The Indian physicist, CV Raman, received the Nobel Prize for his discovery of Raman scattering. Raman found that when monochromatic light from a Hg lamp passed through a solid or liquid sample, such as benzene, a small amount of light was scattered in all directions but at a frequency lower (longer wavelength) than the incident light frequency. We note that a group of Russian scientists independently discovered this effect around the same time. For a good discussion of this fascinating scientific history see reference 1.1 Because it was a fundamental aspect of how light interacts with matter, the theoretical importance of this effect was immediately recognized. In contrast, the practical implications of the Raman effect took many decades to be apparent.
The physical origin of the Raman effect is molecular polarizability. Polarizability is illustrated in Figure 1(a). An intense electric field will apply a force on an electron cloud, causing a distortion of the electron cloud. The distortion of the electron cloud can be described in terms of a dipole moment. In the case of a light beam, which is described by an electromagnetic wave, the oscillating electric field will cause an oscillating distortion of the electron cloud at the frequency of the light beam. Modern Raman spectrometers use a laser beam for the light source, which provides a very intense monochromatic light source.
Figure 1(a): Illustration of polarizability, the electron cloud around the positively charged nuclei of a molecule is distorted by an intense electric field.
Molecules provide an important twist to the above discussion. Molecules consist of atoms, which can vibrate at frequencies characteristic of the specific chemical bonds that make up the molecule, ν0. For the purposes of discussion, our molecule will have one vibrational frequency at ν0. When some chemical bonds vibrate, the polarizability of the electron cloud surrounding the nuclei changes. Changes in the molecular polarizability with nuclear position are the physical origin of the Raman effect.
Figure 1(b): The light scattered from a molecule can contain light at several frequencies VL, VL + V0, and VL – V0.
Molecular vibrational frequencies are much smaller than the frequency of visible or NIR light, which is given by νL (laser). However, the coupling of the incident light field to the vibrating molecule allows for the electron cloud to oscillate at frequencies νL, νL + ν0, νL – ν0. These oscillations of the electron cloud interacting with light field produce light scattered in all directions at these frequencies, Figure 1(b). Light scattered at frequencies greater (νL + ν0) than the incident radiation is called anti-Stokes shifted, the light at lower frequencies (νL – ν0) is called Stokes shifted. In most cases the Stokes shifted light is what is measured by the Raman spectrometer. The Raman spectrum contains peaks typical of the chemical bonding of the molecule under investigation. This information is specific enough to the molecule, that the Raman spectrum can be used for chemical identification in many practical analytical applications.
Figure 2: Simple schematic diagram of a Raman spectrometer.
A modern Raman spectrometer consists of a few critical parts, Figure 2. A laser serves as an intense light source. The light from the laser is focused onto the sample of interest, thereby increasing the photon flux. The flux is the number of incident photons per cm2 per second. The small fraction of photons that are Raman scattered (Stokes shifted) are collected with optics and measured with a sensitive array detector called a CCD (charge-coupled-detector). Drs. Willard Boyle and George Smith received the 2009 Nobel Prize in Physics for their discovery of CCDs.2 CCDs are similar to the detectors found in modern digital cameras and cell phones. Array detection allows for numerous wavelengths to be measured at the same time, in other words an entire spectrum or piece of a spectrum is measured all at once. The use of CCD greatly speeds the data acquisition time for a Raman spectrometer. The availability of high quality, low cost CCD detectors has been a major driver in the development of hand held Raman spectrometers. More details about Raman scattering hardware will be in future articles.
1. R. S. Krishnan and R. K. Shankar, Journal of Raman Spectroscopy 10, 1981, pg 1-8.