Energy anti-Stokes transitions

Suppose that a compound has a Raman-active vibration at vM. If it is illuminated by a probe laser (v) simulataneously with a pump continuum covering the frequency range from v to v + 3,500 cm-1, one observes an absorption at v + vM in the continuum together with emission at v. Clearly, the absorbed energy, h(v + vM), has been used for excitation (/zvM) and emission of the extra energy (hv). This upward transition is called the inverse Raman effect since the normal anti-Stokes transition occurs downward. Because the inverse Raman spectrum can be obtained in the lifetime of the pulse, it may be used for studies of shortlived species (Section 3.5). It should be noted, however, that the continuum pulse must also have the same lifetime as the giant pulse itself. Thus far, the inverse Raman effect has been observed only in a few compounds, because it is difficult to produce a continuum pulse at the desired frequency range. 

A FIGURE 6.17 Raman transitions. An incident photon with frequency may result in a Raman transition to a higher eneigy state (Stokes transition) or a lower energy state (anti-Stokes transition) by way of an intermediate virtual (non-stationary) state. The difference in eneigy between states 3 and 5 shown would be obtained by measuring Vq and the frequency of the emitted radiation and calculating A s 3 =/t(fo — fa). 

In Raman scattering, photons striking a sample are redirected with energies either greater (anti-Stokes transition) or less (Stokes transition) than the original photon energy. 

Hydrogen transfer in excited electronic states is being intensively studied with time-resolved spectroscopy. A typical scheme of electronic terms is shown in fig. 46. A vertical optical transition, induced by a picosecond laser pulse, populates the initial well of the excited Si state. The reverse optical transition, observed as the fluorescence band Fj, is accompanied by proton transfer to the second well with lower energy. This transfer is registered as the appearance of another fluorescence band, F2, with a large anti-Stokes shift. The rate constant is inferred from the time dependence of the relative intensities of these bands in dual fluorescence. The experimental data obtained by this method have been reviewed by Barbara et al. [1989]. We only quote the example of hydrogen transfer in the excited state of... 

Figure 3. Energy schemata of transitions involving vibrational states (a excitation of 1st vibrational state - mid-IR absorption b excitation of overtone vibrations - near-IR absorptions c elastic scattering - Rayleigh lines d Raman scattering - Stokes lines e Raman scattering - Anti-Stokes lines f fluorescence).

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... 

For any vibrational mode, the relative intensities of Stokes and anti-Stokes scattering depend only on the temperature. Measurement of this ratio can be used for temperature measurement, although this application is not commonly encountered in pharmaceutical or biomedical applications. Raman scattering based on rotational transitions in the gas phase and low energy (near-infrared) electronic transitions in condensed phases can also be observed. These forms of Raman scattering are sometimes used by physical chemists. However, as a practical matter, to most scientists, Raman spectroscopy means and will continue to mean vibrational Raman spectroscopy. 

The temperature dependence of the energy transfer rate Wda is related to a changing occupation of the 2E anti-Stokes phonon sidebands and the 4T2 state. On the contrary, pressure significantly increases the energy separation A between the 4T2 and 2E states, whereas the energy of the zero phonon and the vibronic 2E -> 4A2 transitions of Cr3+ change only weakly with pressure. Thus, pressure almost solely influences the occupation of the 4T2 state and with it its contribution to the energy transfer rate, but does not affect the other contributions connected with the 2E state. 

Examples of anti-Stokes data contaminated by an SFG artifact are shown in Fig. 11a and c, where the higher energy C-H stretching transition of neat methanol is pumped at u>ir = 3020 cm-1. The artifact will be centered at col + anti-Stokes emission from methanol vibrational transitions at 3020 cm-1 (actually the higher energy tail of the C-H stretch transition at 2940 cm-1). The spectral and temporal properties of the artifact can be independently characterized by purposely generating SFG in a thin ( 50 pm) slab of KTP placed at the location of the sample. However, the amplitude of the SFG artifact in the spectrum is unknown. 

Figure 11 Examples of transient anti-Stokes spectra with coherent SFG artifacts, obtained from methanol at 300 K with C-H stretch (3020 cm-1) pumping, (a) and (c) Experimental results at two times. The dashed curves represent the (nearly Gaussian) spectrum of the SFG artifact, (b) and (d) Recovered lineshapes with SFG contribution subtracted away. At early time (—1 ps) mainly the higher frequency C-FI stretch is seen. As times passes, energy is redistributed among the two C-FI stretch and the O-FI stretch transitions. (From L. K. Iwaki, unpublished.)...

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). 

In emission the rare-earth vibronic transitions are usually Stokes (i.e., at the lower energy side of the electronic line). The anti-Stokes lines have been observed for some suitable cases, for example LaF3 Gd (143) (see Fig. 34). 

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