Polarized Raman spectroscopy probes information about molecular orientation and symmetry of the bond vibrations, in addition to the general chemical identification which ‘normal’ Raman provides.
Polarized Raman measurements are made acquiring spectra with polarization which is either parallel or perpendicular to the inherent polarization of the excitation laser. The measurement is made by inserting a polarisor in the beam path between the sample and the spectrometer, allowing the Raman polarization to be selected by the user. The polarization of the laser beam can also be kept in its normal state, rotated by 90o, or ‘scrambled’ to remove any polarization by inserting polarizing optics between the laser and the sample.
Polarization measurements provide useful information about molecular shape and the orientation of molecules in ordered materials, such as crystals, polymers and liquid crystals.
An example of the use of polarized Raman spectroscopy is to characterize the symmetry of bond vibrations in a molecule. This is done by calculating the depolarization, p, for a particular peak, where
and is the intensity of the Raman band with polarization perpendicular to the laser beam, and is the intensity with polarization parallel to the laser beam.
If p<0.75 the vibration can be considered to be polarized, and is totally symmetric in nature. If p=0.75 then the vibration can be considered to be depolarized, and is non-symmetric in nature.
Surface enhanced Raman scattering (SERS) is a technique which offers orders of magnitude increases in Raman intensity, overcoming the traditional drawback of Raman scattering – its inherent weakness. Enhancement factors can be as high as 1014-15, which are sufficient to allow even single molecule detection using Raman. SERS is of interest for trace material analysis, flow cytometry and other applications where the current sensitivity/speed of a Raman measurement is insufficient.
The enhancement takes place on a metal surface which has nanoscale roughness, and it is the molecules adsorbed onto that surface which can undergo enhancement. Typical metals used are gold or silver – preparation of the surface can be through electrochemical roughening, metallic coating of a nano-structured substrate, or deposition of metallic nanoparticles (often in a colloidal form). Many researchers create their own SERS substrates, but commercially available kits offer a more routine approach.
Practically, the advantages of SERS can be explored on any Raman system, and the actual measurement is made in the standard way. Typically it is necessary to use a laser wavelength which is compatible with the chosen SERS metal, but beyond this there are no major difficulties. SERS spectra do sometimes differ from a ‘normal’ Raman spectrum of the same material, so interpretation of data must be considered.
SERS and resonant Raman can be combined to further enhance signal (SERRS).
Specialized sample stages designed for microscopes can be used to allow Raman measurements to be made at high/low temperatures, pressures and humidities. A range of stages are available:
Heating/Cooling – typically suitable for temperatures in the range -196ºC to 600ºC, or ambient to 1500ºC, these stages can be used for solids, powders and liquids.
Catalysis– a variant of the heating/cooling stages above, but designed to have preheated gases forced through a catalyst matrix. Suitable for temperatures up to 1000ºC, and gas pressures up to 5 bar.
Tensile Stress – allows structural changes in a sample to be monitored under tensile stress. Forces up to 200 N can be used with these stages. Added heating/cooling from -196ºC to 350ºC ensure complete experimental control.
Pressure– Diamond Anvil Cells (DAC) allow analysis at pressures up to 50 GPa, with elevated temperatures.
Humidity– control of sample temperature and humidity allows analysis of solvent-adsorbate interactions,and the effect of humidity on a sample’s structure.
Low frequency analysis refers to the low Raman shift (low wavenumber, cm-1) region of the spectrum. Most standard Raman spectrometers will allow analysis down to 100-200 cm-1 which allows the standard ‘fingerprint’ spectral range to be detected with ease. However, there are certain materials which exhibit interesting spectral features below 100 cm-1.
Raman systems configured for low frequency analysis allow measurements below 100 cm-1 so that researchers can investigate and characterize these additional spectral features. Research systems can allow low frequency analysis down to 30-50 cm-1(for standard single monochromator instruments) and even 4-5 cm-1 (for triple monochromator instruments and Volume Bragg Gratings filters based systems).
For most routine analyzes, the standard Raman range from 100 cm-1 upwards is sufficient for identification and characterization. However, there are certain materials which exhibit spectral features below 100 cm-1, and being able to measure these peaks is vital for full characterization. Indeed, in some cases, analyzing these low frequency features is the only method to distinguish different materials. Examples where low frequency analysis is important include:
Macro and bulk analyzes are straightforward with modern Raman systems.
Bulk solutions (organic and aqueous) typically require just a simple cuvette cell holder and collection/focusing optics. It is also possible to analyze directly into sample bottles and vials using confocal optics, which minimizes the contribution of the glass container to the spectrum.
Gas samples are more difficult, and the success generally will depend on the type of gaseous sample under investigation, and its pressure. Typically gases have much lower concentrations, which means weaker Raman signals. Examples of more straightforward gas measurements include gaseous inclusions within mi-nerals, and headspace analysis in pharmaceutical vials.
Macro solid samples can also be analyzed but typically this will require a relatively large laser illumination area in order to give an average spectrum from a large area. HORIBA Scientific’s DuoScan™ optics allow the user to define a laser spot size on the sample – it can thus be used for true microscopic analysis with <1 µm spot diameters, as well a bulk analyzes with analysis areas up to 270 x 270 µm2. Transmission Raman is also an ineteresting option to measure bulk samples and get an average information of a large sampling volume.
Raman systems can also be configured with macro chambers, which have special optics designed for large area macro or bulk analysis.
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