Measurements of Isotope Effects in N2 Photoionization And Implications for Titan's Atmosphere

Details Measurements of Isotope Effects in N2 Photoionization And Implications for Titan's Atmosphere Measurements of Isotope Effects in N2 Photoionization And Implications for Titan's Atmosphere

Oct 2010

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010dps....42.3612r&link_type=abstract

American Astronomical Society, DPS meeting #42, #36.12; Bulletin of the American Astronomical Society, Vol. 42, p.1067

Astronomy and Astrophysics

Astronomy

Scientific paper

Isotope effects in the non-dissociative photoionization of molecular nitrogen (N2+hν→N2++e-) are likely to play a role in determining the relative abundances of isotopic species containing nitrogen in interstellar clouds and planetary atmospheres and yet they have not been previously measured. We present measurements of the photoionization efficiency spectra of 14N2, 15N14N, and 15N2 from 15.4 to 18.9 eV (80.5-65.6 nm) using the Advanced Light Source at Lawrence Berkeley National Laboratory. The spectra show significant shifts in peak energies and intensities due to isotopic substitution, with the ratio of the energy-dependent photoionization cross-sections, σ(14N2)/σ(15N14N), ranging from 0.4 to 3.5. Assuming white light and optically thin conditions (i.e., integrating the photoionization cross-sections over all wavelengths measured), the ratio of the photoionization rate coefficients, J, are J(14N15N)/J(14N2) = 1.02±0.02 and J(15N2)/J(14N2) = 1.01±0.02, suggesting that isotopic fractionation between N2 and N2+ should be small under such conditions. However, in a 1D model of Titan's atmosphere, isotopic self-shielding of 14N2 leads to values of J(15N14N)/J(14N2) as large as 1.17, larger than under optically thin conditions but still much smaller than values as high as 29 predicted for N2 photodissociation. Since modeled photodissociation isotope effects overpredict the HC15N/HC14N ratio in Titan's atmosphere, and since both N atoms and N2+ ions can react with hydrocarbons to form HCN, these new results suggest that including N2 photoionization may contribute to a more quantitative explanation of 15N/14N for HCN in Titan's atmosphere without the need to invoke a less well-quantified influx of N atoms with a lower 15N/14N ratio from ion/electron impact of N2 by cosmic rays and Saturnian magnetospheric electrons at the top of the atmosphere.

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