  We have observed a new optical transition in the photoreflectance spectra of
indirect-gap BiOCl thin films, which were grown on SrTiO$_3$ substrates. The
position of this transition is close in energy to its bulk critical point
energy. Moreover, these are significantly lower than a higher-lying direct-type
critical point from an energetic point of view. The spectral line shape
analysis for our observed signal suggests the presence of an excitonic effect
of this compound. We determined its dependence of the optical anomaly on
temperature ranging from 80 K to room temperature. We adopted the Varshni model
for this analysis. At last, we compared phononic properties of BiOCl with those
of an element and binary semiconductors.
