Anisotropy of core-level photoemission from InSe, GaSe, and cesiated W(001)

The angular dependence of photoemission from the $\mathrm{In} 4d$ and $\mathrm{Ga} 3d$ core levels in the layer compounds InSe and GaSe has been measured. Azimuthal anisotropies are found to be quite strong. Asymmetries associated with the strong polarization of the synchrotron radiation source are discussed, and it is shown that more information is obtained with an analyzer sampling photoelectrons propagating out of the plane of incidence rather than in it. Similar measurements of the $5p$ core levels of Cs adsorbed on the (001) face of W show some indications of anisotropy at low coverages. The relevance of these results to the proposed use of photoelectron diffraction for surface-structure determination is discussed.