Composition of He Metastable Beams Formed by Charge Exchange in He + -Alkali-Metal Collisions at Medium Energy

The composition of He metastable beams formed by charge exchange on alkali-metal vapors is determined by a time-of-flight analysis. The relative abundance of the two components He(2/sup 3/S) and He(2/sup 1/S) is measured in the 100--1500-eV energy range. It is shown that a pure He(2/sup 3/S) beam is obtained with sodium as the exchange target, whereas rubidium is found to be the most efficient target to yield a He(2/sup 1/S) beam. Comparison is made with theoretical predictions.