  The photoinduced switching of Fe(II)-based spin-crossover complexes from
singlet to quintet takes place at ultrafast time scales. This a priori
spin-forbidden transition triggered numerous time-resolved experiments of
solvated samples to elucidate the mechanism at play. The involved intermediate
states remain uncertain. We apply ultrafast x-ray spectroscopy in molecular
films as a method sensitive to spin, electronic, and nuclear degrees of
freedom. Combining the progress in molecule synthesis and film growth with the
opportunities at x-ray free-electron lasers, we analyze the transient evolution
of the Fe L3 fine structure at room temperature. Our measurements and
calculations indicate the involvement of an Fe triplet intermediate state. The
high-spin state saturates at half of the available molecules, limited by
molecule-molecule interaction within the film.
