Effusion. VI. Velocity Distributions of Gaseous CsCl from Near-Ideal and Right-Circular Cylindrical Orifices

Abstract

Velocity distributions have been measured for CsCl(g) effusing from double-oven effusion cells equipped with a near-ideal and nonideal right-circular cylindrical orifices. The back oven contained CsCl(s or l), and the front oven possessed an orifice. A multiple slotted-disk velocity selector was used to determine velocity distributions. The disks had slots cut at an angle (with respect to the selector axis) to provide improved transmission. Detectors utilized surface ionization. Data were acquired for states of CsCl(g) (as determined by temperatures of the back and front ovens) ranging from the molecular flow region into the transition region and for several effusion angles. The velocity distribution data may be summarized as follows: near-ideal orifice—data are in accord with the Maxwellian theoretical velocity distribution, and data show no dependence on Knudsen number (ratio of mean free path to orifice diameter) or effusion angle; nonideal cylindrical orifices—deviations from the Maxwellian distribution (with too many high-speed molecules or too few slow molecules) occurred at low pressures under molecular flow conditions, were largest at effusion angles near the axis of the orifice, and were enhanced by increasing pressure in the transition region. These data and previously published angular number distributions may be explained by introducing some specular reflection on the orifice wall for the nonideal orifices. Both sets of data were combined to yield angular momentum distributions.