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

Abstract

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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