Red-shift of the excitonic level in two beam absorption spectra of GaSe

Optical absorption spectra of GaSe samples immersed in superfluid helium have been measured as a function of the optical excitation intensity, by using a two beam technique. As the electron-hole density increases a broadening and a red-shift of the fundamental excitonic absorption line occur. We compare our observation with the results of a simplified first order theory which accounts for electron-hole correlation effects in static screening approximation. The optical spectra, both calculated theoretically and observed experimentally, suggest that the observed Mott transition is, at finite temperatures, smooth and continuous. Our simple model explains surprisingly well the observed optical spectra.