Photoabsorption cross sections of NH3, NH2D, NHD2, and ND3 in the spectral range 110-144 nm.

Cross sections for photoabsorption of NH3, NH2D, NHD2, and ND3 near 298 K were measured in the spectral range of 110-144 nm using radiation from a synchrotron. Absorption cross sections and oscillator strengths of NH3 agree satisfactorily with previous reports; those of ND3 are improved over those in a previous report, whereas those of NH2D and NHD2 are new. The oscillator strengths of transitions to D, D', D", F, and G states are nearly the same among all four isotopic variants, but those to D''' and E states vary substantially. Observed absorption bands are arranged into vibrational progressions in accord with known Rydberg transitions. All progressions show a common trend of vibrational intervals increasing with vibrational quantum numbers. The Rydberg orbitals for states D(3de"), D'(4sa1'), D"(3da1'), D'''(4pe'), and E(4de") are readily assigned with quantum defects determined in these experiments, but assignments for F(5de") and G(6de") are uncertain. Absorption cross sections of dissociative continua underneath discrete structures are larger for NH2D and NHD2 than for NH3 and ND3, indicating that the rate of dissociation of ammonia might increase when its symmetry is broken.