Valley-Orbit Splitting of Free Excitons? The Absorption Edge of Si

Measurements of the wavelength derivative of the indirect absorption edge in Si show features associated with the creation of free excitons in states $n=1\mathrm{} \mathrm{and} 2$ as well as in the exciton continuum. The spectrum shows no valley-orbit splitting of the excitons, contrary to previous suggestions, and it is shown theoretically that such splitting of free excitons cannot exist. It is found also that previous determinations of the energies of higher exciton states ($n>\mathrm{}1\mathrm{}$) were in error. Finally, LO phonon participation has been identified for the first time in the indirect edge of Si.