Comment on ``Inverse exciton series in the optical decay of an excitonic molecule''

Tokunaga et al. [Phys. Rev. B Vol. 59, R7837 (1999)] claim the first successful observation of the inverse exciton M series in the emission spectrum of excitonic molecules. We assert that actually such a series was observed as early as 1989 in the beta-ZnP_2 crystal. We show that the objections of Tokunaga et al. against the biexciton nature of the inverse exciton series in beta-ZnP_2 are ungrounded. In particular, their estimations for the ratio of the intensities of M2 and M1 emission lines in this crystal give two order smaller value because they do not take into account the reabsorption of the M1 line photons. We report the observation of additional inverse exciton M' series in the emission spectrum of excitonic molecules in beta-ZnP_2.