Direct measurement of the electron density in electron-beam-irradiated Ar-F2gas mixtures by time-resolved interferometry

Abstract

The secondary-electron density in electron-beam-irradiated Ar-${\mathrm{F}}_2$ gas mixtures has been measured for the first time using time-resolved interferometry at 9.6 μm. Measurements were performed for gas mixtures with various ${\mathrm{F}}_2$ concentrations between 0.0025% and 2% and total pressures between 0.5 and 2 atm. The e-beam pulse was 350 nsec long at an energy of 150 keV and current density ranging between 0.025 and 6.3 A/${\mathrm{cm}}^2$. From the dependence of the measured steady-state electron density on fluorine concentration, the rate constant for electron attachment to fluorine at zero electric field has been determined for the first time. For a given e-beam current density, the attachment rate constant is found to be a decreasing function of the fluorine concentration, while for a given ${\mathrm{F}}_2$ concentration it is found to be an increasing function of the e-beam current density. The results are interpreted as a manifestation of the variation of the electron energy distribution as a function of ${\mathrm{F}}_2$ concentration and e-beam current density. The predictions of a theoretical kinetic model for calculation of the electron distribution function, taking into account the removal of slow electrons by attachment, are compared with the experimental results. It is concluded that for the range of the experimental parameters presented above, the net effect of adding fluorine to the gas mixture is an increase in the average electron energy, caused by removal of low-energy electrons by attachment.