instrumental techniques applied to mineralogy and geochemistry

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Biliana Gasharova
that electronic transitions occur on a much shorter time scale, and rotational transitions
occur on a much longer time scale, than vibrational transitions.
The energy changes we detect in vibrational spectroscopy are those required to
cause nuclear motion. Vibrational transitions can be observed as IR or Raman spectra.
However, the physical origins of these two spectra are markedly different. In IR
spectroscopy, IR light covering a broad range of frequencies is directed onto the sample.
Absorption occurs where the frequency of the incident radiation matches that of a
vibration so that the molecule is promoted from the electronic ground level to a
vibrational excited state (Fig. 2). The loss of this frequency of radiation from the beam
after it passes through the sample is then detected. From the quantum mechanical point
of view a vibration is IR active if the dipole moment of the vibrating molecule is
changed. It does not matter whether the molecule has a permanent dipole moment or not,
only the change associated with the vibration is important.
FIGURE 2. Spectroscopic transitions underlying several types of vibrational spectroscopy.
On the other hand, Raman spectra have their origin in the electronic polarization
caused by UV, visible or near-IR light. In contrast to IR spectroscopy, Raman
spectroscopy uses monochromatic radiation to irradiate the sample and it is the radiation
scattered from the molecule, which is detected. Thus, unlike IR absorption, Raman
scattering does not require matching of the incident radiation to the energy difference
between the ground and excited state. If only electron cloud distortion is involved in
scattering, the photons will be scattered with very small frequency changes, as the