f−d excitons in the optical spectrum of semiconducting SmS and Sm1−xGdx alloys

Thermomodulation studies of the optical properties of Sm 1− x Gd x S alloys are presented for x ⩽ 0.15. A new interpretation of the low energy absorption peaks as excitons, where a Sm 5 d electron is bound to a localized hole in the 4 f shell, is strongly suggested.