Fluorescence and resonance Raman spectroscopies are closely related. Both are FWM processes and both involve similar excitation frequencies. They differ in the time ordering of the exciting fields and they differ in whether the intermediate state is a coherence or population. Figure 2d shows an example WMEL diagram for stimulated emission of fluorescence. The initial ket and bra side excitations create an excited bb population. This population is stimulated to emit by interaction with the third field which forms the ba output coherence. In contrast, the intermediate state in stimulated Raman spectroscopy is an ag coherence.
Pump-probe methods are also closely related. These methods almost always use FWM processes. Example WMEL diagrams of single color pump-probe methods are shown in Figure 3. Pump-probe methods involve two overlapped excitation pulses- an initial pump pulse that creates the first two interactions in each diagram in figure 3 and a probe pulse that creates the third interaction leading to the final output coherence. Since the output field has the same frequency as the probe field, the two fields interfere. The transmitted intensity of the probe field can either increase because the pump induced stimulated emission (figure 3a) or ground state bleaching (figure 3b) or decrease because the pump created an excited state population that allow excited state absorption. The net change depends on the relative strength of all these processes.
Two color pump-probe experiments use different frequencies for the pump and probe pulses. Figure 3d-f shows the two color pump-probe pathways. The first two electric field interactions represent the pump and create a population. The third interaction is the probe that creates the output coherence. The bleaching, stimulated emission, and excited state absorption pathways are shown in figures 3d; 3e; and 3f, respectively.
The quantum states involved in the pump or probe steps can be electronic/vibronic or vibrational. Electronic states are excited through uv/visible absorption while vibrational states are excited by infrared or Raman (or stimulated Raman) transitions. Consequently, time resolved (TR) pump-probe methods are classified as TR-UV-UV, UV-IR, IR-IR, UV-Raman andIR-Raman, etc. depending upon whether an electronic or vibrational state is pumped or probed. Since Raman is actually a four wave mixing process, the UV-Raman and IR-Raman pump-probe experiments are actually six wave mixing processes. Note that if the two pump interactions are created instead by separate controllable beams, these pump-probe pathways become rephasing stimulated photon echo (figures 4c, e, g) or nonrephasing (figures 4d, f, h) pathways.