Anti-Stokes absorption

Anti-Stokes absorptions can also occur these give a band with higher frequency than the incident light source. However, this requires the system to have initially been in a vibrationally excited state, as shown in Figure 6.10. The thermal population of the excited vibrational states is usually low at normal laboratory temperatures, and so the anti-Stokes bands have lower intensity than the Stokes bands. 

Classical Treatment of Stokes/Anti-Stokes Absorption... 

New metliods appear regularly. The principal challenges to the ingenuity of the spectroscopist are availability of appropriate radiation sources, absorption or distortion of the radiation by the windows and other components of the high-pressure cells, and small samples. Lasers and synchrotron radiation sources are especially valuable, and use of beryllium gaskets for diamond-anvil cells will open new applications. Impulse-stimulated Brillouin [75], coherent anti-Stokes Raman [76, 77], picosecond kinetics of shocked materials [78], visible circular and x-ray magnetic circular dicliroism [79, 80] and x-ray emission [72] are but a few recent spectroscopic developments in static and dynamic high-pressure research. 

The third common level is often invoked in simplified interpretations of the quantum mechanical theory. In this simplified interpretation, the Raman spectrum is seen as a photon absorption-photon emission process. A molecule in a lower level k absorbs a photon of incident radiation and undergoes a transition to the third common level r. The molecules in r return instantaneously to a lower level n emitting light of frequency differing from the laser frequency by —>< . This is the frequency for the Stokes process. The frequency for the anti-Stokes process would be + < . As the population of an upper level n is less than level k the intensity of the Stokes lines would be expected to be greater than the intensity of the anti-Stokes lines. This approach is inconsistent with the quantum mechanical treatment in which the third common level is introduced as a mathematical expedient and is not involved directly in the scattering process (9). 

The hyperpolarizability tensor is obtained in a way similar to the case of SHG. However, the selection rules for an SFG resonance at the IR frequency implies that the vibrational mode is both IR and Raman active, as the SF hyperpolarizability tensor elements involve both an IR absorption and a Raman-anti-Stokes cross-section. Conversely, the DFG hyperpolarizability tensor elements involve an IR absorption and a Raman-Stokes cross-section. The hyperpolarizability tensor elements can be written in a rather compact form involving several vibrational excitations as [117] ... 

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.

For higher-order absorption processes, the intensity of the anti-Stokes luminescence also depends on higher powers of the excitation intensity (see Exercise 1.6). 

If an electronic transition results in both absorption and luminescence, then the Stokes shift is the difference (in either wavelength or frequency units) between the band maxima. If the luminescence occurs at a shorter wavelength, the difference is often referred to as an anti-Stokes shift. 

Occasionally absorption occurs from a higher vibrational level of S0. This leads to anti-Stokes lines, in which the fluorescence is at shorter wavelengths than... 

The inverse Raman effect was detected in liquids 93> soon after the discovery of the stimulated Raman effect. When a medium is irradiated simultaneously by intense monochromatic light from a giant-pulse laser and by a continuum, sharp absorption lines are observed on the anti-Stokes side of the laser line, and under special conditions also on the Stokes side 94 >. McLaren and Stoicheff 95) used the intense fluorescence from a dye solution excited by frequency-... 

Figure 16.18. Energy-level scheme showing the infrared absorption, Rayleigh scattering and Raman scatterings (Stokes and anti-Stokes). Virtual states are not real states of the molecule but are states created when the photons interact with the electrons and cause polarization, being that the energy of these states is determined by the frequency of light.

A schematic energy-level diagram of Cr3+ and Tm3+ in YAG together with the luminescence and absorption spectra of Cr3+ are shown in fig. 18. Three primary Cr3+ - Tm3+ energy transfer pathways can be identified thermally activated energy transfer from the 4T2 state (4T2 ET), thermally activated energy transfer from the 2E anti-Stokes phonon sidebands (2E anti-Stokes ET), and temperature-independent energy transfer from the zero phonon and Stokes phonon sidebands of the 2E state (2E Stokes ET). 

Because Raman scattering involves vibrational and rotational modes within a sample, its explanation must necessarily involve a quantum mechanical treatment [21]. This is certainly true when the incident light corresponds to an intrinsic region of absorption in the sample, but it is also required for a quantitative analysis of the simpler Stokes and anti-Stokes Raman scattering, which is the subject of the discussion in this chapter. A detailed quantum mechanical understanding of Raman scattering, however, is not necessary for the applications that are of interest in this book, and for that reason, only a brief account is offered here. 

M in concentration. This is in the range required for single-molecule detection. These sensitivity levels have been obtained on colloidal clusters at near-infrared excitation. Figure 10.3 is a schematic representation of a single-molecule experiment performed in a gold or silver colloidal solution. The analyte is provided as a solution at concentrations smaller than 10-11 M, Table 10.1 lists the anti-Stokes/Stokes intensity ratios for crystal violet (CY) at 1174 cm-1 using 830-nm near-infrared radiation well away from the resonance absorption of CY with a power of 106 W/cm2 [34]. CV is attached to various colloidal clusters as indicated in the table. Raman cross sections of 10-16 cm2/molecule or an enhancement factor of 1014 can be inferred from the data. 

Up-conversion is a process by which two photons of lower energy are subsequently converted into a luminescence photon of higher energy (typically, two IR photons giving rise to one visible photon, e.g. in Er111-containing compounds). This anti-Stokes process is usually observed for ions embedded in solids and is made possible by various mechanisms, such as the now classical excited state absorption mechanism (ESA), or sequential energy transfers (ETU for... 

Reduction or elimination of fluorescence High resolution High throughput Good frequency accuracy Collect Stokes and anti-Stokes Raman simultaneously Both IR and Raman capabilities on same instrument. Absorptions in the NIR Black-body emissions in IR Lower scattering intensity due to v4 effect Difficult to detect low concentrations of impurities... 

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. 

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