ABSOLUTE DETECTION OF METASTABLE RARE-GAS ATOMS BY A CW LASER PHOTOIONIZATION METHOD

A novel, accurate method for the absolute detection of metastable rare gas atoms is described and demonstrated. It involves a direct in situ determination of the electron emission coefficient gamma for impact of the respective metastable atom on a conducting surface. gamma is reliably obtained by a cw two-photon ionization - depletion technique: the reduction DELTA-I(S) in electron current from the detector surface due to efficient photoionization removal of the metastable flux is compared with the photoelectron current DELTA-I(P) (gamma = DELTA-I(S)/DELTA-I(P). The principle of the method, possible realization schemes for the different metastable rare gas atoms and the apparatus are described in detail. The method has been applied so far to metastable Ne* (3s 3P2), Ar* (4s 3P2), and Kr* (5s 3P2) atoms, and corresponding results for gamma, obtained with five different chemically clean, polycrystalline surface materials and at two surface temperatures (300 K, 360 K) are reported. Whereas for Ne*, the value of gamma (almost-equal-to 0.35) showed only a rather weak dependence on the surface material and temperature (as also found for a mixed He* (2(3)S,2(1)S) beam), strong variations in gamma, especially at 300 K, were detected for Ar* and Kr* (values between 0.25 and 0.003). Some applications of the described method, especially with regard to the determination of absolute reaction cross sections involving metastable rare gas atoms, are discussed.