Determination of the transient electron temperature in a femtosecond-laser-induced air plasma filament

Abstract

The transient electron temperature in a weakly ionized femtosecond-laser-produced air plasma filament was determined from optical absorption and diffraction experiments. The electron temperature and plasma density decay on similar time scales of a few hundred picoseconds. Comparison with plasma theory reveals the importance of inelastic collisions that lead to energy transfer to vibrational degrees of freedom of air molecules during the plasma cooling.

Seeded optically driven avalanche ionization in molecular and noble gases

“…where T b is the background temperature of ions and neutrals [25]. Here the first line on the right-hand side describes heating of the electron gas by the igniter and heater pulses, the second line describes the effect of electron-ion and electron-neutral collisions with the effective scattering rate f e (T e ), and m n is the mass of the neutral atom.…”

“…The key difference for the time scale of our experiment is that a new term must be added to the electron temperature rate equation (2) that describes relaxation of the electron temperature due to the energy transfer to the vibrational degrees of freedom of the molecules. The treatment of the extra vibrational term is described in detail for the case of air by Sun et al [25], and we have adopted their model here. A key observation is that the vibrational term limits the peak electron temperature in air to about 10 000 K, so that ionization in the electron density equation (1) is well described by the Drude term alone.…”

“…Our findings are presented in the next section. The various rate terms and numerical values of the rate constants for the densities and temperatures of the species involved were collected from the open literature [18,19,25]. We confined our simulations to the cases of air and argon as examples of molecular and atomic gases, respectively, since for the case of helium, the peak power of the igniter pulse attainable in the experiments was below the critical power for self-focusing.…”

Characterization of laser-induced air plasmas and thin films using third harmonic generation microscopy

“…In order to investigate the relationship of TH conversion and the electron density of the plasma, we measured the TH signal as a function of the time delay between pump and probe pulses and determined the time-dependent electron density in the plasma by means of a diffraction experiment [6]. We used an focusing lens (L2) for the probe producing a beam waist of .…”

“…1(b), along with the model predictions. The simulations took into account the change of the electron density spatial profile [6] with time delay. To determine the spatial distribution of the electron density, the TH signal was recorded while the plasma was scanned laterally ( direction) across the probe beam.…”