Effect of surrounding gas on velocity distribution function of molecules in molecular beam

Abstract

Molecular-beam methods have become widely used in recent times for the study of flows of rarefied gases [1]. However, the very first experiments with molecular beams for agasdynamic source [2] showed that the measured intensities fell below theoretical predictions. Most devices for the creation of a molecular beam by means of a gasdynamic source have pumping equipment of comparatively low capacity and beam formation in them occurs with residual gas present. It was shown [3] that the residual gas penetrates into the jet and significantly reduces the intensity of the molecular beam. This and subsequent work [4, 5] were confined to measurements of intensity (density) and there are no data in the literature on the effect of residual gas on other parameters of the distribution function. The present work was devoted to a study of the effect of residual gas on the distribution function in a molecular beam defined from a jet in the scattering mode [6]. The work was performed on the small molecular-beam generator [7] and on the VS-4 low-density gasdynamic tube [8] at the Institute of Thermal Physics, Siberian Branch, Academy of Sciences of the USSR. Measurements of the distribution function by the time-of-flight method [9] were performed on the small molecular-beam generator and measurements of gas density on the VS-4.