On the role of the pre-ionization mechanism in the optical breakdown of molecular oxygen induced by CO2 laser: Numerical investigation

A numerical study is presented to investigate the threshold intensity dependence on the gas pressure in the breakdown of molecular oxygen induced by CO2 laser radiation with a wavelength of 10.591 mu m and a pulse FWHM of 64ns [Camacho et al., J. Phys. D: Appl. Phys. 41, 105201 (2008)]. This experiment allowed for a new method of providing an adequate density of the seed electrons required to ignite the breakdown mechanism. The investigations are based on a modification of a previously developed model [Gamal and Omar, Radiat. Phys. Chem. 62(5), 361-370 (2001)], which solves a differential equation and designates the time evolution of the electron energy distribution numerically and a set of rate equations that describe the change of the excited state population. The model considered inverse bremsstrahlung absorption as the main electron energy gain process leading to oxygen breakdown. As an interesting finding, in comparing the calculated and measured thresholds as a function of gas pressure, computations could precisely reveal the densities of the created seed electrons at each value of the tested pressures. The unsystematic variation of these densities with the gas pressure clarified the origin of the slight oscillations observed in the measured thresholds. Analyzing the electron energy distribution function and its parameters in three gas pressure regions could determine the correlation between the gas pressure and the electron gain and loss processes responsible for oxygen breakdown. This analysis explained the violation from the simple p(-1) law observed experimentally in the relation between threshold intensity and gas pressure.