Delayed ionization and excitation dynamics in a filament wake channel in a dense-gas medium

Abstract

A unified theoretical description is developed for the formation of an ionized filament channel in a dense-gas medium and the evolution of electronic degrees of freedom in this channel in the laser pulse wake, as illustrated on an example of high-pressure argon. During the laser pulse, the emerging free electrons gain energy via inverse Bremsstrahlung on neutral atoms, enabling impact ionization and extensive collisional excitation of the atoms. A kinetic model of these processes produces the radial density distributions in the immediate wake of the laser pulse. After the pulse, the thermalized electron gas drives the system evolution via impact ionization (from the ground and excited states) and collisional excitation of the residual neutral atoms, while the excited atoms are engaged in Penning ionization. The interplay of these three processes determines the electron gas cooling dynamics. The local imbalance of the free-electron and ion densities induces a transient radial electric field, which depends critically on the electron temperature. The evolving radial profiles of the electron, ion, and excited-atom densities, as well as the profiles of electron temperature and induced electric field are obtained by solving the system of diffusion-reaction equations numerically. All these characteristics evolve with two characteristic timescales, and allow for measuring the electronic stage of the wake channel evolution via linear and nonlinear light-scattering experiments.