Spectroscopy & Hyperspectral Imaging

About Raman spectroscopy

Raman spectroscopy finds its origins in Planck and Einstein’s formulation that light is not only wavelike in nature, but has the dual character of waves and particles. Once scientists began thinking about the concept of light as particles, the possibility of inelastic scattering of these particles became a method of proof of this new theory. In 1923 Compton showed this with inelastic scattering of x-rays from a graphite target. That same year Smekal theoretically predicted that photons should inelastically scatter from molecular transitions. Five years later, in 1928, C. V. Raman and K. S. Krishnan published an article in the journal Nature with experiments that proved Smekal’s prediction of inelastic scattering of light. For his discovery, Raman was award the Nobel Prize and the inelastic scattering of visible light from molecular transitions has been named after him

One of the characteristics of inelastic scattering is that the intensity of the scattering scales to the fourth power of the energy. This means that Compton’s experiments with x-rays with a wavelength of 0.7 nm and the observation of Raman scattering with visible light at 500 nm will differ by 11 orders of magnitude! Raman was able to observe the weak Raman effect by using the most intense light source available at the time, the sun. He focused a large telescope on the sun and placed a green filter in the intense beams of sunlight. When he used a yellow filter to observe this beam of green light passing through a solution of chloroform, he could see a weak yellow light. The origin of the yellow light was the Raman effect. A small amount of the green light from the sun had inelastically scattered from the chloroform molecules and shifted its energy so that the photons fell within the yellow part of the spectrum.

Raman, light of a different color

Today Raman spectroscopy has become much more sophisticated and much simpler to measure. Major technological advances have transformed Raman spectroscopy from telescopes and visual observation to a highly sensitive spectroscopic technique. The first technological advance was the laser. The sun is pretty bright, but it is sending its energy in all directions and over a very broad range of wavelengths. The Raman effect is observed as a shift in energy of a photon and the shift can be related to a vibrational state of the sample. To observe the shift one needs all of the photons to be a very narrow band of energies. Otherwise, you’ll never be able to distinguish the shifted ones from the source photons. The laser produces quasi-monochromatic light that forms a very narrow band of frequencies. Lasers also produce this light in a small concentrated beam that is very intense.

A second technological advance that has revolutionized Raman spectroscopy is the Charge Coupled Device (CCD). The CCD is millions of times more sensitive than the eye and is able to record a complete Raman spectrum in less than a second. Let’s look back at the history of Raman spectroscopy to understand the advantage that the CCD affords. Not long after Raman’s first experiments scientists started using a photographic plate to record the Raman spectrum. This provided the first hardcopy of a Raman spectrum. The problem was that it didn’t have a softcopy version that could be used for data analysis and spectral manipulation. Early Raman spectroscopy was also like shooting in the dark. The plates were exposed for hours to get a spectrum and you didn’t know if the optics were properly aligned until the plate was developed. The next big advance was the photomultiplier tube. This is a sensitive optical transducer that converts photons into electrons. They are sensitive enough to detect a single photon and a whole technology of “single photon counting” grew around them. Now a Raman spectroscopist could set his instrument to view an energy where Raman scattering should be observed and with an electronic display optimize the optical alignment. When computers became commonly available it was possible to have them record this electronic signal and produce a software file that contained the spectrum. This software file could be manipulated to remove artifacts that otherwise can distort Raman spectra.

However, the photomultiplier’s advantage came at a cost. It was no longer possible to acquire a whole spectrum during a single integration period. These instruments would take very small steps in energy and count the photons at each step. A three thousand point spectrum with photons counted for one second took three thousand seconds. That’s almost an hour!

The solution was to go back to the old concept of a photographic plate, but this time use a digital camera. The CCD does just that. These little integrated circuit chips are extremely sensitive to light and they contain thousands of little picture elements (called pixels) that take the whole spectrum at once in less than a second.

Modern Raman spectroscopy with the Advantage 200A is fun and educational. The Advantage system uses a small HeNe laser to replace Raman’s telescope and colored filters, and a CCD detector to replace out-dated photomultiplier tubes. The result? You get a spectrum in seconds.