Chapter 18
Raman Spectroscopy
When radiation passes through a medium, the species present scatter a fraction of the beam in
every direction. In 1928, the Indian physicist, C. V. Raman discovered that the visible
wavelength of a small fraction of the radiation scattered by certain molecules differs from that of
the incident beam and furthermore the shifts in wavelength depend upon the chemical structure
of the molecules responsible for the scattering. He was awarded the 1931 Nobel Prize in physics
for this discovery.
Background:
The theory of Raman spectroscopy shows that the phenomenon results from the same type of
quantized vibrational changes that are associated with infrared spectroscopy. Thus, the difference
in wavelength between the incident and the scattered visible radiation corresponds to
wavelengths in the midinfrared region. Raman and infrared absorption spectrum often resemble
each other closely. There are, however, enough differences between the kinds of groups that are
Raman active verses IR active.
Advantages:
An important advantage of Raman spectra over infrared lies in the fact that water does not cause
any interference. In addition, glass or quartz cells can be employed thus avoiding working with
sodium chloride or other atmospherically unstable window materials.
Figure:
Internal
working of a spectrometer
Theory of Raman Spectroscopy:
Raman spectra are acquired by irradiating a sample with a powerful laser source of visible or
near IR monochromatic radiation. During irradiation, the spectrum of the scattered radiation is
measured at some angle (often 90 deg) with a suitable spectrometer.
At the very most, the intensity of Raman lines are 0.001% intensity of the source. As a
consequence, their detection and measurement are somewhat difficult than IR spectra.
Mechanism of Raman and Rayleigh Scattering:
In Raman spectroscopy, the spectral excitation is normally carried out by radiation having the
wavelength that is well away from the absorption peaks of the analyte.
Instrumentation:
Instrumentation for modern Raman spectroscopy consists of three components:
1. A laser source
The sources used for Raman spectroscopy are almost always lasers because their high
intensity is necessary to produce Raman scattering of high enough intensity to be
measured with a reasonable signal-to-noise ratio. Five of the most common laser sources
for Raman spectroscopy are tabulated below:
Type Source
Wavelength, nm
Argon ion
488.0 or 514.5
Krypton ion
530.0 or 647.1
Helium/Neon
635.8
Diode Laser
782 or 830
Nd/YAG
1064
2. A sample illumination system
Sample handling for Raman spectroscopic measurements is simpler than for IR
spectroscopy because glass can be used for windows, lens and other optical components,
rather than the more fragile atmospherically unstable crystalline halides.
3. A suitable spectrometer
Both the Fourier Transform instruments and the Dispersion instruments are good for
measurement on samples.
References:
American Chemical Society:
http://www.acs.org
Chemical Abstracts Service:
http://www.cas.org
Chemical Center Home Page:
http://www.chemcenter/org
Science Magazine:
http://www.sciencemag.org
Journal of Chemistry and Spectroscopy:
http://www.kerouac.pharm.uky.edu/asrg/wave/wavehp.html