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Published August 20, 2006 | Published
Journal Article Open

Absorption Cross Sections of NH_3, NH_2D, NHD_2, and ND_3 in the Spectral Range 140-220 nm and Implications for Planetary Isotopic Fractionation


Cross sections for photoabsorption of NH_3, NH_2D, NHD_2, and ND_3 in the spectral region 140-220 nm were determined at ~298 K using synchrotron radiation. Absorption spectra of NH_2D 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_2D, 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_2D, 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_2D into the Caltech/JPL photochemical diffusive model for the atmosphere of Jupiter, we find that the photolysis efficiency of NH_2D is lower than that of NH_3 by as much as 30%. The D/H ratio in NH_2D/NH_3 for tracing the microphysics in the troposphere of Jupiter is also discussed.

Additional Information

© 2006 American Astronomical Society. Received 2006 February 9; accepted 2006 May 2. B.-M. C. and Y.-P. L. thank the National Science Council of Taiwan (grants NSC93-2113-M-213-002 and NSC93-2113- M-009-019) for support. L. C. L. thanks for the National Synchrotron Radiation Research Center in Taiwan for a visiting professorship. M.-C. L. and Y. L. Y. were supported by NASA grant NASA5-13296 to the California Institute of Technology.

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August 22, 2023
October 23, 2023