  By comparing femtosecond laser-pulse-induced spin dynamics in the surface
state of the rare earth metals Gd and Tb we show that the spin polarization of
the valence states in both materials decays with significantly different time
constants of 15\,ps and 400\,fs, respectively. The distinct spin polarization
dynamics in Gd and Tb are opposed by similar exchange splitting dynamics in the
two materials. The different time scales observed in our experiment can be
attributed to weak and strong $4f$ spin to lattice coupling in Gd and Tb
suggesting an intimate coupling of spin polarization and 4f magnetic moment.
While in Gd the lattice mainly acts as a heat sink, it contributes
significantly to ultrafast demagnetization of Tb. This helps explain why all
optical switching is observed in FeGd -- but rarely in FeTb-based compounds.
