 240(50) cm - 1 [ND3], the 1A (5pa1a) state: 0 = 76 674(1) cm - 1 [NH3], 0 = 76 770(5) cm - 1 [ND3], and the 1A (6pa1a) state: 0 = 78 494(1) cm - 1 [NH3]. The present work serves to reinforce the previously noted dominance of np1a Rydberg excitations in the 2 + 1 REMPI spectrum of ammonia. In addition, the adiabatic ionization energy of ND3 is estimated to be 82 280 ± 40 cm - 1 based on the assumption that analogous Rydberg states of NH3 and ND3 will have very similar quantum defects.