Neutral excitations of physisorbed helium

We study neutral excitations of He mono- and bilayers on Pt(111), Ru(001), Ag(111) and Cu(111) surfaces, and sandwich layers of He on well ordered Ne and Ar monolayers on these substrates. We apply narrow bandwidth undulator radiation of variable polarization (BESSY, UE-112-PGM-1) in combination with a Time-Of-Flight technique that enables us to detect electrons, ions, metastable atoms and fluorescence photons. Therewith we obtain comprehensive data sets on excitation and decay processes below the 1s ionization edges of these layers. Compared to the isolated atom, strong inter- and intralayer coupling is found that is well reproduced by calculations. We observe hybridization of s and p final states, and for higher principal quantum numbers also admixture of d states. We demonstrate that the comparison of data from different samples as indicated above is indispensable for a complete understanding of these processes.