Measurements of Isotope Effects in N2 Photoionization

Isotope effects in the non-dissociative photoionization of molecular nitrogen (N2+hν→N2++e-) have not been previously measured, despite the possibility that they may lead to measurable differences in the relative abundances of isotopic species containing nitrogen in interstellar clouds and planetary atmospheres such as Earth, Mars, and Titan. We present measurements of the photoionization efficiency spectra of 14N2, 15N14N, and 15N2 from 15.4 to 18.9 eV (80.5-65.6 nm) using the Advanced Light Source at Lawrence Berkeley National Laboratory. The spectra show that isotopic substitution leads to significant shifts in peak energies and intensities, with the ratio of the energy-dependent photoionization cross-sections, σ(14N2)/σ(15N14N), ranging from 0.4 to 3.5. Photoionization rate coefficients, J, were calculated under white light and optically thin conditions by integrating the photoionization cross-sections over all wavelengths measured. Such conditions yield J(15N14N)/J(14N2) = 1.02±0.02 and J(15N2)/J(14N2) = 1.01±0.02. These results suggest that isotopic fractionation between N2 and N2+ should be small under such conditions, despite ratios of the photionization cross-sections themselves as large as a factor of 3.5 at the experimental energy resolution of 6.5 meV. However, in a 1D model of Titan’s dense, N2-rich atmosphere, isotopic self-shielding of the common isotopologue 14N2 leads to regions of the atmosphere in which the ratio J(15N14N)/J(14N2) is as large as 1.17, ranging from ~1.05 at 1400 km to ~1.17 at 900 km altitude. Implications for Titan's atmosphere will be discussed.