Angle-resolved uv photoemission and electronic band structures of the lead chalcogenides

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Abstract
Angle-resolved photoemission spectra of single-crystal PbS, PbSe, and PbTe have been measured employing 16.85- and 21.22-eV excitation energies. A quite general theory of photoemission, aimed at interpreting the experimental results, is outlined. Two simple models (the direct- and nondirect-transition model) derived from this theory under rather drastic assumptions can only partially account for the experimental data. In contrast, angle-resolved photoemission spectra calculated employing the more sophisticated "weighted-indirect-transition model" agree well with the measured ones. The basic improvement inherent in this model is the consideration of the finite photoelectron lifetimes and the resulting actual relaxation of momentum conservation in the direction normal to the surface. Furthermore, the experimental results are compared in detail with the predictions of four independent band-structure calculations. Best overall agreement is found with the calculations from first principles (augmented plane wave, orthogonal plane wave).