Angle-resolved photoemission spectra of single-crystal PbS, PbSe, and PbTe have been measured employing 16.85and 21.22-eV excitation energies. A quite general theory of photoemission, aimed at interpreting the experimental results, is outlined. Two simple models (the directand 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).
Angular-resolved photoelectron spectra (ARPES) are presented for GeS using 21-eV excitation energy. From the measurements that cover a wide range of angles along the two principal directions in the basal plane of the orthorhombic layered compound GeS E vs k. & curves were constructed. The band structure of GeS was calculated utilizing the empirical pseudopotential method. The E vs ki curves can almost completely be understood in terms of this band structure under the assumption that only peaks in the onedimensional density of states along k1 contribute significantly to the AR, PES spectrum.
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