  We report on optical excitations in the magnetically ordered phases of
multiferroic Fe$_{1.86}$Zn$_{0.14}$Mo$_3$O$_8$ in the frequency range from
10-130 cm$^{-1}$ (0.3-3.9 THz). In the collinear easy-axis antiferromagnetic
phase below $T_N=50$~K eleven optically active modes have been observed in
finite magnetic fields, assuming that the lowest-lying mode is doubly
degenerate. The large number of modes reflects either a more complex magnetic
structure than in pure Fe$_{2}$Mo$_3$O$_8$ or that spin stretching modes become
active in addition to the usual spin precessional modes. Their magnetic field
dependence, for fields applied along the easy axis, reflects the irreversible
magnetic-field driven phase transition from the antiferromagnetic ground state
to a ferrimagnetic state, while the number of modes remains unchanged in the
covered frequency region. We determined selection rules for some of the AFM
modes by investigating all polarization configurations and identified magnetic-
and electric-dipole active modes as well. In addition to these sharp
resonances, a broad electric-dipole active excitation band, which is not
influenced by the external magnetic field, occurs below $T_N$ with an onset at
12 cm$^{-1}$. We are able to model this absorption band as a vibronic
excitation related to the lowest-lying Fe$^{2+}$ electronic states in
tetrahedral environment.
