A new optical method to detect conduction-electron spin resonance in semiconductors is described. Its main interest is its application to undoped samples in which the properties of free carriers are not perturbed by shallow donor impurities. The $g$ factor of ${10}^8$ photocreated electrons at the bottom of the conduction band in $p$-type GaSb was measured to be $\left|g^{*}\right|=9.3\mathrm{}\pm 0.3$. It is different from most of the previous experimental determinations and from the theoretical prediction $g_{\mathrm{th}}^{}{}_{}{}^{*}=-6.66\mathrm{}$.

• Received 24 May 1971

DOI:https://doi.org/10.1103/PhysRevLett.27.373