Raman Spectroscopy. An introduction to Raman Spectroscopy. It is the shift in wavelength of the inelastically scattered radiation that provides the chemical and structural information.

Raman shifted photons can be of either higher or lower energy, depending upon the vibrational state of the molecule under study. A simplified energy diagram that illustrates these concepts is shown on the right. Stokes radiation occurs at lower energy (longer wavelength) than the Rayleigh radiation, and anti- Stokes radiation has greater energy. The energy increase or decrease is related to the vibrational energy levels in the ground electronic state of the molecule, and as such, the observed Raman shift of the Stokes and anti- Stokes features are a direct measure of the vibrational energies of the molecule.

A schematic Raman spectrum may appear as shown below. The energy of the scattered radiation is less than the incident radiation for the Stokes line and the energy of the scattered radiation is more than the incident radiation for the anti- Stokes line. The energy increase or decrease from the excitation is related to the vibrational energy spacing in the ground electronic state of the molecule and therefore the wavenumber of the Stokes and anti- Stokes lines are a direct measure of the vibrational energies of the molecule. In the example spectrum, notice that the Stokes and anti- Stokes lines are equally displaced from the Rayleigh line. This occurs because in either case one vibrational quantum of energy is gained or lost.

Also, note that the anti- Stokes line is much less intense than the Stokes line. This occurs because only molecules that are vibrationally excited prior to irradiation can give rise to the anti- Stokes line. Hence, in Raman spectroscopy, only the more intense Stokes line is normally measured. Raman scattering is a relatively weak process. Bank Soal Matematika Sd Kelas 1 Semester 1 Exam. The number of photons Raman scattered is quite small.

However, there are several process which can be used to enhance the sensitivity of a Raman measurement. Simplified energy diagram. If the wavelength of the exciting laser coincides with an electronic absorption of a molecule, the intensity of Raman- active vibrations associated with the absorbing chromophore are enhanced by a factor of 1.

This resonance enhancement or resonance Raman effect can be extremely useful, not just in significantly lowering the detection limits, but also in introducing electronic selectivety. Thus the resonance Raman technique is used for providing both structural and electronic insight into species of interest. Metalloporphyrins, carotenoids and several other classes of biologically important molecules have strongly allowed electronic transitions in the visible, making them ideal candidates for resonance Raman spectroscopy. Resonance selectivity has a further practical use, in that spectrum of the chromophoric moiety is resonance enhanced and that of the surrounding environment is not. For biological chromophores, this means that absorbing active centres can be specifically probed by visible excitation wavelengths, and not the surrounding protein matrix (which would require UV lasers to bring into resonance). Resonance Raman spectroscopy is also an important probe of the chemistry of metal centred complexes, fullerenes, polydiacetylenes and other . Although many more molecules absorb in the ultraviolet, the high cost of lasers and optics for this spectral region have limited ultraviolet (UV) resonance Raman spectroscopy to a small number of specialist groups.

Schematic Raman spectrum Vibrations which are resonantly enhanced fall into two or three general mechanistic classes. The most common case is Franck- Condon enhancement, in which a component of the normal coordinate of the vibration occurs in a direction in which the molecule expands during an electronic excitation.

The more the molecule expands along this axis when it absorbs light, the larger the enhancement factor. The easily visualized ring breathing (in- plane expansion) modes of porphyrins fall into this class. Vibrations which couple two electronic excited states are also resonantly enhanced, through a mechanism called vibronic enhancement. In both cases, enhancement factors roughly follow the intensities of the absorption spectrum.

The fuller theory of resonance enhancement is beyond the scope of this section. Resonance enhancement does not begin at a sharply defined wavelength. In fact, enhancement of 5x to 1. This surface- enhanced Raman scattering is strongest on silver, but is observable on gold and copper as well.

At practical excitation wavelengths, enhancement on other metals is unimportant. SERS arises from two mechanisms. When the wavelength of the incident light is close to the plasma wavelength of the metal, conduction electrons in the metal surface are excited into an extended surface electronic excited state called a surface plasmon resonance. Molecules adsorbed or in close proximity to the surface experience an exceptionally large electromagnetic field.

Vibrational modes normal to the surface are most strongly enhanced. The electronic transitions of many charge transfer complexes are in the visible, so that resonance enhancement occurs. Molecules with lone pair electrons or pi clouds show the strongest SERS.

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The effect was first discovered with pyridine. Other aromatic nitrogen or oxygen containing compounds, such as aromatic amines or phenols, are strongly SERS active. The effect can also been seen with other electron- rich functionalities such as carboxylic acids. The intensity of the surface plasmon resonance is dependent on many factors including the wavelength of the incident light and the morphology of the metal surface. The wavelength should match the plasma wavelength of the metal.

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This is about 3. 82 nm for a 5. The plasma wavelength is to the red of 6. SERS at wavelengths in the 3. The best morphology for surface plasmon resonance excitation is a small (< 1. SERS is commonly employed to study monolayers of materials adsorbed on metals, including electrodes.

Other popular surfaces include colloids, metal films on dielectric substrates and, recently, arrays of metal particles bound to metal or dielectric colloids through short linkages. Although SERS allows easy observation of Raman spectra from solution concentrations in the micromolar (1. However,standardization in production of SERS active media is steadilyimproving its potential in this area also. UVRRS is a powerful tool in the molecular analysis of complex biological systems. Most biological systems absorb UV radiation and hence have the ability to offer resonance with UV Raman excitation. This results in the highly selective resonance Raman effect enabling enhancement of important biological targets such as protein or DNA.

For example, excitation around 2. Raman peaks from vibrations of amide groups; excitation around 2.

The Raman scatter from water is weak, allowing for analysis of very weak aqueous systems. Fiber optic UVRRS configuration. Due to the selective nature of UVRRS, a tunable laser is typically required as the excitation source. Since truly tunable continuous- wave lasers are not yet available, a Nd: YAG- pumped dye laser with frequency- doubled output is one suitable UVRRS system. Depending on the dyes used, this laser setup can give almost any required UV wavelength. Intensified CCDs (ICCDs) with UV photocathodes, back- illuminated CCDs or CCDs with UV enhancing (BASF lumogen)coatings can be used as detectors for UVRRS.

These detectors are used on account of their high detection efficiency and multichannel capabilities. The primary obstacle to the merging of the worlds of UVRRS and fiber- optic spectroscopy is solarization, the process by which UV radiation causes opacity of fiber- optics (even quite pure silica fibers). This opacity impairs transmission, rendering standard fiber- optics useless for UVRRS. Species of Interest.