Study of Electron Dynamics Controlling the Threshold Intensity Dependence on the Gas Pressure in FIR Laser-Induced Breakdown of Molecular Oxygen: Effect of Loss Processes

The current study is dedicated to investigating the electron dynamics of the breakdown and plasma generation in molecular oxygen at a pressure extended from 4.6 to 75 kPa. The breakdown is motivated by far-infrared laser source operational at λ = 10.591 µm, (hν ~ 0.12 eV) with pulse width 2τ = 64 ns (FWHM) (Camacho et al. in J Phys D Appl Phys 41(10):105201, 2008). This experiment presumed the presence of a new electron ignition mechanism to produce a specific density of seed electrons in the interaction region as a substitution of the negligible involvement of the photoionization process. The analysis is grounded on adapting the electron cascade model given in our past paper (Evans and Gamal in J Phys D Appl Phys 13(8):1447, 1980). This model well thought out the determination of the threshold intensity as a function of gas pressure taken into account the possible physical processes which may take place in the interaction volume. In doing so, the governing differential equation that defines the variation of the energy of electrons during the laser pulse is solved numerically together with a set of rate equations presenting the change of population of the excited states. The calculated breakdown threshold intensity showed a reasonable agreement with the measured ones, where both indicated weak dependence over the tested pressure range. This behavior is resolved by studying the individual effect of each loss processes involved in the model on the threshold intensity concerning the experimentally assumed density of the initial electrons corresponding to the tested gas pressure range. Besides, to evaluate the precise involvement of the action of the single loss process to the mechanism responsible for plasma production, computations of the temporal variation of the density of electrons are carried out in the presence and absence of the individual loss process.