Effusion. I. Angular Number Distributions of Gaseous CsCl from a Near-Ideal Orifice into Vacuum

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Abstract
The angular number distributions of CsCl(g) from a double-oven effusion cell with a near-ideal orifice have been measured as a function of the state of the gas using a surface ionization detector. The orifice was 0.50 mm in diameter with a length of 0.013 mm; L/d equals 0.026. The state of the gas was varied over values of the Knudsen number, K, (ratio of mean-free path to orifice diameter) from 63.38 to 0.16. When K≥8.16, the theoretical angular number distribution given by the cosine law of effusion (slightly corrected for orifice length) was observed to be valid. For runs at K less than 8.16, deviations from the theoretical distributions were observed with increasing probability of effusion in the forward direction. The results are compared with results of previous investigations, and a possible explanation for the observed deviations is suggested.