Intrapulse impact processes in dense-gas femtosecond laser filamentation

The processes of energy gain and redistribution in a dense gas subject to an intense ultrashort laser pulse are investigated theoretically for the case of high-pressure argon. The electrons released via strong-field ionization and driven by oscillating laser field collide with neutral neighbor atoms, thus effecting the energy gain in the emerging electron gas via a short-range inverse Bremsstrahlung interaction. These collisions also cause excitation and impact ionization of the atoms thus reducing the electron-gas energy. A kinetic model of these competing processes is developed which predicts the prevalence of excited atoms over ionized atoms by the end of the laser pulse. The creation of a significant number of excited atoms during the pulse in high-pressure gases is consistent with the delayed ionization dynamics in the pulse wake, recently discovered by Gao et al.[1] This energy redistribution mechanism offers an approach to manage effectively the excitation vs. ionization patterns in dense gases interacting with intense laser pulses and thus opens new avenues for diagnostics and control in these settings.