Absorption Cross Sections of NH3, NH2D, NHD2, and ND3in the Spectral Range 140–220 nm and Implications for Planetary Isotopic Fractionation

Bing‐Ming Cheng, Hsiao‐Chi Lu, Hong‐Kai Chen, Mohammed Bahou, Yuan‐Pern Lee, Alexander M. Mebel, L. C. Lee, Mao‐Chang Liang, Yuk L. Yung
2006 Astrophysical Journal  
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,
more » ... 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.
doi:10.1086/505615 fatcat:lzbg55sfnndwtmqiyrhes5lng4