  We apply grazing-incidence femtosecond x-ray diffraction to investigate the
details of the atomic motion connected with a displacively excited coherent
optical phonon. We concentrate on the low frequency phonon associated with the
charge and orbital order in the mixed valence manganite
La0.25Pr0.375Ca0.375MnO3 for T < 210 K. We measure the response of three
superlattice reflections that feature different sensitivities to the motion of
the unit cell constituents. The results support the assignment to a
translational mode of the Mn4+ atoms together with the oxygen atoms connecting
adjacent Mn4+ sites.
