  We report the ultrafast dynamics of the 47.4 THz coherent phonons of graphite
interacting with a photoinduced non-equilibrium electron-hole plasma. Unlike
conventional materials, upon photoexcitation the phonon frequency of graphite
upshifts, and within a few picoseconds relaxes to the stationary value. Our
first-principles density functional calculations demonstrate that the phonon
stiffening stems from the light-induced decoupling of the non-adiabatic
electron-phonon interaction by creating the non-equilibrium electron-hole
plasma. Time-resolved vibrational spectroscopy provides a window on the
ultrafast non-equilibrium electron dynamics.
