Multiple photon processes in SF₆

Thesis (Ph.D.)--University of Rochester. College of Engineering and Applied Science. Institute of Optics, 1979. This thesis was digitized by the Institute of Optics in 2014 and was determined to have lapsed into the public domain. If you are the author and have questions about the digitization of your work, please contact Kari Brick, Graduate Program Coordinator for the Institute of Optics, at kbrick@ur.rochester.edu. Other contact information for the Institute is available at http://www.optics.rochester.eduThis thesis is an investigation of multiple-photon phenomena which may occur during the interaction of certain polyatomic molecules with powerful infrared lasers. The present work is aimed at elucidating the basic mechanisms involved in the multiple photon absorption (MPA) and multiple photon dissociation (MPD) in these molecules. During MPA some molecules absorb more than one photon, whose frequency is close to that of a vibrational normal mode of the molecule. In certain instances, if a high level of vibrational excitation of the molecule is attained through MPA, the molecule may dissociate. This process is then called MPD. The molecule SF₆ was chosen as representative of a whole class of molecules, in conformity with the common practice in the relevant literature. The laser employed in the experiments was a pulsed CO₂ laser. The investigation involved experimental as well as theoretical work. Experimentally, the quantitative dependence of MPA and MPD on system parameters, such as gas pressure, laser energy fluence and laser wavelength, was observed and analyzed. A marked influence of collisions on MPA/MPD was observed. A few collisions (sometimes only a fraction of a collision per molecule on the average) were shown to significantly enhance MPA/MPD. "Collisionless" MPA and MPD have aroused the widest interest for applicative as well as fundamental reasons. The results of the experiments presented in this thesis shed light on the importance of collisions and on the conditions under which truly "collisionless" MPA/MPD may be achieved. The theoretical part of this thesis consists of the development of a phenomenological model for "collisionless" MPA. The model was based on the assumption that the absorbed energy was confined to the pumped vibrational mode and was not redistributed among the other modes as long as MPA/MPD took place. A phenomenological absorption bandwidth of the excited molecule was introduced. The bandwidth was construed as being the result of vibration-vibration and vibration-rotation interactions. Quantum mechanical formalism for a modified harmonic oscillator, together with computer calculations, were applied to predict the average level of excitation and the population distribution in energy space. Finally, these predictions were compared with the experimental results obtained previously. With only one variable parameter at our disposal, a remarkable fit between theory and experiments was obtained. The narrow population distribution predicted by the model has potential applications in various fields, including laser induced chemistry and laser up- and down- conversion