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MEASUREMENTS OF ISOTOPE EFFECTS IN THE PHOTOIONIZATION OF N{sub 2} AND IMPLICATIONS FOR TITAN'S ATMOSPHERE
Isotope effects in the non-dissociative photoionization of molecular nitrogen (N{sub 2} + h{nu} {yields} N{sub 2} {sup +} + e {sup -}) may play a role in determining the relative abundances of isotopic species containing nitrogen in interstellar clouds and planetary atmospheres but have not been pre...
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Published in: | Astrophysical journal. Letters 2011-02, Vol.728 (2) |
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Main Authors: | , , , , , , |
Format: | Article |
Language: | English |
Subjects: | |
Online Access: | Get full text |
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Summary: | Isotope effects in the non-dissociative photoionization of molecular nitrogen (N{sub 2} + h{nu} {yields} N{sub 2} {sup +} + e {sup -}) 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 {sup 14}N{sub 2}, {sup 15}N{sup 14}N, and {sup 15}N{sub 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, {sigma}({sup 14}N{sub 2})/{sigma}({sup 15}N{sup 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({sup 15}N{sup 14}N)/J({sup 14}N{sub 2}) = 1.00 {+-} 0.02 and J({sup 15}N{sub 2})/J({sup 14}N{sub 2}) = 1.00 {+-} 0.02, suggesting that isotopic fractionation between N{sub 2} and N{sub 2} {sup +} should be small under such conditions. In contrast, in a one-dimensional model of Titan's atmosphere, isotopic self-shielding of {sup 14}N{sub 2} leads to values of J({sup 15}N{sup 14}N)/J({sup 14}N{sub 2}) as large as {approx}1.17, larger than under optically thin conditions but still much smaller than values as high as {approx}29 predicted for N{sub 2} photodissociation. Since modeled photodissociation isotope effects overpredict the HC{sup 15}N/HC{sup 14}N ratio in Titan's atmosphere, and since both N atoms and N{sub 2} {sup +} ions may ultimately lead to the formation of HCN, estimates of the potential of including N{sub 2} photoionization to contribute to a more quantitative explanation of {sup 15}N/{sup 14}N for HCN in Titan's atmosphere are explored. |
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ISSN: | 2041-8205 2041-8213 |
DOI: | 10.1088/2041-8205/728/2/L32 |