Microwave scattering from a plasma produced by REMPI in argon

This paper explores the properties of signals generated by microwave scattering trom a laser generated Resonance Enhanced Multi-Photon lonization (REMPI) plasma in low density neutral argon. The ionization process occurs through a nanosecond pulsed laser driven three photon resonant absorption to an excited electronic state of argon followed by a single photon ionization from that state. By using time dependent perturbation theory and empirical values of the Einstein coefficients of different levels of neutral argon, the three photon excitation cross section is calculated. The single photon ionization cross section from the excited state is then calculated using quantum defect theory. The time dependence of the ionization and recombination processes are monitored with the microwaves and modeled from the rate equations of electrons, ions and excited atoms, which are solved numerically. Good agreement has been achieved between the results of the theory, the computational model, and the experimental data.