The ultrathin (17-25 nm thick) continuous Ni films deposited on K8 glass an
d ZrO2 substrates are experimentally studied. The degenerate four-photon sp
ectroscopy showed typical resonances in the dependence of the self diffract
ion efficiency on the wavelength of pump components. It is shown within the
framework of the model of electronic cubic nonlinear susceptibility - taki
ng into account the real structure of the spectrum of electronic states of
Ni, its quantum-size renormalisation and spin splitting, the inter- and int
raband relaxation, saturation, and the selection rules for the electronic t
ransitions - that the electronic subsystem makes the dominant contribution
to the nonlinear response of Ni films to picosecond laser pulses (20 ps lon
g) in the 620-634 nm spectral range. The interband-polarisation relaxation
time T-2 was found to be similar to 200 fs.