  Time dynamics of photoexcited electron-hole pairs is important for a number
of technologies, in particular solar cells. We combined ultrafast pump-probe
Raman scattering and photoemission to directly follow electron-hole excitations
as well as the G-phonon in graphite after an excitation by an intense laser
pulse. This phonon is known to couple relatively strongly to electrons.
Cross-correlating effective electronic and phonon temperatures places new
constraints on model-based fits. The accepted two-temperature model predicts
that G-phonon population should start to increase as soon as excited
electron-hole pairs are created and that the rate of increase should not depend
strongly on the pump fluence. Instead we found that the increase of the
G-phonon population occurs with a delay of $\sim$65 fs. This time-delay is also
evidenced by the absence of the so-called self-pumping for G phonons. It
decreases with increased pump fluence. We show that these observations imply a
new relaxation pathway: Instead of hot carriers transferring energy to
G-phonons directly, the energy is first transferred to optical phonons near the
zone boundary K-points, which then decay into G-phonons via phonon-phonon
scattering. Our work demonstrates that phonon-phonon interactions must be
included in any calculations of hot carrier relaxation in optical absorbers
even when only short timescales are considered.
