Arabian Journal for Science and Engineering https://doi.org/10.1007/s13369-020-05077-2 RESEARCH ARTICLE-PHYSICS 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 Yosr E. E.-D. Gamal 1 · Khaled A. Elsayed 2 · Olodia A. Nassef 1 Received: 19 June 2020 / Accepted: 23 October 2020 © King Fahd University of Petroleum & Minerals 2020 Abstract 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. Keywords Oxygen breakdown · CO 2 laser · Vibrational and rotational excitations · Inverse bremsstrahlung absorption · Stepwise collisional ionization · Dissociative attachment 1 Introduction Since the invention of lasers, massive theoretical models [3–8] are performed to investigate the experimental measure- ments carried out to study spark ignition in gases under the single effect of laser beams [9–13]. The models are centered on the numerical solution of the equations, which describe the variation of the electron energy adopting classical and B Khaled A. Elsayed Kaelsayed@iau.edu.sa 1 National Institute of Laser Enhanced Science (NILES), Cairo University, Giza, Egypt 2 Department of Basic Engineering Sciences, College of Engineering, Imam Abdulrahman Bin Faisal University, P.O. Box 1982, Dammam 31441, Saudi Arabia quantum aspects. In solving these equations, a density of seed electrons must be present in the focal volume before or along with firing on the laser source. These electrons accumulate energy from the laser field through the inverse bremsstrahlung absorption process to initiate a cascade ion- ization mechanism, leading ultimately to gas breakdown. For a detailed description of the breakdown development, the models include the various loss processes which might take place in the interaction region. These losses are acting to retard or offset the electron growth rate and play a vital role in the determination of the breakdown threshold concerning the nature of the irradiated gas, its pressure, as well as the focusing geometry of the laser beam used in the experiment. For laser sources operating in the near infrared (NIR) to the vacuum ultra-violet (VUV) of the electromagnetic spec- trum range, these seed electrons are expected to be generated 123