Electron densities and temperatures in a xenon afterglow with heavy-ion excitation

The time dependence of electron densities and temperatures in a heavy-ion-beam-excited gas target has been determined. A pulsed beam of 89-MeV ³²S ions, 2-ns pulse width, was used for the excitation of xenon at pressures of 500, 1000, and 1500 hPa. This type of excitation leads to the formation of a unique afterglow plasma. Electron densities and temperatures are of the order of those found in typical low-pressure gas discharges, but the electrons, ions, and excited atoms are embedded in a cold, dense gas. Molecule formation is therefore an important process. The dissociative recombination of Xe₂+* molecules, which leads to the emission of the second excimer continuum of xenon at a wavelength of 172 nm, was used as a probe at times longer than 200 ns after termination of the beam pulses. The emission of light at shorter times was modeled by solving the coupled rate equations of the collisional and radiative processes involved.