  We study ultrafast magnetization quenching of ferromagnetic iron following
excitation by an optical vs a terahertz pump pulse. While the optical pump
(photon energy of 3.1 eV) induces a strongly nonthermal electron distribution,
terahertz excitation (~4 meV) results in a quasi-thermal perturbation of the
electron population. The pump-induced spin and electron dynamics are
interrogated by the magneto-optic Kerr effect (MOKE). A deconvolution procedure
allows us to push the time resolution down to 130 fs, even though the driving
terahertz pulse is more than 0.5 ps long. Remarkably, the MOKE signals exhibit
an almost identical time evolution for both optical and terahertz pump pulses,
despite the three orders of magnitude different number of excited electrons. We
are able to quantitatively explain our results using a model based on
quasi-elastic spin-flip scattering. It shows that in the small-perturbation
limit, the rate of demagnetization of a metallic ferromagnet is proportional to
the excess energy of the electrons, independent of the precise shape of their
distribution. Our results reveal that the dynamics of ultrafast demagnetization
and of the closely related terahertz spin transport do not depend on the pump
photon energy.
