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Published May 26, 2004 | Published
Journal Article Open

A semianalytic model for photo-induced isotopic fractionation in simple molecules


We have developed a semianalytic model for computing the photo-induced isotopic fractionation in simple molecules of interest to the atmospheric science community. The method is based on the Born-Oppenheimer approximation and the Reflection Principle. It has the main advantage of using commonly available input data, namely, the photolysis cross sections for the standard isotopologue/isotopomer and the ground state isotope-specific spectroscopic constants. The isotopic fractionation arises principally from the spectral shift induced by the small difference in zero point energy between isotopologues/isotopomers and the contraction of the wave function due to heavier isotope substitution. The latter effect dominates photolytic fractionation away from the cross section maxima. Our new approach is demonstrated with applications to the diatomic molecules HCl and HI, and the triatomic molecules N_2O and O_3. Agreement between the model and measurements is excellent. New modeling results for the fractionation of ^(15)N^(15)N^(16)O in the stratosphere using the Caltech/JPL two-dimensional model are presented.

Additional Information

© 2004 American Geophysical Union. Received 14 January 2004; revised 29 March 2004; accepted 2 April 2004; published 26 May 2004. We thank C. D. Camp, M. Gerstell, X. Jiang, V. Natraj, C. D. Parkinson, R. L. Shia, and J. D. Weibel for helpful comments and discussions. We also thank M. S. Johnson for sharing the ab initio calculation of N_2O's cross sections. This work was supported in part by an NSF grant ATM-9903790. The development of the Caltech/JPL 2-D model was supported by NASA grant NAG1-1806.

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