Characterization Techniques in Nanotechnology …
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1.6 Raman Spectroscopy
Raman spectroscopy is a method used to study vibrational, rotational, and other
low-frequency modes in materials thereby providing information about molecular
signature needed to identify and quantify samples (Gardiner 1989). In this technique,
a monochromatic light source (usually laser) is shined on a sample, and the scattered
light is detected. In most cases, the frequency of the scattered light and that of
excitation source is the same for large fraction of the scattered light (Rayleigh or
elastic scattering). About 5–10% of the light intensity incident on the sample which
constituted a small fraction of the scattered light has a shift in energy from the laser
frequency because of the interactions between the incident electromagnetic wave
and the vibrational energy levels of the molecules in the sample (Raman scattering).
Plotting the intensity of this “shifted” light against frequency results in a Raman
spectrum of the sample (Nakamoto 2008). Stokes lines are noticed in the Raman
spectrum whenever the frequency of incident radiation is greater than frequency
of scattered radiation. Otherwise, anti-Stokes lines become visible in the Raman
spectrum. Scattered radiation is usually estimated at right angle to incident radiation
(Skoog et al. 2007; Smith and Dent 2005; Willard et al. 1952). The schematic diagram
of Raman spectrometer is shown in Fig. 12.
In the past, the experimental and theoretical efforts in Raman spectroscopy have
been focused on the fundamentals of inelastic scattering, and it is employed for
studying molecular structure. However, several developments of chemical measurements raise the interest in Raman spectroscopy especially for analysis of a wide range
of forensic samples (Laserna 2001). Raman spectroscopy can be used for both qualitative as well as quantitative purpose. While qualitative analysis involves measurement of frequency of scattered radiations, quantitative analysis has to do with the
measurement of intensity of scattered radiations (Laserna 2001; Skoog et al. 2007;
Fig. 12 Schematic diagram of Raman spectrometer (Downes and Elfick 2010)
33
1.6 Raman Spectroscopy
Raman spectroscopy is a method used to study vibrational, rotational, and other
low-frequency modes in materials thereby providing information about molecular
signature needed to identify and quantify samples (Gardiner 1989). In this technique,
a monochromatic light source (usually laser) is shined on a sample, and the scattered
light is detected. In most cases, the frequency of the scattered light and that of
excitation source is the same for large fraction of the scattered light (Rayleigh or
elastic scattering). About 5–10% of the light intensity incident on the sample which
constituted a small fraction of the scattered light has a shift in energy from the laser
frequency because of the interactions between the incident electromagnetic wave
and the vibrational energy levels of the molecules in the sample (Raman scattering).
Plotting the intensity of this “shifted” light against frequency results in a Raman
spectrum of the sample (Nakamoto 2008). Stokes lines are noticed in the Raman
spectrum whenever the frequency of incident radiation is greater than frequency
of scattered radiation. Otherwise, anti-Stokes lines become visible in the Raman
spectrum. Scattered radiation is usually estimated at right angle to incident radiation
(Skoog et al. 2007; Smith and Dent 2005; Willard et al. 1952). The schematic diagram
of Raman spectrometer is shown in Fig. 12.
In the past, the experimental and theoretical efforts in Raman spectroscopy have
been focused on the fundamentals of inelastic scattering, and it is employed for
studying molecular structure. However, several developments of chemical measurements raise the interest in Raman spectroscopy especially for analysis of a wide range
of forensic samples (Laserna 2001). Raman spectroscopy can be used for both qualitative as well as quantitative purpose. While qualitative analysis involves measurement of frequency of scattered radiations, quantitative analysis has to do with the
measurement of intensity of scattered radiations (Laserna 2001; Skoog et al. 2007;
Fig. 12 Schematic diagram of Raman spectrometer (Downes and Elfick 2010)