| Abstract
| - Nitrous oxide (N2O) plays an important role in greenhouse warming and ozone depletion. Yung and Miller'szero point energy (ZPE) model for the photolysis of N2O isotopomers was able to explain atmosphericisotopomer distributions without invoking in situ chemical sources. Subsequent experiments showed enrichmentfactors twice those predicted by the ZPE model. In this article we calculate the UV spectrum of the key N2Oisotopomers to quantify the influence of factors not included in the ZPE model, namely, the transition dipolesurface, bending vibrational excitation, dynamics on the excited state potential surface, and factors related toisotopic substitution itself. The relative cross-sections are calculated as the Fourier transform of the correlationfunction of the initial vibrational wave function and the time-propagated wave function, using a Hermiteexpansion of the time evolution operator. The model uses the electronic structure data recently published byBalint-Kurti and co-workers and makes several predictions. (a) The absolute values of the enrichment factorsdecrease with increasing temperature. (b) Photolysis of N2O will produce “mass-independent” enrichment inthe remaining sample. (c) Much of the enrichment is due to decreased heavy isotopomer cross-section overthe entire absorption band, in contrast to the wavelength shift predicted by the ZPE model. Consequently, towithin the error of the calculation, we predict only minor enrichments at λ < 182 nm. The smaller bendingexcursion of heavy isotopomers combines with the transition dipole surface to produce a smaller integratedcross-section. This effect is partially countered by the larger fraction of heavy isotopomers in excited bendingstates; the first three bending states have an integrated intensity ratio of ca. 1:3:6. The model agrees withavailable experimental enrichment factors and stratospheric balloon infrared remote sensing data to withinthe estimated error.
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