Molecular Beam Studies of Stratospheric Photochemistry

Citation

Moore, Teresa Anne (1998) Molecular Beam Studies of Stratospheric Photochemistry. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/nkqt-qj82. https://resolver.caltech.edu/CaltechTHESIS:07302025-155308477

Abstract

Photochemistry of chlorine oxide containing species plays a major role in stratospheric ozone depletion. This thesis discusses two photodissociation studies of the key molecules ClONO₂ and ClOOCl which were previously thought to only produce Cl-atom ( ozone depleting) products at wavelengths relevant to the stratosphere. The development of a molecular beam source of ClOOCl and the photodissociation dynamics of the model system Cl₂O are also discussed.

In the first chapter, the photochemistry of ClONO₂ is examined at 308 run using the technique of photofragment translational spectroscopy. Two primary decomposition pathways, leading to Cl+ NO₃ and ClO + NO₂, were observed, with a lower limit of 0.33 for the relative yield of ClO. The angular distributions for both channels were anisotropic, indicating that the dissociation occurs within a rotational period.

Chapter two revisits the photodissociation dynamics of Cl₂O at 248 and 308 nm, on which we had previously reported preliminary findings. At 248 nm, three distinct dissociation pathways leading to Cl + ClO products were resolved. At 308 nm, the angular distribution was slightly more isotropic that previously reported, leaving open the possibility that Cl₂O excited at 308 run lives longer than a rotational period.

Chapter three describes the development and optimization of a molecular beam source of ClOOCl. We utilized pulsed laser photolysis of Cl₂O to generate ClO radicals, and cooled the cell to promote three body recombination to form ClOOCl. The principal components in the beam were Cl₂, Cl₂O, and ClOOCl.

In the fourth chapter, the photodissociation dynamics of ClOOCl are investigated at 248 and 308 nm. We observed multiple dissociation pathways which produced ClO + ClO and 2Cl + O₂ products. The relative Cl:ClO product yields are 1.0:0.13 and 1.0:0.20 for ClOOCl photolysis at 248 and 308 run, respectively. The data at 308 nm was more difficult to interpret because of extensive interference from Cl₂ and Cl₂O byproducts. The upper limit for the relative yield of the ClO + ClO channel was 0.19 at 248 nm and 0.31 at 308 nm. These results substantially confirm the current assumption but decrease somewhat the efficiency of the ClOOCl ozone-depleting catalytic cycle. At 248 run, ClOOCl photolysis exhibited novel dissociation dynamics which appeared to depend on the symmetry of the excited state.

Thesis Files