Survival probabilities of metastable He atoms scattered from clean and adsorbate-covered Pd, Cu, and W surfaces were found to be only of the order between 10 -6 and 10 -3 . A novel theoretical formalism is presented which on the basis of a "dimensionality effect" is able to qualitatively explain the experimental observations. The radiationless de-excitation of metastable inertgas atoms at solid surfaces gives rise to electron emission containing information on the electronic structure of the Outmost atomic layer [1]. For the fraction of metastable atoms reflected without de-excitation the available results are unclear and contradictory [2-7] : Craig and Dickinson [3] observed no reflection from Ge(100) surfaces which were either clean or covered by adsorbed oxygen in the monolayer range. He* reflection of the order of a few percent could only be detected with oxidized samples. Directed reflection was reported for Mo(110), but without any information on the absolute yield [4]. Titley and Delchar [5] reported on rather high reflection coefficients (up to ~50%) for He* on W(110). However, since in this work no time-of-flight measurements could be made, it was suggested that the effects were mainly caused by groundstate He with high kinetic energy [6]. Quite recently Roussel et al. [7] were unable to detect any reflection from Ni surfaces and estimated an upper limit of the order of 10 -4 .
This paper reports on the probability SP of rectastable He* (2 1S and 2 3S) atoms with thermal kinetic energy to survive the collision with both clean and adsorbate-covered metal surfaces in their electronically excited state. The numbers were found to be extremely