  We present a general analysis for the interaction of a probe laser radiation
with a coherently prepared molecular Raman medium. We describe a general
formalism which includes dispersive effects, such as group velocity and group
velocity dispersion (GVD). When dispersion is negligible, the analysis is
especially simple and insightful. We show that molecular oscillations result in
a modulated instantaneous susceptibility of the medium. The effect of the
time-varying susceptibility on a probe laser pulse is two-fold: the output
frequency becomes modulated because of the time-varying phase velocity, and the
pulse shape becomes deformed because of the time-varying group velocity. We
identify two mechanisms for pulse compression: (1) Frequency chirping with
subsequent pulse compression by normal linear GVD (possibly in the same medium)
and (2) Compression due to the time-varying group velocity. We analyze various
aspects of pulse compression in the coherent Raman medium and derive
conservation relations for this process. When we consider a probe laser pulse
which is much shorter than the molecular oscillation period, we observe
frequency chirping, compression, or stretching of this pulse, depending on its
relative timing with respect to the molecular oscillations. Based on our
analysis, we propose a method for selective compression or frequency conversion
of single ultrashort pulses.
