Anti-Stokes transitions, energy level

Fig. 2 Jablonski energy level diagram illustrating possible transitions, where solid lines represent absorption processes and dotted lines represent scattering processes. Key A, IR absorption B, near-IR absorption of an overtone C, Rayleigh scattering D, Stokes Raman transition and E, anti-Stokes Raman transition. S0 is the singlet ground state, S, the lowest singlet excited state, and v represents vibrational energy levels within each electronic state.

As already introduced in section I of this chapter, in a CARS process (Figures 7.9a-c see also Figure 7.1c), a Raman transition between two vibrational energy levels of a molecule is coherently driven by two optical laser fields (frequencies co and co) and subsequently probed by interaction with a third field at frequency co, . This generates the anti-Stokes signal at the blue-shifted frequency cars = p- The... 

Let us now turn to two-photon excitation via an intermediary level. In this chapter we restrict ourselves to processes without energy transfer, that is, typical one-ion processes. A recent and intensity-rich example is Eu " in LaOCl (41). Excitation of the Do level of Eu (cf. Fig. 6) does not only yield the usual emission transitions from the Dq level, but also yields anti-Stokes emission from the higher Di,2,3 levels. The intensities of these emissions were at least one order of magnitude smaller (for excitation with a continuous dye laser pumped with an argon ion laser). 

Although each Stokes line and its anti-Stokes counterpart are equally separated from the Rayleigh line, they are not of equal intensity. This is because the intensity of each transition is proportional to the population of the energy level from which the transition originates under equilibrium conditions the ratio of populations is given by the Boltzmann distribution. With the fourth-power dependence on the scattering frequency, the ratio of intensities of the Stokes line and its anti-Stokes partner in a Raman spectrum is given by... 

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