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 an 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 reported by Gao et al. [X. H. Gao et al., Phys. Rev. A 95, 013412 (2017)]. 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 avenues for diagnostics and control in these settings.

Figure 1

The rates of collisional excitation (blue solid line) and the impact ionization (magenta dashed line) as functions of the electron energy, affected by the respective total cross sections presented by Eqs. (7) and (11) in the text. $\nu$ is in units of $t_0^{-1}$ and $\varepsilon$ is in units of ${\varepsilon }_0$.

Figure 2

Evolution of the electron density distribution over energy during the laser pulse. The dimensionless energy and time variables are described in the paragraph after Eq. (11). The pulse envelope is modeled by the cosine-square function centered at $\tau =12$; the time-dependent laser intensity is shown symbolically as the gray curve. The initial rise of the electron density is due to strong-field ionization; the following buildup is due to impact processes energized by inverse bremsstrahlung.

Figure 3

The total density of the ionized atoms (magenta curve) and the excited atoms (blue curve) vs time during the laser pulse. The normalized laser intensity is shown for reference as the gray curve. The dimensionless time variable is described in the paragraph after Eq. (11).