Isotope effects in N₂O photolysis from first principles

Abstract. For the first time, accurate first principles potential energy surfaces allow N₂O cross sections and isotopic fractionation spectra to be derived that are in agreement with all available experimental data, extending our knowledge to a much broader range of conditions. Absorption spectra of rare N- and O-isotopologues (¹⁵N¹⁴N¹⁶O, ¹⁴N¹⁵N¹⁶O, ¹⁵N₂¹⁶O, ¹⁴N₂¹⁷O and ¹⁴N₂¹⁸O) calculated using wavepacket propagation are compared to the most abundant isotopologue (¹⁴N₂¹⁶O). The fractionation constants as a function of wavelength and temperature are in excellent agreement with experimental data. The study shows that excitations from the 3rd excited bending state, (0,3,0), and the first combination state, (1,1,0), are important for explaining the isotope effect at wavelengths longer than 210 nm. Only a small amount of the mass independent oxygen isotope anomaly observed in atmospheric N₂O samples can be explained as arising from photolysis.