FIG. 2.

Feynman diagram describing the evolution of the density matrix for FSRS. The time delay, Δt, between the actinic pump (ω₁) and probe (ω₃) beams determines the delay between the electronic excitation of the sample and the initiation of the Raman transition. The vibrational dephasing time ${\mathit{T}}_2^{\mathrm{vib}}$ determines the linewidth of the Raman peaks. Use of a picosecond duration Raman pump (ω₂) allows the Raman spectrum to sample the full T₂ time of the vibrational transition and thereby produces lineshapes determined by the intrinsic molecular limits.