Studies of Electron Energy Distribution Function (EEDF) in Lithium Vapor Excitation at 2S→3D Two-Photon Resonance

Mohamed A. Mahmoud, Kholoud A. Hamam

Physics Department, Faculty of Science, Sohag University, Sohag, Egypt.
Physics Department, Sciences Faculty for Girls, King AbdUlaziz University, Jeddah, KSA.

DOI: 10.4236/opj.2014.48020                

Abstract

We have developed a computational model which quantitatively studies the Electron Energy Distribution Function (EEDF) in laser excited lithium vapor at 2s→3d two-photon resonance. A kinetic model has been constructed which includes essentially all the important collisional ionization, photoionization, electron collisions and radiative interactions that come into play when lithium vapor (density range 10¹³ - 10¹⁴ cm^-3) is subject to a sudden pulse of intense laser radiation (power range 10⁵ - 10⁶ W·cm^-2) at wavelength 639.1 nm and pulse duration 20 ns. The applied computer simulation model is based on the numerical solution of the time-dependent Boltzman equation and a set of rate equations that describe the rate of change of the formed excited states populations. Using the measured values for the cross-sections and rate coefficients of each physical process considered in the model available in literature, relations are obtained as a function of the electron energy and included in the computational model. We have also studied the time evolution and the laser power dependences of the ion population (atomic and molecular ions) as well as the electron density which are produced during the interaction. The energy spectra of the electrons emerging from the interaction contains a number of peaks corresponding to the low-energy electrons produced by photoionization and collisional ionization such as assosicative and Penning ionization processes. The non-equilibrium shape of these electrons occurs due to relaxation of fast electrons produced by super-elastic collisions with residual excited lithium atoms. Moreover, a reasonable agreement between McGeoch results and our calculations for the temporal behaviour of the electron density is obtained.

Keywords

Two-Photon Resonance Excitation, Laser, Lithium, Collisional Ionization, Energy Pooling Photoionization, Electron Energy Distribution Function

Conflicts of Interest

The authors declare no conflicts of interest.

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