Cross sections for photoabsorption of NH 3 , NH 2 D, NHD 2 , and ND 3 in the spectral region 140Y220 nm were determined at $298 K using synchrotron radiation. Absorption spectra of NH 2 D and NHD 2 were deduced from spectra of mixtures of NH 3 and ND 3 , of which the equilibrium concentrations for all four isotopologues obey statistical distributions. Cross sections of NH 2 D, NHD 2 , and ND 3 are new. Oscillator strengths, an integration of absorption cross sections over the spectral lines, for both A X and B X systems of NH 3 agree satisfactorily with previous reports; values for NH 2 D, NHD 2 , and ND 3 agree with quantum chemical predictions. The photolysis of NH 3 provides a major source of reactive hydrogen in the lower stratosphere and upper troposphere of giant planets such as Jupiter. Incorporating the measured photoabsorption cross sections of NH 3 and NH 2 D into the Caltech /JPL photochemical diffusive model for the atmosphere of Jupiter, we find that the photolysis efficiency of NH 2 D is lower than that of NH 3 by as much as 30% . The D/H ratio in NH 2 D/ NH 3 for tracing the microphysics in the troposphere of Jupiter is also discussed.
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.
Laboratory measurements of the photoabsorption cross section of NH 3 at wavelengths between 140 and 220 nm 15 are presented for the first time. Incorporating the measured photoabsorption cross sections of NH 3 and NH 3 into a 15 14 one-dimensional photochemical diffusive model, we find that at 400 mbar, the photolytic efficiency of NH 3 is about 15 38% greater than that of NH 3. In addition, it is known that ammonia can condense in the region between 200 and 14 700 mbar, and the condensation tends to deplete the abundance ratio of NH 3 and NH 3. By matching the observed 15 14 ratio of NH 3 and NH 3 at 400 mbar, the combined effect of photolysis and microphysics produces the ratio of 15 14 in the deep atmosphere, in excellent agreement with the Galileo spacecraft measurements. The Ϫ3 (2.42 ע 0.34) # 10 usefulness of the isotopic composition of ammonia as a tracer of chemical and dynamical processes in the troposphere of Jupiter is discussed.
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