Strong Isotope-dependent Photodissociation Branching Ratios of N2 and Their Potential Implications for the 14N/15N Isotope Fractionation in Titan's Atmosphere

The origin and evolution of the 14 N/ 15 N ratio of Titan’s atmosphere has long been a subject of debate. Clearly a better understanding of the N isotopic fractionation mechanism would greatly help resolve this. Photodissociation of N 2 by solar radiation has been suggested to either play a negligible role in fractionating the N isotopes in Titan, due to its rather low escape velocity, or to preferentially remove 15 N through self-shielding controlled photochemical reactions. Here, we systematically measure the branching ratios of 14 N 15 N between N( 4 S)+N( 2 P) and N( 4 S)+N( 2 D) channels. We find that many of its absorption states predominantly dissociate into N( 4 S)+N( 2 P) with a strong isotope effect between 14 N 2 and 14 N 15 N. Since N atoms produced from N( 4 S)+N( 2 P) acquire velocities close to Titan’s escape velocity, these findings provide a new N isotope fractionation mechanism for Titan that has not been considered before, potentially providing important constraints on the origin and evolution of Titan’s N 2 -dominated atmosphere.