Measurement and Full Model of Isotope Fractionation During Photodissociation and Applications in Cosmo and Geochemistry

The application of isotope effects from photodissociation processes in nature date back to Viking and the observation of a massive ¹⁵N in the Martian atmosphere, derived from combined photolysis and gravitational escape. Large observed effects in meteorites, interstellar molecular clouds, and pre solar nebulae utilize photodissociation as a source of the wide range in isotopic composition. CO, which is isoelectronic with nitrogen, has also been widely used, but models do not agree with experiments suggesting models do not include all parameters.

We report precise novel measurements of the isotopic branching ratio in the photodissociation of N₂ in the VUV at the advanced light source, Berkeley with quantitative scavenging of the nascent N atoms. We here report an integration of these measurements with state-of-the-art dynamics modeling and light shielding. The measured photodissociation enrichment in ¹⁵N with wavelength with a down trend above 90 nm is shown to arise from dynamical effects. There are two effects identified by the computations, the branching between exit channels and the more subtle role of the non-monotonic variation in the individual line widths that in the higher energies begin to significantly overlap. The widths have a significant effect on both the shielding computations at the higher energies and on the cross sections themselves. The modeling requires accurate quantum dynamical simulations using state of the art multireference potential energies and their state-dependent couplings. As the excitation energy increases, competition between different coupled exit channels, some leading to reactive N (²D) and some leading to significantly less reactive N (²P) in an isotope dependent way, modulates the selectivity for the ¹⁵N atoms. As a result, the dissociation lifetimes of initial states close in energy vary in a nonmonotonic isotopic dependent manner as a function of energy. Our work shows that modelling can interpret the novel experimental observations and account for the exceptionally high selectivity. Additional progress requires accurate high resolution UV spectra for entire UV bands, both measured and computed to complement fractionation measurements. The complexity of the non-statistical dynamics and the role of the light shielding make such high-resolution work necessary for the detailed understanding of isotope enrichment fractions in the higher energy regime for nitrogen and also for other molecules of interest in cosmochemistry such as CO. Given the massive range in isotopic composition, the interpretation of e.g the Mars atmosphere and photolysis intersection, meteoritic nitrogen may be modeled better. Samples from the earth’s interface with space where N₂ photolysis occurs would be an interesting application and testing of the model.