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The spectroscopy of high Rydberg states of ammonia

1998; American Institute of Physics; Volume: 108; Issue: 16 Linguagem: Inglês

10.1063/1.476082

1520-9032

Stephen R. Langford, Andrew J. Orr‐Ewing, Ross A. Morgan, Colin M. Western, Michael N. R. Ashfold, Arjan Rijkenberg, C.R. Scheper, Wybren Jan Buma, C. A. de Lange,

Spectroscopy and Quantum Chemical Studies

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 n⩽8 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 Ẽ″ 1A2″ (5sa1′←1a2″) state: ν0=74 118(2) cm−1 [NH3], ν0=74 258(2) cm−1 [ND3], the F̃″ 1E″ (5pe′←1a2″) state: ν0=76 220(50) cm−1 [NH3], ν0=76 240(50) cm−1 [ND3], the F̃′ 1A1′ (5pa2″←1a2″) state: ν0=76 674(1) cm−1 [NH3], ν0=76 770(5) cm−1 [ND3], and the G̃′ 1A1′ (6pa2″←1a2″) state: ν0=78 494(1) cm−1 [NH3]. The present work serves to reinforce the previously noted dominance of np←1a2″ 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.