Ultrathin (5-25 nm thick) nickel films were investigated experimentally by
the picosecond biharmonic pumping method. Characteristic side resonances in
the wing of the dependence of the self-diffraction efficiency on the frequ
ency detuning of the biharmonic pumping components were observed. A model o
f the electronic part of the cubic nonlinear susceptibility was constructed
. It takes into account the real structure of the spectrum of the electroni
c states of nickel, its quantum-size renormalisation and spin splitting, in
terband and intraband relaxation, as well as selection rules for the electr
onic transitions. For interband polarisation relaxation times T-2 > 300 fs,
a qualitative agreement was obtained between the calculated relationships
and the experimental data. In the frequency-degenerate case, points at whic
h the nonlinear response should be absent were found on the frequency scale
.