  We observe anharmonic decay of the photoexcited coherent A1g phonon in
bismuth to points in the Brillouin zone where conservation of momentum and
energy are satisfied for three-phonon scattering. The decay of a coherent
phonon can be understood as a parametric resonance process whereby the atomic
displacement periodically modulates the frequency of a broad continuum of
modes. This results in energy transfer through resonant squeezing of the target
modes. Using ultrafast diffuse x-ray scattering, we observe build up of
coherent oscillations in the target modes driven by this parametric resonance
over a wide range of the Brillouin zone. We compare the extracted anharmonic
coupling constant to first principles calculations for a representative decay
channel.
