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

Abstract

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.

Multi-channel photodissociation dynamics of 14N2 in its b′ 1Σ+u(ν = 20) state

“…The VUV-VUV-TS-VMI setup has been described before, 17,25 and detailed quantum state-to-state experimental schemes are similar to those by Jackson and coworkers. 20,26 A pulsed supersonic molecular beam of N 2 was generated through a pulsed valve (Parker, series 9) and then passed through two skimmers before entering the photodissociation/photoionization (PD/PI) region.…”

“…11–13 The strengths of these couplings are strongly isotope dependent, 14 which results in strongly isotope dependent absorption line oscillator strengths, predissociation rates and branching ratios of different channels. 15–17…”

“…First, it provides a unique opportunity for studying competitions between spin-forbidden and spin-allowed channels, which could undergo drastically different predissociation pathways; second, this high energy region is already close to the first ionization threshold of N 2 , and detailed investigations of photodissociation dynamics in this region can provide useful information on the relevant potential energy curves for understanding the dissociative recombination process of N 2 + ions, which is a key photochemical process in many planetary atmospheres; 22 third, the competitions between spin-forbidden and spin-allowed channels and the possible isotope dependent properties constitute important information for modeling N atom escape processes above the exobases of many planetary atmospheres, due to the fact that N atoms produced from different channels possess different velocities, and this can be an efficient N fractionation process in these environments. 5,17,23,24…”

“…[11][12][13] The strengths of these couplings are strongly isotope dependent, 14 which results in strongly isotope dependent absorption line oscillator strengths, predissociation rates and branching ratios of different channels. [15][16][17] Thus far, most previous studies aiming for investigating the photodissociation dynamics of N 2 have been done below the spin-allowed limit N( 2 D 3/2,5/2 ) + N( 2 D 3/2,5/2 ). Since the lowest dissociation channel N( 4 S) + N( 4 S) has never been observed in any experiments yet, the only two energetically available channels are N( 4 S) + N( 2 D 3/2,5/2 ) and N( 4 S) + N( 2 P 1/2,3/2 ), which compete with each other in the energy range below the threshold of the channel N( 2 D 3/2,5/2 ) + N( 2 D 3/2,5/2 ).…”

“…12,13 Indeed, the overall dependence pattern of branching ratios between the two channels N( 4 S) + N( 2 D 3/2,5/2 ) and N( 4 S) + N( 2 P 1/2,3/2 ) on the excitation photon energy is largely modulated by the shapes of the potential energy curves of the manifold of 3 P u states below the N( 2 D 3/2,5/2 ) + N( 2 D 3/2,5/2 ) limit. [17][18][19][20] While systematic investigations on the photodissociation dynamics of N 2 below the N( 2 D 3/2,5/2 ) + N( 2 D 3/2,5/2 ) limit have been widely reported, dedicated studies on photodissociation dynamics of N 2 above it have been rare. b 0 1 S + u (n = 20) is the first vibronic state above the dissociation limit N( 2 D 3/2,5/2 ) + N( 2 D 3/2,5/2 ) of 14 N 2 that has been observed in the absorption spectrum.…”