Spatial profiles of effusive molecular beams and their dependence on gas species

The spatial profiles of molecular beams formed by both single and multicapillary sources have been investigated for He, Ne, Ar, Kr, H 2 and N 2 . The profile measurements were undertaken at distances from the source exit and driving pressures which are typical of those used in atomic collision experiments (1-5 mm and 0.02-10 Torr respectively). The experimental results indicate the clear superiority of a capillary array source over a single tube source, of similar overall dimensions, in producing a well collimated molecular beam over a wide range of driving pressures and distances from the capillary exit. They also indicate that there is a measurable dependence of the beam profile on gas species, particularly at the higher pressures studied. The significance of this result for the relative flow, cross section normalization technique is discussed.