Massive isotopic effect in vacuum UV photodissociation of N ₂ and implications for meteorite data

Abstract

Nitrogen isotopic distributions in the solar system extend across an enormous range, from −400‰, in the solar wind and Jovian atmosphere, to about 5,000‰ in organic matter in carbonaceous chondrites. Distributions such as these require complex processing of nitrogen reservoirs and extraordinary isotope effects. While theoretical models invoke ion-neutral exchange reactions outside the protoplanetary disk and photochemical self-shielding on the disk surface to explain the variations, there are no experiments to substantiate these models. Experimental results of N 2 photolysis at vacuum UV wavelengths in the presence of hydrogen are presented here, which show a wide range of enriched δ 15 N values from 648‰ to 13,412‰ in product NH 3 , depending upon photodissociation wavelength. The measured enrichment range in photodissociation of N 2 , plausibly explains the range of δ 15 N in extraterrestrial materials. This study suggests the importance of photochemical processing of the nitrogen reservoirs within the solar nebula.nitrogen isotopes | organic molecules | perturbation

Constraints from Comets on the Formation and Volatile Acquisition of the Planets and Satellites

“…, respectively; after Aléon 2010). Data are consistent with different processes of isotope fractionation, e.g., ion-molecule isotope fractionation during low temperature isotope exchange between organics and the protosolar gas, kinetic isotope fractionation proportional to the square root of mass, or self-shielding enhancement of the isotope fractionation through photodissociation of protosolar N 2 by UV light (Clayton 2002;Chakraborty et al 2013). The Earth shares H and N isotope signatures with bulk chondrites, whereas comets are richer in D and 15 N, thus suggesting an asteroidal, rather than cometary, origin for terrestrial volatiles (Alexander et al 2012;Marty 2012).…”

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

“…13 The summarized data are shown in Table I. The wavelength dependent fractionation profile is significantly different from that measured at 200 K and 300 K, 13 in each of these relatively higher temperature profiles, there only one 15 N enrichment peak was measured at 90 nm (111 111 cm −1 , 13.78 eV). However, the low temperature (80 K) profile shows multiple peaks (Figure 2).…”

“…2. 15 N enrichment profiles of the photodissociation product of N 2 at three different temperatures: 80 K (new data), 200 K, and 300 K. 13 N 2 dissociation product atomic N was captured as NH 3 (and NH 2 ) by reacting with hydrogen inside the chamber. At the end of the reaction, NH 3 was isolated cryogenically and decomposed to N 2 (see text for details) and measured in the mass spectrometer for isotopic composition.…”

“…4. 15 N enrichment profiles over the wavelength range have been computed using a MATLAB program described by Chakraborty et al, 13 based on the concept of isotopologue specific photon absorption (e.g., self-shielding using Beer-Lambert law). In these computations the available cross-sectional data sets from different research groups (DL = Liang et al; 3 SM = Spelsberg and Meyer, 21 Muskatel et al; 31 ANU = Li et al, 16 Heays et al 39 ) are inserted separately for the 100, 200, and 300 K temperatures.…”

“…We recently reported a large nitrogen isotopic fractionation in the vacuum ultraviolet (VUV) photodissociation of N 2 performed at temperatures between ambient (300 K) and dry ice (∼200 K). 13 Here we present new data on the N-isotopic composition in ammonia (measured as N 2 ) formed by VUV photodissociation of N 2 at 80 K temperature using the VUV photons from Advanced Light Source (ALS) synchrotron at the Lawrence Berkeley National Laboratory (LBNL). The lower temperatures are more relevant for astrophysical environments.…”