Observation of Selective Isotope Effect in the Ultraviolet excitation of N2: A Computational Study

TL;DR

This computational study reveals a strong wavelength-dependent isotope effect in the ultraviolet photodissociation of N2, with implications for interpreting atmospheric and extraterrestrial isotopic signatures.

Contribution

The paper provides ab initio calculations of isotope effects in N2 photodissociation across 12.5-15 eV, identifying two physical mechanisms and predicting experimental conditions for observation.

Findings

Significant wavelength dependence of isotope effects observed.

Narrower laser beams enhance the isotope effect, enabling experimental testing.

Larger fractionation for 15N15N compared to 15N14N.

Abstract

Isotope effects associated with gas phase N2 photolysis are used to interpret Martian atmospheric evolution, icy satellite atmospheric chemistry and meteorite isotopic anomalies from nebular N2 photochemistry. To interpret observations at the highest level, fundamental understanding of the precise wavelength dependency of the process must be known. In this paper VUV isotopic photodissociation effects are calculated as a function of wavelength at different wavelength slices in the 12.5-15 eV range. A very strong wavelength dependence is observed, which is significant for experiments. An observable effect is possible for the width of the beam profile at the advanced light source, ALS that may produce sufficient photolysis product for high precision isotopic analysis. A significantly more pronounced effect is predicted for a beam narrower by a factor of four providing a potential experimental test of the model. The spectrum is computed ab initio. It manifests two physical mechanisms for the isotope effect and they can be discriminated using a narrow beam. The fractionation is larger for the rarer heaviest isotopomer 15N15N and half as large for 15N14N.