A simple atomic or molecular beam as target for ion-atom collision studies

The performance of simple molecular beam sources as targets for ion-atom collision studies is analysed in respect of beam density, angular and spatial distribution in relation to background gas. The directional properties of single tubes with various length/diameter ratios is compared with that of multi-capillary arrays when both are operating in the molecular flow regime. Conditions are derived for obtaining a well localised gas beam when gas flow is limited by the available pumping speed; the corresponding ratio of molecular densities in the directed beam to the random background density is evaluated for a number of possible arrangements, without the complication of differential pumping.