ATOMIC VELOCITY DISTRIBUTIONS IN A HYDROGEN BEAM EFFUSING OUT OF A RADIO-FREQUENCY DISCHARGE DISSOCIATOR

We have measured atomic hydrogen velocity distributions in an effusive beam coming out of a rf discharge dissociator by using a magnetic deflection technique. Dissociator pressures varied between 0.028 and 0.340 Torr. At low dissociator pressures the measured atomic velocity distributions were narrower than the expected beam Maxwellians; at higher pressures they were indistinguishable from beam Maxwellians at the dissociator wall temperature, indicating full thermalization of the atoms prior to exiting the dissociator. Monte Carlo simulations of the thermalization process within the dissociator reproduce these results, and point out the important role of vibrational excitation of the background hydrogen molecules as an energy loss mechanism. Our results are significant when designing magnetic state selectors for spin- or hyperfine-polarized atomic hydrogen beams.