Thesis (M.Sc.) - Cairo University - National Institute of Laser Enhanced Sciences - Department of Laser Applications in Environmental Metrology, Photochemistry , Photobiology and Agriculture

This work presents a study on the breakdown of molecular oxygen by CO₂ laser radiation to determine the threshold intensity dependence on gas pressure as well as the elementary physical processes contributing to the gas breakdown. In doing so, a previously developed model by Evans and Gamal is modified and applied[1, 2]. The model numerically solves a differential equation designates the time evolution of the electron energy distribution function and a set of rate equations that describe the change of excited states population. Owing to the minimal value of the photon energy associated with the used far-IR laser radiation (~ 0.12 eV), photo-ionization (PI) of the ground and excited molecules are excluded. The modified model investigated the results of the experimental measurements carried out by Camacho et al.[3]. In this experiment, a CO₂ laser source operating at a wavelength of 10.591 om and pulse duration of 64 ns is used to irradiate oxygen gas over a pressure range 30-570 torr, for two different experimental conditions namely: presence and absence of a previous breakdown. The former case is used as a source for the generation of seed electrons in the focal volume before firing on the laser at each gas pressure value. These electrons play the same role as the Photo-ionization process

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