X-Ray-Like Singularities for Nonequilibrium Fermi Sea

Absorption and emission edge singularities are calculated in the presence of a nonequilibrium Fermi sea ${\mathrm{\mu }}_1$–${\mathrm{\mu }}_2$ instead of 0–μ. The result is used to derive the absorption spectra of highly excited semiconductors. It is also of importance in the problem of x-ray-like singularities as it shows that the well-accepted physical interpretation proposed by Hopfield is actually invalid.