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.
Highlights
Generation of plasma in gases by intense laser pulses is a well-known optical phenomenon
The electron energy distribution function is a topic of continuing interest to the plasma physics, fusion, and astrophysical communities because they can play an important role in the formation, evolution, and radiative properties of a wide variety of plasma sources
The set of equations which was mentioned previously is solved numerically under the experimental conditions of Labazan and Milosevic [21]. In this instance the initial lithium vapor density was assumed to be 3 × 1013 - 7 × 1014 cm‒3, the laser power changes from 1 × 105 to 1 × 106 W·cm‒2, the temperature varied from 760 K - 880 Kand the energy of single pulse (E) varied from 1→4 m Joule
Summary
Generation of plasma in gases by intense laser pulses is a well-known optical phenomenon. Labazan and Milosevic [21] studied the processes which occurred in lithium vapor under two-photon excitation of the Li(3d) state at 639.1 nm. In their experiment they used a pulsed dye laser pumped with excimer laser to excite the Li(3d) level by two-photon absorption at 639.1 nm. Our interest in this work is primary to investigate the Electron Energy Distribution Function (EEDF) in lithium vapour excitation at 2s→3d two-photon resonance to determine conditions (namely lithium atom densities, laser energy, etc.) for the excitation processes because of its eventual importance in understanding ionization-guiding discharge experiments. The molecular channel does not contribute to ionization processes according to the experimental conditions [20] [21]
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