Electronic Spectrum of Crystalline Copper

The optical spectrum of Cu is calculated in the random-phase approximation. Energy levels and model wave functions are obtained from Mueller's combined interpolation scheme. It is shown how oscillator strengths can be obtained throughout the Brillouin zone to an accuracy of about 20%. The optical energy differences and oscillator strengths have been computed from a Monte Carlo sample of 2716 independent points distributed throughout the Brillouin zone. The smoothed spectrum is in good agreement with the experimental spectrum above 4 eV. The second peak in the interband spectrum near 5 eV is assigned to $\mathrm{conduction}-\mathrm{band} \left(L_{2^{\prime }}\right)\to \mathrm{conduction}-\mathrm{band} \left(L_1\right)$ transitions, in agreement with the suggestion of Beaglehole. It is proposed that the large peak near 2 eV should be regarded as a virtual exciton resonance induced by final-state vertex corrections.