MEASUREMENTS OF ISOTOPE EFFECTS IN THE PHOTOIONIZATION OF N(2) AND IMPLICATIONS FOR TITAN'S ATMOSPHERE

Please use this identifier to cite or link to this item: http://ir.lib.ncu.edu.tw/handle/987654321/50888

Title:	MEASUREMENTS OF ISOTOPE EFFECTS IN THE PHOTOIONIZATION OF N(2) AND IMPLICATIONS FOR TITAN'S ATMOSPHERE
Authors:	Croteau,P;Randazzo,JB;Kostko,O;Ahmed,M;Liang,MC;Yung,YL;Boering,KA
Contributors:	天文研究所
Keywords:	PLANETARY-ATMOSPHERES;EXTREME-ULTRAVIOLET;CROSS-SECTIONS;ION CHEMISTRY;NITROGEN;HCN;N2;TEMPERATURE;COLLISIONS;C-12/C-13
Date:	2011
Issue Date:	2012-03-27 18:12:12 (UTC+8)
Publisher:	國立中央大學
Abstract:	Isotope effects in the non-dissociative photoionization of molecular nitrogen (N(2) + h --> 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, sigma((14)N(2))/sigma((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 similar to 1.17, larger than under optically thin conditions but still much smaller than values as high as similar to 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.
Relation:	ASTROPHYSICAL JOURNAL LETTERS
Appears in Collections:	[Graduate Institute of Astronomy] journal & Dissertation

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