Figure 2.

(A) Representative optical process involved in time-domain Raman spectroscopy. The ultrashort Raman pump pulse inherently possesses a bandwidth that exceeds the energy spacing between vibrational energy levels. Therefore, upon interaction with the molecule, it can induce a Raman transition within its bandwidth, creating a coherent superposition of the vibrational eigenstates. This process is called the impulsive stimulated Raman process. (B) The created superposition state evolves in time, meaning that the coherent nuclear wavepacket moves back and forth along the vibrational coordinate. This can be interpreted as a classical nuclear (vibrational) motion of the molecule.