  We report inelastic light scattering experiments on CaFe_2As_2 in the
temperature range of 4 to 290 K. In in-plane polarizations two Raman-active
phonon modes are observed at 189 and 211 cm-1, displaying A_1g and B_1g
symmetries, respectively. Upon heating through the tetragonal-to-orthorhombic
transition at about T_S=173 K, the B_1g phonon undergoes a discontinuous drop
of the frequency by 4 cm-1 whereas the A_1g phonon shows a suppression of the
integrated intensity. Their linewidth increases strongly with increasing
temperature and saturates above T_S. This suggests (i) a first-order structural
phase transition and (ii) a drastic change of charge distribution within the
FeAs plane through T_S.
