Optical spectroscopy of crystal-field transitions in the molecular magnet Fe8

Dipolar transitions in the magnetic cluster ${\mathrm{Fe}}_{8}$ and its derivative ${\mathrm{Fe}}_{8}$-perchlorate $({\mathrm{Fe}}_{8}\mathrm{PCL})$ have been studied by coherent-source spectroscopy at frequencies 1.5\char21{}33 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$. Four magnetic transitions are observed between 2 and 4 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ in ${\mathrm{Fe}}_{8}$ and ${\mathrm{Fe}}_{8}\mathrm{PCL}.$ The temperature behavior of the resonance frequencies, linewidths, and intensities identifies them as transitions inside the spin $S=10$ ground state which is split by the crystal field. In both compounds these transitions are characterized by a Gaussian rather than a Lorentzian line shape, which originates from a random distribution of the internal dipolar magnetic fields with a width of approximately 70 mT for ${\mathrm{Fe}}_{8}$ and 140 mT for ${\mathrm{Fe}}_{8}\mathrm{PCL}.$ We find an excellent agreement of experimental findings and theoretical description. Several absorption lines are discovered in ${\mathrm{Fe}}_{8}$ at frequencies above 15 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ which could originate from electronic transitions between different multiplets or lattice vibration modes. No corresponding lines are observed in ${\mathrm{Fe}}_{8}\mathrm{PCL}.$