By making use of effects of the image charge on projectile trajectorie
s in scattering of a fast beam of metastable He-atoms with keV energie
s from an Al (111) surface under a glancing angle of incidence, we hav
e developed a new method to analyze fast atomic beams with respect to
relative populations of excited metastable states. Based on our experi
mental studies we propose the construction of a corresponding detector
. (C) 1997 American Institure of Physics.