Time-resolved photoemission spectroscopy, using high
order harmonics, is used to measure the energy relaxation
rate of hot electrons in α-SiO2 with sub-picosecond
time resolution. Our results indicate that electrons of
30 eV kinetic energy in the conduction band relax at a
rate which is at least two orders of magnitude lower than
the one of photo-excited carriers of a few eV.
As a result, we give insight in the relaxation process
of hot electrons and show that impact ionization probability
per unit time is only of the order of 1/40 ps−1,
in very strong contrast with the much higher value generally
assumed in models of optical breakdown.