  The temperature dependence of ultrafast photoinduced reflectivity transients
is reported in Nd$_{0.5}$Sr$_{0.5}$MnO$_{3}$ thin film. The photoinduced
reflectivity shows a complex response with very different temperature
dependences on different timescales. The response on the sub-ps timescale
appears to be only weakly sensitive to the 270K-metal-insulator phase
transition. Below $\sim 160$ K the sub-ps response displays a two component
behavior indicating inhomogeneity of the film resulting from the substrate
induced strain. On the other hand, the slower response on the 10-100 ps
timescale is sensitive only to the metal-insulator phase transition and is in
agreement with some previously published results. The difference in the
temperature dependences of the responses on nanosecond and $\mu $s timescales
indicates that thermal equilibrium between the different degrees of fredom is
established relatively slowly - on a nanosecond timescale.
