Measurement of the 2(3)S population in a He-4 beam by means of laser-induced fluorescence

The population of the 2(3)S level in a He-4 beam (E(Lab) = (12.5-17.5) keV) has been measured by means of laser-induced fluorescence. For optical pumping the 2(3)S --> 3(3)P(o) transition (lambda = 389 nm) was chosen. The 2(3)S projectiles were produced by quasi-resonant charge transfer collisions of He+ in lithium vapour. Densities between 0.5 x 10(6) cm(-3) and 12 x 10(6) cm(-3) were investigated. By comparison with the beam neutralization it could be shown that the fraction of 2(3)S particles in the beam can be manipulated by variation of the lithium density in the neutralization chamber. An explanation of this behaviour is proposed on the basis of collisionally-induced de-excitation of the 2(3)S state. An effective cross section of the de-excitation could be derived with a simple model of the de-excitation processes.