Photodissociation Rate Constants for VUV Processes of CF3Cl and CF2Cl2 in the Upper Atmosphere. A MQDO Study

Abstract

The availability of data concerning the upper atmosphere is essential for an understanding of both the change in solar activity and the different processes that have direct effects on the biosphere, in particular, those with harmful environmental consequences. The main goal of the present work is the theoretical analysis of the photodissociation of two chlorofluorocarbon (CFC) compounds that play an important role in the chemical and energetic balance of the upper atmosphere, CF3Cl and CF2Cl2. Given that the molecular absorption cross section is directly linked to a molecule's photodissociation rate, we have first calculated cross sections and then have used the achieved values to determine the mechanisms of the photofragmentation processes that CF3Cl and CF2Cl2 undergo at specific vacuum-UV (VUV) wavelengths that are present in the ionosphere. We have focused our study on the calculation of the kinetic rate constants for the processes that can give rise to cations upon photoabsorption, because they are difficult to determine experimentally. Through the analysis of the photodissociation rate constants, we have been able to make a comparative study of the dissociative behavior of CF3Cl and CF2Cl2 when these two CFC's undergo absorption within the VUV spectral region. The atmospheric photodissociation rate constants of CF3Cl and CF2Cl2 have been calculated with the Molecular-adapted Quantum Defect Orbital (MQDO) approach as a function of the altitude and at different solar zenith angles. Altitudes from 60 to 150 km, which fall within the D and E layers of the ionosphere, have been considered. No earlier data of this kind have been found in the literature.