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

• Published in: The Astrophysical journal Vol. 923; no. 2
• Main Authors: Liu, Min, Jiang, Pan, Lu, Liya, Yin, Tonghui, Ma, Liying, Cheng, Min, Yin, Qing-Zhu, Gao, Hong
• Format: Journal Article
• Language: English
• Published: Philadelphia The American Astronomical Society 01.12.2021; IOP Publishing
• Subjects: Astrophysics; Escape velocity; Evolution; Fractionation; Interstellar molecules; Isotope effect; Isotope fractionation; Isotopes; Laboratory astrophysics; Molecular clouds; Molecular physics; Nitrogen isotopes; Photochemical reactions; Photochemicals; Photodissociation; Photodissociation of water; Photodissociation regions; Planetary atmospheres; Planetary science; Satellite atmospheres; Solar radiation; Solar radiation shielding; Titan; Titan atmosphere
• ISSN: 0004-637X; 1538-4357
• DOI: 10.3847/1538-4357/ac2f97

The origin and evolution of the 14N/15N 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 N2 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 15N through self-shielding controlled photochemical reactions. Here, we systematically measure the branching ratios of 14N15N between N(4S)+N(2P) and N(4S)+N(2D) channels. We find that many of its absorption states predominantly dissociate into N(4S)+N(2P) with a strong isotope effect between 14N2 and 14N15N. Since N atoms produced from N(4S)+N(2P) 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 N2-dominated atmosphere.