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

Abstract

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.

Vacuum-UV spectroscopy of interstellar ice analogs

“…2, was measured by directly coupling the VUV-lamp to the spectrometer with a MgF 2 window acting as the interface. The proportion of Ly-α in 110−180 nm range is 5.8%, lower than the 8.4% value estimated by Chen et al (2013). This difference could be due to the lower transmittance of the MgF 2 window used as interface between the MDHL and the UHV-chamber, although the different position of the pressure gauge measuring the H 2 flow in the MDHL may also play a role.…”

“…The VUV-spectrum is measured routinely in situ during the irradiation experiments with the use of a McPherson 0.2 m focal length VUV monochromator (model 234/302) with a photomultiplier tube (PMT) detector equipped with a sodium salicylate window, optimized to operate from 100−500 nm (11.27−2.47 eV), with a resolution of 0.4 nm. The characterization of the MDHL spectrum has been studied before by Chen et al (2010) and will be discussed in more detail by Chen et al (2013).…”

“…VUV-absorption cross section as a function of photon wavelength (bottom X-axis) and VUV-photon energy (top X-axis) of pure NH 3 ice deposited at 8 K, black trace. The blue trace is the VUVabsorption cross-section spectrum of gas phase NH 3 adapted from Cheng et al (2006) and Wu et al (2007). The fit, red trace, is the sum of two Gaussians, dashed trace.…”

“…The VUV-absorption cross sections of NH 3 ice at 121.6 nm, 157.8 and 160.8 nm are 9.1 +0.7 −1.1 × 10 −18 cm 2 , 3.8 +0.3 −0.5 × 10 −18 cm 2 , and 4.1 +0.3 −0.5 × 10 −18 cm 2 . Gasphase data from Cheng et al (2006) and Wu et al (2007) were adapted, see Fig. 5.…”

ABSORPTION CROSS SECTION OF GASEOUS ACETYLENE AT 85 K IN THE WAVELENGTH RANGE 110-155 nm

“…To assess the abundances and photochemically induced changes in space, one must have quantitative information about the absorption cross sections of interstellar molecules in the VUV range (Miller & Yung 2000). For this purpose, we determined the absorption cross sections of several interstellar molecular species in the VUV range (Cheng et al 1999(Cheng et al , 2002(Cheng et al , 2004(Cheng et al , 2006Bahou et al 2001;Lee et al 2001;Liang et al 2007), but most such measurements were made near 300 K.…”