  Here we report the experimental evidence of the interactions between the
excitations of the $\pi\to\pi^*$ optical transition and the image potential
states (IPS) of highly oriented pyrolitic graphite (HOPG). By using non-linear
angle resolved photoelectron spectroscopy (NL-ARPES) we show that the IPS
photoemission intensity, the effective mass, and the linewidth exhibit a strong
variation when the photon energy is tuned across the $\pi\to\pi^*$ saddle
points in the 3.1 - 4.5 eV photon energy range. A model based on the
self-energy formalism is proposed to correlate the effective mass and the
linewidth variations to transient many body effects, when a high carriers
density (in the $10^{20}$ cm$^{-3}$ range) is created by the absorption of a
coherent light pulse. This finding brings a clear evidence of a high IPS-bulk
coupling in graphite and opens the way for exploiting the IPS as a sensitive,
nonperturbing probe for the many-body dynamics in materials.
