Isotopomer fractionation in the UV photolysis of N₂O: Comparison of theory and experiment

Abstract

[1] In the photodissociation of N 2 O, absorption cross sections differ with isotopic substitution, leading to a wavelength-dependent fractionation of the various isotopomers. Several models ranging from shifts by zero-point energy differences to propagation of wave packets on the excited electronic state potential energy surface have been proposed to explain the observed fractionations. We present time-independent fractionation calculations for the isotopomers 447, 448, 456, 546, and 556. Besides largely agreeing with the experimental data, these calculations have the advantage of not being computationally intensive, as well as satisfying the physical facts that the asymmetric stretch and the doubly degenerate bending vibration are the principal Franck-Condon active modes in the photodissociation. The latter is reflected in the actual dissociation and in the high rotational excitation and lack of vibrational excitation of the N 2 product. The calculations are based on a multidimensional reflection principle using an ab initio potential energy surface. The theory for the absorption cross section and isotopomer fractionation accompanying photodissociation is described. The absolute value of the theoretically calculated absorption cross section is very close (90%) to the experimentally observed value. The present computations also provide data for the slope of a threeisotope plot of the fractionation of 447/446 relative to 448/446, using the fractionations at different wavelengths. The resulting slope is compared with a perturbation theoretical expression for direct photodissociation given elsewhere.

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

“…Stratospheric photolysis of OCS occurs near the center of the cross section where the fractionation constant is close to zero. In contrast, N 2 O photolysis occurs on the low energy shoulder of the cross section giving rise to a large enrichment of heavy N 2 O in the stratosphere (Kim and Craig, 1993;Rahn and Wahlen, 1997;Toyoda et al, 2001;Röckmann et al, 2001;Prakash et al, 2005;Chen et al, 2008Chen et al, , 2010.…”

Three-isotope plot of fractionation in photolysis: A perturbation theoretical expression

“…11,13 We use a form of the reflection principle 10 for the absorption cross section at a frequency which is more accurate than an earlier 11,13 version of the principle. There are several expressions in the literature 10,13 and the one we found 9 to represent the absorption cross section data best for a spin-allowed transition is given by 10…”

“…We expressed the harmonic-oscillator wave function in terms of the mass-dependent normal-mode coordinates Q and vibrational frequencies, and Q was then transformed to R with the appropriate transformation matrix that depends on isotopic substitution. 9,15 The force constants obtained from Ref. 26 were used to form the force-constant matrix F. Using the results from those detailed calculations the fractionation factors ⑀ 447 and ⑀ 448 were obtained as functions of wavelength.…”

“…II A, of oxygen isotopes in the photolysis of N 2 O at different wavelengths. 9 In the present paper we present these wavelength-dependent fractionation factors as a three-isotope plot. To our surprise the slope of the plot was the same as the mass-dependent value predicted by Bigeleisen-Mayer theory for a purely chemical equilibrium system.…”

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.…”