Raman Spectroscopy: The Basics
Discovered by Krishna and Raman in 1928, Raman spectroscopy has given rise to a multitude of specific techniques, from Linear Raman Spectroscopy to Coherent Anti-Stokes Raman Spectroscopy, and proven itself to be a powerful tool for spectroscopic analysis. One of the most common applications of Raman spectroscopy is to measure vibrational, rotational, and other low-frequency modes of a system (e.g., molecules).
In molecules, photons (from a laser) undergo Raman scattering from the molecules. This is a form of inelastic scattering in which the final energy state, Ef, is different from the initial energy state, Ei. This type of scattering is in contrast to Rayleigh scattering, which is an elastic scattering event in which the final and initial energy states are the same. Both Rayleigh and Raman scattering are dependent upon the polarizability of a molecule; however, the stronger the polarizability of a molecule, the larger the scattering cross section. While both Rayleigh and Raman scattering are second order processes that scale as 1/λ4, the scattering rate for Rayleigh scattering is on the order of 103 times greater than that for Raman scattering . Typically in Raman spectroscopy, the stronger Rayleigh signal must be extricated since it carries little pertinent information on vibrational modes.
Since Raman spectroscopy requires that the polarizability change as a function of normal coordinate, one of its limitations is that it cannot measure direct dipole transitions. Because of this, Raman spectroscopy is sometimes utilized with other techniques to fully measure the vibrational and rotational states of a molecule. For example, in the CO2 molecule, of the three vibrational states depicted in the figure to the right, only ν1 (symmetric streching) is Raman active. The other two vibrational states (bending and anti-symmetric streching) are infrared active ; thus Raman and infrared spectroscopy comprise complementary measurements.
Raman scattering is a two-photon process where the incident photon (hνi) is absorbed by the molecule, and the molecule is excited to a "virtual" level (not necessarily a stationary Eigenstate). Once promoted to this virtual level, the molecule will decay to an excited state and emit a "scattered" photon (hνs). In general, the molecule begins in the ground state, and thus, the energy of the scattered photon is less than that of the incident photon. The energy difference is related to the vibrational, rotational, or electronic energy of the molecule . The emission of a scattered photon possessing less energy than the incident photon is called Stokes radiation, whereas the emission of a scattered photon possessing more energy than the incident photon is known as anti-Stokes radiation. The figure to the left depicts Stokes and anti-Stokes radiation. Since anti-Stokes radiation requires that the molecule already be in the excited state before scattering, the peak intensity of the anti-Stokes signal is lower than of the Stokes signal.
The graph on the bottom left of this presentation shows the results of a typical Raman spectrum for acetone (taken with Thorlabs' DJ532-40 laser diode) and compared to published results. For standard linear Raman spectroscopy, information about the molecule is obtained through several measurements. The linewidth of the scattered radiation can yield a plethora of diverse information about the system. For example, in a gas sample, the linewidth can represent Doppler width, collisional broadening, natural linewidth, etc. Polarization analysis of the Raman spectrum also yields additional information about anisotropy and the polarizability tensor. Additionally, information about molecular orientation or vibrational symmetry can be extracted from polarization analysis. Finally, the intensity of the Raman lines relates to the scattering cross section and population density of molecules in the initial state.
Raman Spectroscopy: The Approach
Since Raman scattering is relatively weak compared to Rayleigh scattering, one of the main historical problems with Raman spectroscopy had been separating out the weak Raman signal from the strong Rayleigh signal. Today, this problem is easily remedied with notch or edgepass filters. Similarly, recording the Raman spectrum has been aided greatly by the advent of CCD spectrometers. The image to the right shows a Raman Spectrometer system constructed here at Thorlabs. This particular Raman spectrometer was designed for 780 nm light using a TLK-780M Tunable Laser Kit fed into a TPA780P20 Tapered Amplifier (sold below).
In this side-scattering configuration, the polarization of the laser was set vertically with respect to the table (horizontally polarized light cannot scatter horizontally). The sample, held in a cuvette, was mounted in Thorlabs' CVH100 Cuvette Holder, which allowed optical access on all four sides of the cuvette, making it ideal for a Raman spectrometer. The scattered light was collected by a fiber and fed into Thorlabs' CCS200 spectrometer. The Raman spectra for Isopropyl Alcohol, measured with this 780 nm Raman spectrometer, is presented at the bottom right.
The Tapered Amplifier
Power is important in Raman measurements. Not only is Raman scattering weak to begin with, but it scales as 1/λ4. Sensitivity and integration time for data accumulation improve with increasing power, as long as the notch or edgepass filter can sufficiently attenuate the strong Rayleigh signal and the power is below the damage/saturation thresholds of the devices. Our tapered amplifiers provide excellent optical amplification. The TPA780P20 used in our Raman spectrometer can provide up to 2 W of power, sufficient to resolve Raman spectra over a 60 s integration time. The bottom right graph shows the results for Isopropyl Alcohol obtained with the 780 nm Raman spectrometer. The tapered amplifier is able to produce enough power to produce well resolved Raman spectra, as demonstrated below.
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Raman spectrum for Isopropyl Alcohol, measured with the 780 nm Raman spectrometer discussed above
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Raman spectrum for Acetone, measured with a 532 nm Raman Spectrometer (bottom), and compared to published results (top)
 D. W. Ball, Spectroscopy 16(2), 28 - 30 (2001)
 W. Demtroder: Laser Spectroscopy Volume 2, 4th Edition (Springer-Verlag, Berlin, Heidelberg, 2008)
 G. Dent and E. Smith: Modern Raman Spectroscopy: A Practical Approach, (Wiley, Chichester, United Kingdom, 2005)
 I. R. Lewis and H. Edwards: Handbook of Raman Spactroscopy, (CRC Press, 2001)
 R. L. McCreery: Raman Spectroscopy for Chemical Analysis, (John Wiley & Sons, Inc., 2000)