A self-inversion of emission lines of free excitons in CdS single crys
tals has been observed for the first time. The inversion occurs after
a heating from 77 to 295 K and then from 295 to 395 K, followed by a r
apid cooling to liquid-nitrogen temperature. This inversion is attribu
table to a self-absorption of the radiation in a thin surface layer in
which the exciton concentration is relatively low. The exciton distri
bution is nonuniform because of electric fields which arise during the
charging of surface states.