Silicon absorption spectrum
The effect of correlations in the optical excitations of electronic systems is known to be important in a large number of systems, and cannot be addressed within the Random-Phase Approximation (RPA), one of the simplest theories that describe excitations optical excitations. With a two-particle description, based on solving the Bethe-Salpeter equation (BSE), we can introduce correlations between an optically excited electron and the vacancy it leaves in the previously filled electronic state. This way, it is possible to get a good agreement between the calculated absorption spectrum and the experimental spectrum (below). The same method is being used to describe the properties of bound excitons in a number of systems, like GaAs and crystalline polyacetylene, a semiconducting polymer, with promising results.
Silicon absorption spectrum