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Introduction 10
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Understanding Raman Spectroscopy
Raman spectroscopy (named after C. V. Raman), is a technique for studying vibrational, rotational, and
other low-frequency aspects of molecular systems. To use the technique, monochromatic light, usually
from lasers in the visible, near infrared, or near ultraviolet ranges, is aimed onto a substance. The laser
light interacts with the molecular vibrations, phonons, and other excitations of the substance’s
molecular systems. This interaction generates a unique spectral response that is analyzed to identify the
substance.
The typical spectral response of molecular systems to laser light includes three modes of light refraction
or scattering. The most intense of these scattering modes is elastic (or Rayleigh) scattering. This occurs
when the light interacting with a molecule scatters at the same energy, or frequency, it had when it
encountered the molecule. While the intensity of this response is millions of times more than that of the
other modes, it usually does not help identify the substance. Typically, it has been filtered out of the
spectral response with holographic gratings and multiple dispersion stages. Current setups use more
efficient notch or edge filters to reject this type of spectral response.
In addition to the elastic scattering, two less-intense scattering modes, first observed by C. V. Raman in
1928, also occur. Currently called Raman shifts, these light scattering modes happen when photons from
the laser light impinge upon and interact with the electron clouds and atomic bonds of a molecule. This
interaction causes bonds within the molecule to vibrate, and excites the molecule from its ground state
to a virtual energy state. In these modes, when the molecule then relaxes and emits photons, it returns
to a different rotational or vibrational state by either retaining some of the energy added by the
photons, or losing some of its ground state energy to the emitted light. Raman shifts are called inelastic
scattering because of this energy transfer between molecules and photons during their interaction.
This energy transfer shifts the energy level, or frequency, of the scattered laser light up or down. When
the molecule retains energy from the photons the emitted light shifts to a lower frequency than the
laser stimulus. This type of Raman shift is called a Stokes shift. When the molecule loses energy to the
emitted photons the emitted light shifts to a higher frequency than the laser stimulus. This type of
Raman shift is called an Anti-Stokes shift. As a result, these energy transfers change the rotational and
vibrational states of the molecule and determine the shifted pattern of emitted light frequencies.
To display these Raman effects, the molecule requires a change in the molecular polarizability—or
amount of deformation of the electron cloud—with respect to the vibrational coordinates. When this
occurs, the amount of change in the molecular polarization potential determines the Raman scattering
intensity. Raman scattering should not be confused with absorption (as with fluorescence) where the
molecule is excited to a discrete, or non-virtual, energy level.
The wavelengths of these frequency shifts are separated with transmission volume phase grating and
then recorded with array detectors to produce the Raman spectra. The energy shifts shown in the
spectra data provide information about the molecular structure of the sample. With this information the
substance can be identified by matching it to the energy shift data from spectral libraries of known
substances.
What You Can Do with Progeny Analyzers
Progeny Analyzers provide a portable platform for analyzing a wide range of substances. Their
customizable interfaces and flexible operating properties make it simple to verify or identify the
substances you encounter. The following sections outline how Progeny Analyzers can integrate the
power of Raman spectroscopy into your development, manufacturing, or security protocols. With your