Photoemission spectral lineshapes from metal overlayers

Photoelectron spectral peaks derived from bulk band-to-band transitions contain information on band structure and quasiparticle lifetime. However, the lineshapes can be significantly distorted by surface photoemission and final-state broadening. The latter is often a dominant source of peak width and may be strongly dependent on the experimental geometry. The use of a thin film sample provides an alternate method of resolving k and removes the dependence on the matrix element and final states from the measurement. Many thin film systems display quantum-well peaks in their photoemission spectra, but in the typical case, the line widths are dominated by interfacial and surface roughness. It has been demonstrated that films that are atomically uniform over the area used for measurement can be grown. In such films broadening due to roughness is eliminated, and the observed peak widths are directly related to the quasiparticle lifetime. The film behaves like an optical interferometer. Imperfection of this interferometer, such as scattering or leakage at the boundaries, introduces additional broadening, but this contribution can be accounted for using a Fabry-Pérot analysis. Quantum-well peaks derived from the sp band can be resolved in films of Ag on Fe(100) to thicknesses in excess of 100 ML. Application of the Fabry-Pérot model to data covering a large thickness range yields an accurate band-structure determination, quasiparticle lifetimes, as well as properties of the well including the interfacial reflectivity and phase shift. Through a study of the temperature dependence of the spectra, the contribution to the lifetime width due to phonon scattering can be determined. This results in a value for the electron-phonon mass-enhancement parameter λ. The observation of quantum well states is not limited to sp-derived states: the Ag d bands also provide discrete states, although the range of film thicknesses over which they are observable is more limited than for the sp-derived states.