Ultrafast signatures of exciton-phonon coupling in TiSe$_2$

The complex coupling between charge carriers and phonons is responsible for diverse phenomena in condensed matter. We apply ultrafast electron diffuse scattering to unravel these couplings in both momentum and time by following the response of semi-metallic 1T-TiSe$_2$ to selective photo-carrier doping. Wavevector-specific renormalization of a phonon frequency is followed by wavevector-independent electron-phonon equilibration. These results implicate strong exciton-phonon coupling and reveal the underlying mechanism driving the second order charge-density wave transition in the normal phase.