TITLE:
Studies of Electron Energy Distribution Function (EEDF) in Lithium Vapor Excitation at 2S→3D Two-Photon Resonance
AUTHORS:
Mohamed A. Mahmoud, Kholoud A. Hamam
KEYWORDS:
Two-Photon Resonance Excitation, Laser, Lithium, Collisional Ionization, Energy Pooling Photoionization, Electron Energy Distribution Function
JOURNAL NAME:
Optics and Photonics Journal,
Vol.4 No.8,
August
22,
2014
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.