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Isotopomer fractionation in the UV photolysis of N_2O: Comparison of theory and experiment

Prakash, Meher K. and Weibel, Jason D. and Marcus, R. A. (2005) Isotopomer fractionation in the UV photolysis of N_2O: Comparison of theory and experiment. Journal of Geophysical Research D, 110 (D21). Art. No. D21315. ISSN 0148-0227. doi:10.1029/2005JD006127.

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In the photodissociation of N_2O, 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 three-isotope 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.

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Marcus, R. A.0000-0001-6547-1469
Additional Information:© 2005 by the American Geophysical Union. Received 24 April 2005; accepted 18 August 2005; published 12 November 2005. It is a pleasure to acknowledge the support of this research by the National Science Foundation. We would like to particularly thank G. G. Balint-Kurti for immediately providing us with the results of their calculations on N_2O potential energy surfaces. We thank Wei-Chen Chen for all the valuable discussions and his tutelage (of M.K.P.) in chemical concepts. We also thank M. C. Liang, G. A. Blake, and Y. L. Yung for sharing with us the results of their work before publication. We are indebted to Matt Johnson for his very helpful comments.
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Issue or Number:D21
Record Number:CaltechAUTHORS:20150316-150216501
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Official Citation:Prakash, M. K., J. D. Weibel, and R. A. Marcus (2005), Isotopomer fractionation in the UV photolysis of N2O: Comparison of theory and experiment, J. Geophys. Res., 110, D21315, doi:10.1029/2005JD006127
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:55816
Deposited By: Tony Diaz
Deposited On:16 Mar 2015 22:27
Last Modified:10 Nov 2021 20:50

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