AN ELECTRON SPIN POLARIZED HELIUM-EXCITED(TRIPLET-2S) METASTABLE BEAM FOR USE IN SURFACE STUDIES

A source of electron spin polarized He*(2('3)S) metastable atoms is described that provides a thermal energy beam with a flux up to 8 x 10('12) metastables/sec/sterad at a polarization P(,z) (TURN) 50%. The polarization is created by optical pumping and can be simply reversed or modulated without changing the beam trajectory or flux. The source is housed in a system of three differentially pumped vacuum chambers which allows the beam to be used for surface experiments in an ultra high vacuum environment. The polarization is measured with a Stern-Gerlach type analyzer which also serves as a useful diagnostic of beam composition and verifies the purity of the 2('3)S beam. The beam is demonstrated not to contain any significant admixture of 2('1)S metastables, ions, fast neutrals, or photons. Unless care is exercised, however, it is possible to produce a beam of energetic ground state helium which is of much greater intensity than the metastable beam. The conditions under which this occurs are described. A brief overview of metastable deexcitation spectroscopy (MDS) is given and a proposed first experiment involving the investigation of surface magnetism of Ni(110) and the necessary apparatus for this experiment are described