الفهرس 
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Abstract
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 μm 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. Since no experimental definition was given for the values of the initial electrons density, a computer program was first to run to calculate the threshold intensity as a function of gas pressure assuming different values of initial electrons density ranging from 1011 cm-3 down to 104cm-3 for each gas pressure value. The calculated thresholds are taking to be those values which showed reasonable agreement with the experimentally measured ones corresponding to the tested gas pressures. Although this agreement validates the applied model, despite the observed slight fluctuations, the obtained results showed different behavior for the variation of the threshold intensity with gas pressure. This relation exhibited obvious violation than that expected from the electron cascade theory (Ithα p-1). This is observed from the almost unvarying values of the threshold intensity over the tested gas pressure range. The slight fluctuations shown in the experimentally measured threshold intensities and confirmed by the calculated ones are referred to the unpredictable values of the initial electrons density achieved in the focal volume at each gas pressure. Moreover, calculations clarified the effect of these initial free electrons on the threshold intensity. To find out the origin of the behavior observed experimentally for the threshold intensity variation with gas pressure, calculations are carried out to determine the electron energy distribution function as well as the temporal variation of its parameters (viz, the electrons density, excited molecules density, excitation rate, ionization rate, electron mean energy ….and so on). The results of these calculations enabled the correlation between gas pressure and the gain and loss processes that control the breakdown of oxygen corresponding to the experimental conditions under investigation. In addition, the analysis indicates the important role played by each loss process to the breakdown phenomenon. It was shown that at high pressures (exceeding 300.0 torr) electron losses due to three body attachment and dissociative attachment have a significant effect on the behavior of the threshold intensity in previous and non-previous breakdown conditions.