Mary Masterman - Oklahoma Biological Survey - University of ...

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Introduction
Objectives
• To construct a Raman spectrograph system using a Raman head consisting of a set of filters and a sample holder,
appropriate lenses, and an appropriate excitation laser, to be attached to a spectrograph.
• To build a medium- to high-resolution inexpensive amateur Littrow spectrograph to be attached to the Raman head.
• To test the Raman system with various Raman standards and make any modifications as necessary.
Theory
Raman spectroscopy, like infrared spectroscopy, is a form of vibrational spectroscopy. The Raman effect occurs when a photon
striking a molecule excites an electron into a higher “virtual” energy level, and the electron decays back to a lower level, emitting a
scattered photon (6). Unlike infrared spectroscopy, which results from a change in the dipole moment, Raman scattering results
from changes in the polarizability of a molecule.
In Raman, a high-intensity beam of light is directed to a sample. The vast majority of the light is scattered elastically (Rayleigh
scattering) and the reflected light has the same wavelength as the incident light. A very small portion of the scattered radiation,
however (on the order of 10^-7) is shifted to a different wavelength. The scattered photons are known as Raman scattering. A few
are shifted to higher energy wavelengths (anti-Stokes radiation) but most are shifted to lower frequencies (Stokes radiation) (6). If a
molecule is in its ground state when it interacts with the beam of light, some energy from the colliding photon can be channeled into
the vibrational mode of the molecule, causing the light to be absorbed and then re-emitted at a lower frequency (Stokes scattering).
If a molecule is in a vibrationally excited state when it interacts with the beam of light, the interaction can cause it to drop to the
ground state and lose some of its vibrational energy to the re-emitted (high frequency) light. (3) Stokes scattering is much more
common than anti-Stokes, because at normal temperatures, electrons are most likely to be in their lowest energy state (a result of the
Boltzmann energy distribution) (6). A spectrum of anti-Stokes scattering is identical in pattern, but much less intense, than a
spectrum of Stokes scatterings;for this reason, only Stokes scattering is typically used in Raman spectroscopy. (Fig. 1)
Fig. 1 - Spectrum of CCl 4 demonstrating the symmetry of Stokes and Anti-Stokes bands (4).