ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-11-8965-2011 Isotope effects in N<sub>2</sub>O photolysis from first principles Schmidt J. A. 1 Johnson M. S. 1 Schinke R. 2 Department of Chemistry, University of Copenhagen, Universitetsparken 5, 2100 Copenhagen Ø, Denmark Max-Planck-Institut für Dynamik und Selbstorganisation, 37073 Göttingen, Germany 02 09 2011 11 17 8965 8975 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys.net/11/8965/2011/acp-11-8965-2011.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/11/8965/2011/acp-11-8965-2011.pdf

For the first time, accurate first principles potential energy surfaces allow N<sub>2</sub>O 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 (<sup>15</sup>N<sup>14</sup>N<sup>16</sup>O, <sup>14</sup>N<sup>15</sup>N<sup>16</sup>O, <sup>15</sup>N<sub>2</sub><sup>16</sup>O, <sup>14</sup>N<sub>2</sub><sup>17</sup>O and <sup>14</sup>N<sub>2</sub><sup>18</sup>O) calculated using wavepacket propagation are compared to the most abundant isotopologue (<sup>14</sup>N<sub>2</sub><sup>16</sup>O). 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 N<sub>2</sub>O samples can be explained as arising from photolysis.

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