Rotational levels of shallow acceptor states: The undulation spectra of N in GaP

We report the first direct evidence of rotational levels of vibronic states in solids. These levels are associated with the ground states of shallow acceptors and provide the basis of a detailed understanding of the fine structure or "undulations" in emission spectra derived from bound excitons in GaP. This explanation is contrasted with one proposed recently by Hopfield, Kukimoto, and Dean. © 1972 The American Physical Society.