  We employed femtosecond pump probe technique to investigate the dynamics of
coherent optical phonons in iron garnet. A phenomenological symmetry-based
consideration reveals that oscillations of the terahertz T2g mode are excited.
Selective excitation by a linearly polarized pump and detection by a circularly
polarized probe confirm that impulsive stimulated Raman scattering (ISRS) is
the driving force for the coherent phonons. Experimental results obtained from
ISRS measurements reveal excellent agreement with spontaneous Raman
spectroscopy data, analyzed by considering the symmetry of the phonon modes and
corresponding excitation and detection selection rules.
