Cooper-minimum effects in the photoionization cross sections of4dand5delectrons in solid compounds

We present experimental determinations of the cross section for photoionization for $4d$ and $5d$ valence states in transition and noble metals and at the interfaces that these metals form with elemental semiconductors (group IV) in the photon-energy range $h\nu =70-200\mathrm{}$ eV. A description of the technique used to obtain cross sections $\sigma$ from photoemission measurements is provided, and a phenomenological discussion of the strong dependence of $\sigma$ on the distortion of the initial-state $d$ wave function in the solid environment with respect to the atomic case and in solid-state compounds with respect to pure metals is proposed. A strong reflection of the chemistry involving the $d$ states on the photoionization probability is found, in particular, at the Cooper minimum. The use of photoemission at the Cooper minimum to study interfaces and compounds involving $4d$ and $5d$ states is discussed with the support of data relevant to commonly investigated systems, as well as an application to a less-studied refractory-metal—Si interface, namely, Mo/Si.