Abstract
High‐resolution spectra at 4.2°K show that the shallow impurity 13CC5H6 shifts the C6H6 factor‐group components by 2 cm−1 and increases the linewidth by 5 cm−1 in going from 6% to 50% 13CC5H6. The effect is explained qualitatively by an extension of the Frenkel exciton theory to the mixed‐crystal system. Although transitions to virtual bound exciton states have been predicted, none was observed.