We have found that the frequency of in-plane C-C stretching (G mode) evolves with time; it jumps up to higher frequency immediately after photoexcitation (delay time=0), and then gradually decreased to an equilibrium value on picosecond time scale.  The initial upshift arises from the sudden weakening of electron-phonon coupling, or breakdown of Born-Oppenheimer approximation. 

For more detail see http://link.aps.org/abstract/PRB/v77/er121402

On picosecond (ps) timescale, reflectivity of graphite is modified at two different frequencies.  The higher frequency (47 THz) corresponds to the in-plane C-C stretching (period 0.021ps), and the lower frequency (1.3 THz) to the interlayer shear (period 0.77 ps).  Both coherent phonons are generated via Impulsive stimulated Raman scattering (ISRS) process.