  Far-infrared spectroscopy has been performed on [Fe(2-picolylamine)3]Cl2EtOH
(Fe-pic) single crystals, to probe changes in the molecular vibrations upon the
photoinduced and temperature-induced spin crossovers. Synchrotron radiation has
been used as the farinfrared source to overcome the strong absorption and the
small sizes of the samples. Absorption lines due to FeN6 cluster vibrations,
observed below 400 cm-1, show strong intensity variations upon the crossover
due to the deformation of FeN6 between high-spin and low-spin states. However,
they remain almost unchanged between the photo- and temperature-induced
high-spin states. This is in sharp contrast to the lines at 500-700 cm-1 due to
intramolecular vibrations of the picolylamine ligands, which show marked
variations between the two high-spin states. It is concluded that the most
important microscopic difference between the two high-spin states arises from
the ligands, which is likely to reflect different states of intermolecular
bonding between them.
