The anomalous temperature dependence of photoemission from Cu(111)

The theory of temperature dependence of photoemission spectra is reviewed in a systematic manner and an attempt to identify the essential components is made. Using synchrotron light the authors have explored the temperature dependence of the photoemission from the valence band of Cu(111) over a broad energy range (h nu =20 to 85 eV). Remarkable differences in the thermal responses were observed as the photon energy was varied. Particularly unusual was the observation of a direct transition peak which increased in intensity as the temperature was raised. The experimental data support the idea that, under certain circumstances, phonon-assisted transitions from k-space regions where the initial-state bands are flat, e.g. at band edges, may result in spectral peaks, which compensates for the reduced intensities of direct transitions as the temperature is raised. The authors have also performed full numerical calculations in a time-reversed LEED formalism, and the results of these are compared with their experimental data.