  For nearly a decade, it has been a mystery why the small average number of
photons absorbed per atom from an ultrashort laser pulse is able to induce a
strong magnetization within a few hundred femtoseconds. Here we resolve this
mystery by directly computing the number of photons per atom layer by layer as
the light wave propagates inside the sample. We find that for all the 24
experiments considered here, each atom has more than one photon. The so-called
photon shortage does not exist. By plotting the relative demagnetization change
versus the number of photons absorbed per atom, we show that depending on the
experimental condition, 0.1 photon can induce about 4% to 72% spin moment
change. Our perturbation theory reveals that the demagnetization depends
linearly on the amplitude of laser field. In addition, we find that the
transition frequency of a sample may also play a role in magnetization
processes. As far as the intensity is not zero, the intensity of the laser
field only affects the matching range of the transition frequencies, but not
whether the demagnetization can happen or not.
