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Published December 16, 2000 | Published
Journal Article Open

Photo-induced isotopic fractionation


This paper presents a systematic method for the analysis of photo-induced isotopic fractionation. The physical basis for this fractionation mechanism centers on the fact that isotopic substitution alters the energy levels, molecular symmetries, spin statistical weights and other fundamental molecular properties, producing spectroscopic signatures distinguishable from that of the parent isotopomer. These mass-dependent physical properties are identical to those invoked by Urey to explain stable isotope fractionation in chemical systems subject to thermodynamic equilibrium. Photo-induced isotopic fractionation is a completely general phenomenon and should be observable in virtually all gas phase photochemical systems. Water photo-induced isotopic fractionation has been examined in detail using experimental and theoretical data. These results illustrate the salient features of this fractionation mechanism for molecules possessing continuous UV absorption spectra and unit photodissociation quantum yields. Using the photo-induced isotopic fractionation methodology in conjunction with standard photochemical models, we predict substantial deuterium enrichment of water vapor in the planetary atmospheres of Earth and Mars.

Additional Information

© 2000 by the American Geophysical Union. Received April 17, 2000: revised June 13, 2000. Paper number 2000JD900388. The authors would like to thank Y. P. Lee for the H2O, HDO, and D2O absorption cross section data, R. van Harrevelt and M. van Hemert for sharing their ab initio absorption cross sections prior to publication, and D. Schwenke for the water isotopomer zero point energies.

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