MEASUREMENTS OF ISOTOPE EFFECTS IN THE PHOTOIONIZATION OF N2 AND IMPLICATIONS FOR TITAN'S ATMOSPHERE

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 N-14(2), (N14N)-N-15, and N-15(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(N-14(2))/sigma((NN)-N-15-N-14), 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((NN)-N-15-N-14)/J(N-14(2)) = 1.00 +/- 0.02 and J(N-15(2))/J(N-14(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 N-14(2) leads to values of J((NN)-N-15-N-14)/J(N-14(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 (HCN)-N-15/(HCN)-N-14 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 N-15/N-14 for HCN in Titan's atmosphere are explored.