Direct Observation of Hot-Electron Spectra from Laser-Excited Sodium Vapor

A sodium atomic beam with density ${10}^{13}$ ${\mathrm{cm}}^{-3\mathrm{}}$ was illuminated by cw dye laser radiation (a few watts per square centimeter) tuned to the $D_2$ resonance line. In the energy spectrum of the emitted electrons, several lines were observed between 4 and 7 eV. Their positions and intensities indicate that seed electrons are produced either via associative ionization or via collisional ionization from excited states populated by energy-pooling collisions. These electrons are then heated through successive superelastic collisions with excited $3p$ atoms.