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Published February 20, 2011 | Published
Journal Article Open

Measurements of Isotope Effects in the Photoionization of N_2 and Implications for Titan's Atmosphere


Isotope effects in the non-dissociative photoionization of molecular nitrogen (N_2 + hν → N_2^+ + e^−) may play a role in determining the relative abundances of isotopic species containing nitrogen in interstellar clouds and planetary atmospheres but have not been previously measured. Measurements of the photoionization efficiency spectra of ^(14)N^2, ^(15)N^(14)N, and ^(15)N_2 from 15.5 to 18.9 eV (65.6–80.0 nm) using the Advanced Light Source at Lawrence Berkeley National Laboratory show large differences in peak energies and intensities, with the ratio of the energy-dependent photoionization cross sections, σ(^(14)N_2)/σ (^(15)N^(14)N), ranging from 0.4 to 3.5. Convolving the cross sections with the solar flux and integrating over the energies measured, the ratios of photoionization rate coefficients are J(^(15)N^(14)N)/J(^(14)N_2) = 1.00 ± 0.02 and J(^(15)N_2)/J(^(14)N_2) = 1.00 ± 0.02, suggesting that isotopic fractionation between N_2 and N_2^+ should be small under such conditions. In contrast, in a one-dimensional model of Titan's atmosphere, isotopic self-shielding of ^(14)N_2 leads to values of J(^(15)N^(14)N)/J(^(14)N_2) as large as ~1.17, larger than under optically thin conditions but still much smaller than values as high as ~29 predicted for N_2 photodissociation. Since modeled photodissociation isotope effects overpredict the HC^(15)N/HC^(14)N ratio in Titan's atmosphere, and since both N atoms and N_2^+ ions may ultimately lead to the formation of HCN, estimates of the potential of including N_2 photoionization to contribute to a more quantitative explanation of ^(15)N/^(14)N for HCN in Titan's atmosphere are explored.

Additional Information

© 2011 American Astronomical Society. Received 2010 June 18; accepted 2010 December 28; published 2011 January 28. We gratefully acknowledge support from NASA Planetary Atmospheres grant NNX08AE69G to UC Berkeley and a Dreyfus Teacher-Scholar Award for K.A.B.; from the Director, Office of Energy Research, Office of Basic Energy Sciences, Chemical Sciences Division of the U.S. Department of Energy under contract no. DE-AC02-05CH11231 for O.K., M.A., and the ALS; from NSC grant 98-2111-M-001-014-MY3 to Academia Sinica for M.-C.L.; and NASA grant NX09AB72G to the California Institute of Technology for Y.L.Y.

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