  The exciton-phonon coupling in highly oriented pyrolytic graphite is studied
using resonant inelastic X-ray scattering (RIXS) spectroscopy. With ~ 70 meV
energy resolution, multiple low energy excitations associated with coupling to
phonons can be clearly resolved in RIXS spectra. Using resonance dependence and
the closed form for RIXS cross-section without considering the intermediate
state mixing of phonon modes, the dimensionless coupling constant g is
determined to be 5 and 0.4, corresponding to the coupling strength of 0.42 eV
+/- 40 meV and 0.21 eV +/- 30 meV, for zone center and boundary phonons
respectively. The reduced g value for zone-boundary phonon may be related to
its double resonance nature.
