520 Measurement Techniques of Femtosecond Spectroscopy
state |2 by a succession of optical excitations via the dipole allowed transitions
|1→| and |→|2, involving photons of frequencies ω
1
and ω
2
.
Because of the broad bandwidth of the fs pulse, stimulated Raman scattering
can occur through the mixing of various spectral components of the ultrashort
optical pulse [Fig. 10.20(b)]. For example, a molecular vibration is initiated
by the sudden impulse exerted by the electric field of the pulse. The sample
selects a pair of frequency components whose difference is in resonance with
the eigenfrequencies of a Raman transition. This type of Raman scattering is
called “impulsive stimulated Raman scattering” [26]. The fs excitation makes
it possible