A comparison of electron beam and heavy ion excitation of rare gas-halogen gas mixture

Rare gas-halogen lasers (eg., XeF, KrF) are commonly excited by electron beams having incident energies of 300 keV - 1 meV. New methods of pumping these lasers use heavy ion beams, either directly, or in the form of energetic fission fragments, having energies of 0.5 - 2.0 MeV/amu. Power deposition proceeds by momentum transfer between the projectile (beam electrons or heavy ions) with an orbital electron. Due to the mass difference of the projectiles of the two excitation methods, the kinetics of the power deposition also differ. In this paper, first principles models for the slowing of electron beams and heavy ions in gas mixtures of interest to the excitation of excimer lasers are presented. The authors compare the secondary electron spectra, W values (energy/ion pair), and the performance of an XeF laser excited by each method. Due to the more favorable mass ratio for e-beam excitation, the secondary electron spectrum is more energetic than with heavy ion excitation. As a result, the efficiency of ionization is higher and the W value for e-beam excitation is lower. The fraction of ionization caused directly by the beam projectiles is larger with heavy ion excitation, as the ions slow in increments of energy not significantly greater than the ionization potential. These differences in excitation methods, though, do not significantly affect the efficiency of excitation of excimer lasers due to the efficiency with with deposited energy flows to the upper laser level, through either the neutral or ion channels