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.

Photochemistry of CH3Cl: Dissociation and CH···Cl Hydrogen Bond Formation

“…8 Although a large percentage of these molecules photodissociate in the stratosphere via this low-lying channel, the surviving molecules can reach the ionosphere, where vacuum UV (VUV) opens new dissociation channels after excitation into higher states. 9 Moreover, because some of these high excitations have large transition moments, 10,11 the action spectrum is shifted to high energies, making them specially relevant from the photochemical point of view.…”

“…The lowest photodissociation channel of clorofluorcarbons (CFCs) and halocarbons usually involves excitations from chlorine lone pairs ( n ) into C–Cl antibonding (σ*) orbitals − induced by UVB or low UVC radiation . Although a large percentage of these molecules photodissociate in the stratosphere via this low-lying channel, the surviving molecules can reach the ionosphere, where vacuum UV (VUV) opens new dissociation channels after excitation into higher states . Moreover, because some of these high excitations have large transition moments, , the action spectrum is shifted to high energies, making them specially relevant from the photochemical point of view.…”

Valence and Rydberg states of CH3Cl: a MR-CISD study

“…7 Although a large percentage of these molecules photodissociate while in the stratosphere, the surviving molecules can reach the ionosphere, where vacuum UV (VUV) is present, and in this case the Rydberg states play a very important role. 8 Thus, a detailed knowledge of its excited states is of fundamental importance to understand its photochemistry.…”

Photodissociation of the δ(0,0) and δ(1,0) bands of nitric oxide in the stratosphere and the mesosphere: A molecular‐adapted quantum defect orbital calculation of photolysis rate constants

“…[10] The theoretical tool employed in the calculation of the photoabsorption cross sections has been the Molecular Orbital Quantum Defect (MQDO) approach, formulated within our group a decade ago [Martı ´n et al, 1996a]. This methodology has proved, in a number of works [Velasco et al, 2003a[Velasco et al, , 2003bMayor et al, 2004Mayor et al, , 2005aBustos et al, 2004], to yield quantitative intensities for electronic transitions involving one or two Rydberg states. Very recently, rotational resolution has been accurately introduced in the procedure [Mayor et al, 2005b].…”