Resonance Raman enhancement

Quantum effects are observed in the Raman spectra of SWCNTs through the resonant Raman enhancement process, which is seen experimentally by measuring the Raman spectra at a number of laser excitation energies. Resonant enhancement in the Raman scattering intensity from CNTs occurs when the laser excitation energy corresponds to an electronic transition between the sharp features (i.e., (E - ,)" type singularities at energy ,) in the ID electronic DOS of the valence and conduction bands of the carbon CNT. 

Bolis et al (43) reported volumetric data characterizing NH3 adsorption on TS-1 that demonstrate that the number of NH3 molecules adsorbed per Ti atom under saturation conditions was close to two, suggesting that virtually all Ti atoms are involved in the adsorption and have completed a 6-fold coordination Ti(NH3)204. The reduction of the tetrahedral symmetry of Ti4+ ions in the silicalite framework upon adsorption of NH3 or H20 is also documented by a blue shift of the Ti-sensitive stretching band at 960 cm-1 (43,45,134), by a decrease of the intensity of the XANES pre-edge peak at 4967 eV (41,43,134), and by the extinction of the resonance Raman enhancement of the 1125 cm-1 band in UV-Raman spectra (39,41). As an example, spectra in Figs. 15 and 16 show the effect of adsorbed water on the UV-visible (Fig. 15), XANES (Fig. 16a), and UV-Raman (Fig. 16b) spectra of TS-1. 

The resonance Raman enhancement profiles In Figures 7 and 8 show that the maximum Intensity of the Fe-O-Fe symmetric stretch falls to correspond to a distinct absorption maximum In the electronic spectrum. This Implies that the 0x0 Fe CT transitions responsible for resonance enhancement are obscured underneath other, more Intense bands. Although strong absorption bands In the 300-400 nm region (e > 6,000 M" cm"l) are a ubiquitous feature of Fe-O-Fe clusters, the Raman results make It unlikely that they are due to 0x0 -> Fe CT. An alternative possibility Is that they represent simultaneous pair excitations of LF transitions In both of the... 

The different schemes above can also be distinguished by using TRRR techniques. At the moment this technique might take more effort than the optical methods. However, it can be done with more accuracy since vibrational Raman bands are better resolved than optical absorption bands. A detailed study of the observed change of the resonance Raman spectrum with time and with probe laser frequency should, in principle, enable one to distinguish between the different schemes given above. This will be possible if the photoproducts in a certain scheme are produced with different rates or have different optical absorption maxima (and thus different resonance Raman enhancement profiles). 

Various possible time resolved techniques are discussed which enable one to measure the vibrational spectra (and what they entail of structural information) of the distinct transient intermediates formed in different photochemical decomposition schemes and at different times (in the sec-picosec range). The techniques make use of 1) the difference in the time development behavior of the different intermediates, 2) the difference in the absorption maxima and thus the difference in the resonance Raman enhancements for the different intermediates, and 3) the laser power. The techniques use one or two lasers for the photolytic and probe sources as well as an optical multichannel analyzer as a detector. Some of the results are shown for the intermediates in the photosynthetic cycle of bacteriorhodopsin. 

In summary, resonance Raman enhancement occurred in reduced molybdenum oxides when the exciting frequency corresponded to the Mo5+ Mo6+ IVCT transition at about 2 eV. This resonance enhancement was found to be crucial for catalyst characterization during operation. Excitation of the Raman spectra of such reduced oxides is of course also possible with other laser frequencies (vide supra). However, then the overall Raman scattering efficiency is much smaller, and small concentrations (e.g., in a catalytic reaction experiment) may not be detectable. 

The visible spectrum of this intermediate consists of a band at Amax = 614 nm. Excitation at 614 nm gives resonance Raman enhanced bands at 416 and 666 cm-1 that shift to 408 and 638 cm-1 upon addition of H21sO, indicating exchange with water. These bands are unaffected by the addition of D20. This behavior is consistent with an FeO stretch and the shift observed upon substitution agrees with the expected shift of 29 cm-1. The second peak at 416 cm-1 was attributed to a metal-ligand vibration coupled to the iron-oxo stretch. 

Cotton, T.M., Uphaus, R.A., and Mobius, D. (1986) Distance dependence of surface-enhanced resonance Raman enhancement in Langmuir-Blodgett dye multilayers. Journal of Physical Chemistry, 90, 6071-6073. 

The origin of resonance Raman enhancement is explained in terms of Eq. 1.201. In normal Raman spectroscopy, Vo is chosen in the region that is far from the electronic absorption. Then, v vq, and a.p is independent of the exciting frequency vq. In resonance Raman spectroscopy, the denominator, Vq, becomes very small as vq approaches v. Thus, the first term in the square brackets of Eq. 1.201 dominates all other terms and results in striking enhancement of Raman lines. However, Eq. 1.201 cannot account for the selectivity of resonance Raman enhancement since it is not specific about the states of the molecule. Albrecht [103] derived a more specific equation for the initial and final states of resonance Raman scattering by... 

Raman studies have been carried out on the potassium, cesium and ammonium salts and on the [OsjOClio] " ion in acid solution resonance Raman enhancement was observed for the v, (symetric 6s—6—Os stretch) mode. IR data have also been reported. A molecular orbital scheme for the bonding in [OsjOCfo] " can be derived from that proposed by Dunitz and Orgel for the analogous [RU2OCI10] species. Such a scheme has been used to interpret the electronic and vibrational spectra of [OsjOCljJ ... 

The reorganization energies for ET between the dinuclear Cua center and cytochrome a in cytochrome c oxidase have been estimated to be between 0.15-0.5 eV from ET experiments. This estimate means that the A for the Cua center is even lower than that for the mononuclear cupredoxin center. An electron transfer study of an engineered Cua azurin showed that the calculated A for Cua in the engineered azurin is 0.4eV, which is about half that of the blue copper center in native azurin. " Furthermore, an excited-state distortion analysis of a resonance Raman enhancement profile for Cua also confirmed that the inner-sphere A for Cua is about 50% that of a blue copper center., i7,ioi lower A for Cua can be attributed to the valence delocalization of the dinuclear center the bond length distortions associated with the redox reaction can be spread over two metal ions and thus reduced to 1/2 that of the mononuclear copper or valence-trapped dinuclear centers. 

---