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

TL;DR

This paper challenges a previous claim about the first observation of an inverse exciton series, showing it was observed earlier in 1989 in beta-ZnP2 and providing new spectral observations.

Contribution

It corrects the historical record on inverse exciton series and reports new spectral features in beta-ZnP2, clarifying the nature of the emission lines.

Findings

Inverse exciton series observed in 1989 in beta-ZnP2

Objections to biexciton interpretation are unfounded

Additional inverse exciton M' series detected

Abstract

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.