Energy 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. 

Figure 6.1-5 Stokes transition for continuum resonance Raman scattering in from the initial vibrational state />= 0> to the final state f >= 6> via electronic state B( 77o+). (A) Absolute value of the time overlap < 6 0(t) > as a function of time, (B) excitation profile of this transition [square of the half Fourier transform of < f i t) > as a function of energy] (Ganz and Kiefer, 1993 a).

Fig. 10. Energy level diagram for electronic resonance enhancement in the CARS spectrum of hydroxyl, OH, in which the pump laser is tuned into resonance. Strong enhancement occurs only for allowed downward Stokes transitions leading to a triplet spectrum.

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... 

The quantum mechanical view of Raman scatering sees a radiation field hvo inducing a transition from a lower level A to a level n. If vnlc is the transition frequency, then the inelastically scattered light has frequency v0 — v t. That is, the molecule removes energy hv k from an incident photon. This process corresponds to Stokes scattering. Alternatively, a molecule under-... 

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.

A detailed study of the electronic structure and optical properties was published for the spiro derivative of f-Bu-PBD, Spiro-PBD (40) [108]. The vibronic structure of the lowest energy absorption band is well resolved, in solution as well as in the amorphous him. The 0-0 transition is at 351 nm (3.53 eV), the 0-1 and 0-2 vibronic bands that have a higher oscillator strength, are at 336 nm (3.69 eV) and 318 nm (3.90 eV), respectively. The fluorescence spectrum of this compound is symmetrical to the absorption spectrum with a Stokes shift of 43 nm. 

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