  We report on the low-frequency optical excitations in the multiferroic ground
state of polycrystalline FeCr2S4 in the frequency range 0.3-3~THz and their
changes upon applying external magnetic fields up to 7~T. In the ground state
below the orbital-ordering temperature T_OO=9K we observe the appearance of
several new modes. By applying the external magnetic field parallel and
perpendicular to the propagation direction of the THz radiation, we can
identify the strongest absorptions to be of predominantly electric-dipole
origin. We discuss these modes as the low-energy electronic excitations of the
Fe^{2+} ions (3d^6, S=2) in an tetrahedral environment. The eigenfrequencies
and relative intensities of these absorption lines are satisfactorily
reproduced by our calculation assuming an effective exchange field of 12.8 cm-1
at the Fe^{2+}-ions sites. The direction of the exchange field is found to be
slightly tilted out of the ab-plane. With our approach we can also describe
previously reported results from M\"ossbauer studies and the order of magnitude
of the electric polarisation induced by orbital and non-collinear spin
ordering.
