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

Abstract

Nitrous oxide (N 2 O) plays an important role in greenhouse warming and ozone depletion. Yung and Miller's zero point energy (ZPE) model for the photolysis of N 2 O 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 N 2 O 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 N 2 O 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.

OCS photolytic isotope effects from first principles: sulfur and carbon isotopes, temperature dependence and implications for the stratosphere

“…more narrow) the cross sections for these isotopologues will be smaller than that of light OCS at long wavelengths. A similar effect is seen in N 2 O (Johnson et al, 2001). The smaller cross section leads to a negative fractionation constant at long wavelengths.…”

Ultra-violet absorption cross sections of isotopically substituted nitrous oxide species: &lt;sup&gt;14&lt;/sup&gt;N&lt;sup&gt;14&lt;/sup&gt;NO, &lt;sup&gt;15&lt;/sup&gt;N&lt;sup&gt;14&lt;/sup&gt;NO, &lt;sup&gt;14&lt;/sup&gt;N&lt;sup&gt;15&lt;/sup&gt;NO and &lt;sup&gt;15&lt;/sup&gt;N&lt;sup&gt;15&lt;/sup&gt;NO

“…If there were a dynamical effect on the cross section, one would expect it to produce a simultaneous change in the cross section and the partitioning of energy between rotation and translation. The change in zero point energy of 14 N 15 N 16 O is 39 cm −1 , and for 15 N 14 N 16 O, 22 cm −1 (frequencies may be found in Johnson et al, 2001). Thus the ZPE theory predicts that the fractionation constant for 15 N 14 NO should be about 22/39=56% that of 14 N 15 NO.…”

“…Also, new theoretical models of the absorption cross section and the resulting isotopic fractionation constants have been presented (Blake et al, 2003;Johnson et al, 2001;Morgan et al, 2004;Nanbu and Johnson, 2004).…”

“…A secondary purpose is to help refine theoretical models of the effect of isotopic substitution on absorption cross sections (Blake et al, 2003;Johnson et al, 2001;Morgan et al, 2004;Nanbu and Johnson, 2004). We report the absorption spectra of 14 N 14 NO,14 N 15 NO,15 N 14 NO and 15 N 15 NO at 233 and 283 K over the wavelength range 181 to 218 nm.…”

Sources of the oxygen isotopic anomaly in atmospheric N2O

“…It is known that atmospheric O 3 is enriched in heavy isotopologues and isotopomers with mass-independent fractionation [Mauersberger, 1987;Krankowsky et al, 2000;Mauersberger et al, 2001;Lämmerzahl et al, 2002;Brenninkmeijer et al, 2003;Thiemens, 2006]. The isotopic composition of atmospheric O 3 [Liang et al, 2006] can be well explained by two processes: formation [Thiemens and Heidenreich, 1983;Mauersberger et al, 1999;Gao and Marcus, 2001] and photolysis [Johnson et al, 2001;Bhattacharya et al, 2002;Blake et al, 2003;Liang et al, 2004;Miller et al, 2005;Prakash et al, 2005] [8] In this paper, we simulate the isotopic fractionation of O 3 and N 2 O in one-dimensional (height) and twodimensional (latitude and height) modes. The modeling of the longitudinal variation requires a three-dimensional model and will be deferred to a later paper.…”