Electronic excitations of helium bilayers on a metal substrate

Neutral one-hole, one-electron excitations converging to the He1s ionization limit of He bilayers on a metal surface are studied with linearly polarized synchrotron radiation and time-of-flight techniques. Comparing signals of emitted electrons, emitted He atoms in metastable excited states, and fluorescence photons as a function of photon energy and polarization with theoretical results, a detailed picture of the photoexcitation is obtained. We show that the width of the photoabsorption peaks is governed in an anisotropic way by the large zero point motion of the He atoms; broadening directly correlates with the shape of the electronic clouds of the absorbing states.