Raman scattering molecules

Billmann and Otto have shown that the potential dependence of SERS depends also on the presence of other constituents beside the Raman scattering molecule. Thus, pyridine produced a lower signal at -0.8 V than at -1.0 V in the presence of cyanide, while in its absence the inverse was seen. Similarly, the cyanide signal (at 2113 cm ) was higher at -0.8 V than at -1.0 V in the presence of pyridine, and vice versa in its absence. This behavior can perhaps be understood on the basis of competition of the two species over the available surface. The cyanide is more strongly adsorbed at the less cathodic potentials. 

Considered is a Raman scattering molecule located at coordinate r outside of a spherical particle of radius a. If the electromagnetic wave of frequency oil incident on that small sphere, the total field outside the sphere is equivalent to EJir, l) plus the field Eij (r, l). This field polarizes the molecule and induces a dipole moment. 

Figure Bl.2.2. Schematic representation of the polarizability of a diatomic molecule as a fimction of vibrational coordinate. Because the polarizability changes during vibration, Raman scatter will occur in addition to Rayleigh scattering.

Nie S and Emory S R 1997 Probing single molecules and single nanoparticles by surface-enhanced Raman scattering Science 275 1102-6... 

Kneipp K, Wang Y, Kneipp H, Itzkan I, Dasari R R and Feld M S 1996 Approach to single molecule detection using surface-enhanced Raman scattering ICORS 98 XVth Int. Conf on Raman Spectroscopy ed S A Asher and P B Stein (New York Wley) pp 636-7... 

Xu FI, B]erneld E J, Kail M and Bdr]esson L 1999 Spectroscopy of single hemoglobin molecules by surface enhanced Raman scattering Phys. Rev. Lett. 83 4357-60... 

Another related issue is the computation of the intensities of the peaks in the spectrum. Peak intensities depend on the probability that a particular wavelength photon will be absorbed or Raman-scattered. These probabilities can be computed from the wave function by computing the transition dipole moments. This gives relative peak intensities since the calculation does not include the density of the substance. Some types of transitions turn out to have a zero probability due to the molecules symmetry or the spin of the electrons. This is where spectroscopic selection rules come from. Ah initio methods are the preferred way of computing intensities. Although intensities can be computed using semiempirical methods, they tend to give rather poor accuracy results for many chemical systems. 

In a diatomic or linear polyatomic molecule rotational Raman scattering obeys the selection rule... 

The similarity between a two-photon absorption and a Raman scattering process is even closer. Figure 9.27(a) shows that a Raman transition between states 1 and 2 is really a two-photon process. The first photon is absorbed at a wavenumber to take the molecule from state 1 to the virtual state V and the second photon is emitted at a wavenumber Vj,. 

Resonance Raman Spectroscopy. If the excitation wavelength is chosen to correspond to an absorption maximum of the species being studied, a 10 —10 enhancement of the Raman scatter of the chromophore is observed. This effect is called resonance enhancement or resonance Raman (RR) spectroscopy. There are several mechanisms to explain this phenomenon, the most common of which is Franck-Condon enhancement. In this case, a band intensity is enhanced if some component of the vibrational motion is along one of the directions in which the molecule expands in the electronic excited state. The intensity is roughly proportional to the distortion of the molecule along this axis. RR spectroscopy has been an important biochemical tool, and it may have industrial uses in some areas of pigment chemistry. Two biological appHcations include the deterrnination of helix transitions of deoxyribonucleic acid (DNA) (18), and the elucidation of several peptide stmctures (19). A review of topics in this area has been pubHshed (20). 

Fig. 4.56. Schematic diagram of a SERS-active substrate and the measurement arrangement. Alumina nanoparticles are deposited on a glass surface and produce the required roughness. A thin silver layer is evaporated on to the nanoparticles and serves for the enhancement. Organic molecules adsorbed on the silver surface can be detected by irradiation with a laser and collecting the Raman scattered light.

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