In order to gain some insight on the electronic relaxation mechanisms
occuring in diamond under high intensity laser excitation and/or VUV
excitation, we studied experimentally the pulsed conductivity induced by
femtosecond VUV pulses, as well as the energy spectra of the photoelectrons
released by the same irradiation. The source of irradiation consists in highly
coherent VUV pulses obtained through high order harmonic generation of a high
intensity femtosecond pulse at a 1.55 eV photon energy (titanium-doped sapphire
laser). Harmonics H9 to H17 have been used for photoconductivity (PC) and
harmonics H13 to H27 for photoemission experiments (PES). As the photon energy
is increased, it is expected that the high energy photoelectrons will generate
secondary e-h pairs, thus increasing the excitation density and consequently
the PC signal. This is not what we observe : the PC signal first increases for
H9 to H13, but then saturates and even decreases. Production of low energy
secondary e-h pairs should also be observed in the PES spectrum. In fact we
observe very few low energy electrons in the PES spectrum obtained with H13 and
H15, despite the sufficient energy of the generated free carriers. At the other
end (H21 and above), a very intense low energy secondary electron peak is
observed. As a help to interprete such data, we realized the first ab initio
calculations of the electronic lifetime of quasiparticles, in the GW
approximation in a number of dielectrics including diamond. We find that the
results are quite close to a simple "Fermi-liquid" estimation using the
electronic density of diamond. We propose that a quite efficient mechanism
could be the excitation of plasmons by high energy electrons, followed by the
relaxation of plasmons into individual e-h pairs