Nitrogen isotopic fractionations in the low temperature (80 K) vacuum ultraviolet photodissociation of N2 (original) (raw)

Abstract

N2 is a diatomic molecule with complex electronic structure. Interstate crossings are prominent in the high energy domain, introducing significant perturbations to the system. Nitrogen mainly photodissociates in the vacuum ultraviolet (VUV) region of the electromagnetic spectrum through both direct and indirect predissociation. Due to the complexity introduced by these perturbations, the nitrogen isotopic fractionation in N2 photodissociation is extremely hard to calculate and an experimental approach is required. Here we present new data of N-isotopic fractionation in N2 photodissociation at low temperature (80K), which shows a distinctly different 15 N enrichment profile compared to that at relatively higher temperatures (200 and 300 K). The new data is discussed in light of the knowledge of N2 photochemistry and calculated photoabsorption cross-sections in the VUV. This data, important to understanding the N-isotopic compositions measured in meteorites and other planetary bodies, is discussed in light of the knowledge of N2 photochemistry and calculated photoabsorption cross-sections in the VUV. I. INTRODUCTION Nitrogen is the fifth most abundant element in the universe and an essential component as a prebiotic molecular building block. In interstellar clouds, nitrogen exists in atomic and molecular 2 forms (N and N2) in gas-phase reservoirs. Atomic nitrogen is very reactive and takes part in chemical reactions leading to ammonia, nitriles, and other nitrogen compounds. No such reactions occur when nitrogen is in N2 form. Nitrogen isotopic analyses of meteorites, terrestrial planets, atmospheres of giant planets and their moons, solar wind, comets, and interplanetary dust particles advance understanding of volatile chemistry and prebiotic processes in the early solar system 1. Direct astronomical observations of N2 is difficult because of the absence of strong pure rotational or vibrational lines. It is well studied through the electronic transitions at ultraviolet wavelengths 2, 3. The isotopic inventory of nitrogen in astronomical environments is also reasonably well known 1, 4. The solar system was formed with an initial 15 N/ 14 N ratio acquired from parent molecular clouds from the interstellar medium (ISM). The near-identical compositions measured in the solar wind and the Jovian atmosphere (δ 15 N ~-400 ‰) may indicate the formation of the gas giant with the initial solar system materials of the same N-isotopic composition 5, 6. Bulk meteorite analysis exhibits a variation in the range of few hundred permil in δ 15 N (wrt to air-N2) 7-9 with occasional exceptionally high values (as well as a range of variation) in some carbonaceous

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