Photolysis of Nitrous Oxide Isotopomers Studied by Time-Dependent Hermite Propagation

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