Cars time resolved measurement of H- rovibrational populations in an H- source

The presence of high fractions of negative hydrogen ions in magnetic multicusp discharges has been attributed to the dissociative attachment of slow electrons to highly vibrationally excited molecules. Since the formation of atomic hydrogen requires a time of the order of 10-3s, the calculation predicts that the transient populations in vibrational levels v > 3 can exceed steady state populations. The study of transient populations by CARS has been therefore undertaken. The experiments have been made at a discharge current of 10 A and two values of the gas pressure, 55 μbar and 5.5 μbar, which we will denote as ''high'' and ''low'' pressure, respectively. Measurements with a steady state discharge have also been performed and are reported. Excitation temperatures and relaxation time are measured