  We report the observation of photo-induced plasmon-phonon coupled modes in
the group IV-VI semiconductor PbTe using Fourier-transform inelastic X-ray
scattering at the Linac Coherent Light Source (LCLS). We measure the
near-zone-center dispersion of the heavily screened longitudinal optical (LO)
phonon branch as extracted from differential changes in x-ray diffuse
scattering intensity following above band gap photoexcitation.
