Raman Spectroscopy

Raman Scattering: the Basics

Raman scattering is a powerful light scattering technique used to diagnose the internal structure of molecules and crystals. In a light scattering experiment, light of a known frequency and polarization is scattered from a sample. The scattered light is then analyzed for frequency and polarization. Raman scattered light is frequency-shifted with respect to the excitation frequency, but the magnitude of the shift is independent of the excitation frequency. This "Raman shift" is therefore an intrinsic property of the sample.

Because Raman scattered light changes in frequency, the rule of conservation of energy dictates that some energy is deposited in the sample. A definite Raman shift corresponds to an excitation energy of the sample (such as the energy of a free vibration of a molecule). In general, only some excitations of a given sample are "Raman active," that is, only some may take part in the Raman scattering process. Hence the frequency spectrum of the Raman scattered light maps out part of the excitation spectrum. Other spectroscopic techniques, such as IR absorption, are used to map out the non-Raman active excitations.

Additional information, related to the spatial form of the excitation, derives from the polarization dependence of the Raman scattered light. The shape of an excitation in a material, for example a vibration pattern of the atoms in a molecule, and the polarization dependence of the scattering, are determined by the equilibrium structure of the material through the rules of group theory. By this route one gleans valuable and unambiguous structural information from the Raman polarization dependence.

As an example of some actual Raman data taken here, consider the intensity/frequency spectra presented in the figure below.

Here, the frequency is plotted relative to the laser frequency, so the frequency scale represents the Raman shift. The peaks in the intensity occur at the frequencies of the Raman active modes. The spectra differ because of the different polarization conditions enforced on the incident and scattered light. Different polarization conditions select different sets of Raman active excitations.

Experimental Considerations: the NIST Raman Apparatus
Raman scattering is, as a rule, much weaker than Rayleigh scattering (in which there is no frequency shift) because the interactions which produce Raman scattering are higher order. Therefore most experiments require an intense source which is as monochromatic as possible--a laser with a narrow linewidth is usually used--and the collected light must be carefully filtered to avoid the potentially overwhelming Rayleigh signal. Other potentially large sources of non-Raman signal include fluorescence (the decay of long-lived electronic excitations) and of course light from ambient sources. Fluorescence can be particularly pernicious to a Raman measurement because the fluorescence signal is also shifted from the laser frequency, and so can be much more difficult to avoid. (Note that although the fluorescence spectrum is shifted from the laser frequency, the fluorescence shift depends on the laser frequency whereas the Raman shift does not).

So the relative weakness of the Raman signal dictates the organization of the data-taking apparatus. The original Raman setup here at NIST is fairly standard. The photo below shows the basic Raman configuration.

The laser beam from the Argon-ion laser is filtered for monochromaticity and directed by a system of mirrors to a focussing/collecting lens. The beam is focussed onto the sample; the scattered light which passes back through the same lens is then passed through a second lens into the first stage of the spectrometer. One point to notice is that a sample should be oriented such that the specular reflection from the sample passes outside of the collection lens--otherwise, the laser signal might damage the detector which is sensitive enough to see the weak Raman signal.

The Spectrometer and Detector
The spectrometer itself is a commercial "triple-grating" system. Physically, it is separated into two stages which are shown schematically here.

The first stage is called a monochromator, but is really used as a filter. Its structure is basically two diffraction gratings, separated by a slit, with input and output focussing mirrors. The incoming signal from the collecting lenses is focussed on the first grating, which separates the different wavelengths. This spread-out light is then passed through a slit. Because light of different wavelengths is now travelling in different directions, the slit width can be tuned to reject wavelengths outside of a user-defined range. This rejection is often used to eliminate the light at the laser frequency. The light which makes it through the slit is then refocussed on the second grating, whose purpose is only to compensate for any wavelength-dependence in the dispersion of the first grating. This grating is oriented such that its dispersion pattern is the mirror image of that from the first grating. Finally the light is refocussed and sent out to the second stage.

The second stage focusses the filtered light on the final grating. The dispersed light is now analyzed as a function of position, which corresponds to wavelength. The signal as a function of position is read by the system detector. In the present case the detector is a multichannel charge-coupled device array (CCD) in which the different positions (wavelengths) are read simultaneously. The wavelength/intensity information is then read to a computer and converted in software to frequency/intensity. This is the Raman spectrum which appears as the raw data.

Frequency Resolution; Intensity Limits
The Raman data comes out as an intensity/frequency plot. To resolve a Raman peak of a certain width, the resolution of the spectrometer should be smaller than the peak width. In the system described above, the resolution is determined by a final slit between the third grating and the CCD array. The final dispersed image of the sample spot is focussed in the plane of the CCD array; the slit width determines the extent to which the image may shift along the face of the CCD array, and hence the frequency resolution. When the apparatus is properly aligned, the intensity is a function of four factors: the applied laser power, the sample properties (how absorptive/reflective the sample is, and the intrinsic strength of the Raman modes), the width of the spectrometer's admission slit, and the width of the resolution slit. There is a tradeoff between resolving power and intensity. As for signal noise, statistically speaking, Raman is like a random decay process, so the noise in the Raman spectrum follows Poisson statistics. Finally, the CCD array has a certain dark current which is a function of the detector temperature. Typically, reducing the CCD array temperature to about 150 K with liquid nitrogen as a cryogen reduces the variation in dark current to about 20 counts per CCD pixel. Hence 20 counts is the practical limit of a measured signal.

Spectrometer Response; Raman Intensity Standards
Ideally, first stage of the spectrometer filters out the laser frequency, while leaving the rest of the frequencies unaffected, and the second stage spreads the filtered light onto the CCD array, which then reponds uniformly to each frequency. Of course, nearly the opposite is true: every spectrometer in its parts and as a whole has a wavelength (or frequency) dependent transmittance. The actual spectrum displayed by the software is the product of the spectrometer frequency response with the actual spectrum of the scattered light. To know not only the energies of the Raman-active excitations, but also the relative magnitudes of the scattering at different frequencies, one needs a calibration of the spectrometer response to a source with a known spectrum. Typically, one uses a NIST-traceable standard lamp; a recent NIST project concentrated on using the spectrum from a well-characterized piece of luminescent glass, and this is one of the calibrations we use with our instrument.

The material presented in this post have been taken from the webite mentioned below.For more in-depth coverage,please visit : http://physics.nist.gov/Divisions/Div844/facilities/raman/Ramanhome.html