A time resolved study of velocity distributions in pulsed molecular beams

We have studied with 12-us resolution the time evolution of the characteristic flow velocity and parallel temperature of a supersonic gas jet produced by a commercial pulsed valve. The gases studied were H2> He, Ne, O2, Ar, and a seed mixture of 5% O2in He. The gases were supplied to the pulsed valve at 100- and 1000-Torr inlet pressure. Our results indicate that the time required to reach an equilibrium flow condition is essentially independent of the gas species used, but is a function of the inlet pressure. Furthermore, the static Mach number obtained in the expansion is found to deviate from that predicted for a cw expansion with the same nozzle Knudsen number Kn0with increasing inlet pressure. This leads to the conclusion that the effective diameter Deffthrough which the gas expands is a function of inlet pressure, and that the observed equilibration time is in fact the time required for the valve opening to reach this effective value. The results obtained should be applicable to any pulsed valve with similar mechanical bandwidth and gas passage geometry. © 1988, American Vacuum Society. All rights reserved.