Absorption Cross Sections of NH₃, NH₂D, NHD₂, and ND₃ in the Spectral Range 140-220 nm and Implications for Planetary Isotopic Fractionation

Cross sections for photoabsorption of NH₃, NH₂D, NHD₂, and ND₃ in the spectral region 140-220 nm were determined at ~298 K using synchrotron radiation. Absorption spectra of NH₂D and NHD₂ were deduced from spectra of mixtures of NH₃ and ND₃, of which the equilibrium concentrations for all four isotopologues obey statistical distributions. Cross sections of NH₂D, NHD₂, and ND₃ are new. Oscillator strengths, an integration of absorption cross sections over the spectral lines, for both A ← X and B ← X systems of NH₃ agree satisfactorily with previous reports; values for NH₂D, NHD₂, and ND₃ agree with quantum chemical predictions. The photolysis of NH₃ 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 NH₃ and NH₂D into the Caltech/JPL photochemical diffusive model for the atmosphere of Jupiter, we find that the photolysis efficiency of NH₂D is lower than that of NH₃ by as much as 30%. The D/H ratio in NH₂D/NH₃ for tracing the microphysics in the troposphere of Jupiter is also discussed.