  We study the relaxation dynamics of photoexcited Fe-II complexes dissolved in
water and identify the relaxation pathway which the molecular complex follows
in presence of a hydration shell of bound water at the interface between the
complex and the solvent. Starting from a low-spin state, the photoexcited
complex can reach the high-spin state via a cascade of different possible
transitions involving electronic as well as vibrational relaxation processes.
By numerically exact path integral calculations for the relaxational dynamics
of a continuous solvent model, we find that the vibrational life times of the
intermittent states are of the order of a few ps. Since the electronic
rearrangement in the complex occurs on the time scale of about 100 fs, we find
that the complex first rearranges itself in a high-spin and highly excited
vibrational state, before it relaxes its energy to the solvent via vibrational
relaxation transitions. By this, the relaxation pathway can be clearly
identified. We find that the life time of the vibrational states increases with
the size of the complex (within a spherical model), but decreases with the
thickness of the hydration shell, indicating that the hydration shell acts as
an additional source of fluctuations.
