  We investigated photoinduced ultrafast transient dynamics in different
magnetic phases in CeSb by means of the time resolved magneto-optical
spectroscopy. We observe a distinctive coherent oscillations in the
ground-state antiferromagnetic (AF) phase and the high-magnetic field
ferromagnetic (F) phase. While the AF-phase oscillations frequencies match the
recent Raman scattering findings the F-phase oscillation frequency does not
correspond to the previously observed magnetic excitation. The large
spectroscopic factor, g=3.94, and optical polarization properties suggest that
it corresponds to a previously undetected Ce^{3+} coherent crystal-field state
excitation. The AF-phase oscillations show no magnetic field dependence so
their lattice origin cannot be entirely excluded. The non-oscillatory part of
the transients is qualitatively similar in all investigated magnetic phases
with a faster sub-picosecond dynamics in the ferromagnetic and
ferro-para-magnetic phases and is attributed to differences in the electronic
structure, which affect the photo-excited quasiparticle energy relaxation
kinetics.
