MEASUREMENTS OF ISOTOPE EFFECTS IN THE PHOTOIONIZATION OF N{sub 2} AND IMPLICATIONS FOR TITAN'S ATMOSPHERE

Journal Article · Sun Feb 20 04:00:00 EST 2011 · Astrophysical Journal Letters

DOI:https://doi.org/10.1088/2041-8205/728/2/L32· OSTI ID:21560487

Croteau, Philip; Randazzo, John B; Boering, Kristie A ^[1]; Kostko, Oleg; Ahmed, Musahid ^[2]; Liang, Mao-Chang ^[3]

---

1. Department of Chemistry, University of California, Berkeley, CA 94720 (United States)
2. Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720 (United States)
3. Research Center for Environmental Changes and Institute of Astronomy and Astrophysics, Academia Sinica, Taipei 115, Taiwan (China)

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.

• OSTI ID: 21560487
• Journal Information: Astrophysical Journal Letters, Journal Name: Astrophysical Journal Letters Journal Issue: 2 Vol. 728; ISSN 2041-8205
• Country of Publication: United States
• Language: English

Related Subjects

74 ATOMIC AND MOLECULAR PHYSICS
ATMOSPHERES
CHEMICAL REACTIONS
CLOUDS
CROSS SECTIONS
DECOMPOSITION
DISSOCIATION
EFFICIENCY
ELEMENTS
ENERGY DEPENDENCE
FRACTIONATION
IONIZATION
ISOTOPE EFFECTS
ISOTOPES
LIGHT NUCLEI
NITROGEN
NITROGEN 14
NITROGEN 15
NITROGEN ISOTOPES
NONMETALS
NUCLEI
ODD-EVEN NUCLEI
ODD-ODD NUCLEI
PHOTOCHEMICAL REACTIONS
PHOTOIONIZATION
PHOTOLYSIS
PLANETARY ATMOSPHERES
RADIATION FLUX
SEPARATION PROCESSES
SOLAR FLUX
SPECTRA
STABLE ISOTOPES