  In this work, we study the heat transfer from electron to phonon system
within a five monolayer thin epitaxial Pb film on Si(111) upon fs-laser
excitation. The response of the electron system is determined using
time-resolved photoelectron spectroscopy while the lattice excitation is
measured by means of the Debye-Waller effect in time-resolved reflection
high-energy electron diffraction. The electrons lose their heat within 0.5 ps
while the lattice temperature rises slowly in 3.5 to 8 ps, leaving a gap of 3-7
ps. We propose that the hidden energy is transiently stored in high-frequency
phonon modes for which diffraction is insensitive and which are excited in 0.5
ps. Within a three-temperature model we use three heat baths, namely electrons,
high-frequency and low-frequency phonon modes to simulate the observations. The
excitation of low-frequency acoustic phonons, i.e., thermalization of the
lattice is facilitated through anharmonic phonon-phonon interaction.
