instrumental techniques applied to mineralogy and geochemistry

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Biliana Gasharova
Even though Raman spectroscopy probes vibrational transitions indirectly by light
scattering, the Raman shift has the same energy range as IR absorption.
Selection rules
Different vibrational modes have different relative intensities in IR and Raman
spectra – some modes are active in one and not the other, and some modes are not
observed at all. The IR and Raman activities of particular modes are determined by the
quantum mechanical selection rules for the vibrational transitions, and by the mode
symmetry. In a simple model, the selection rules can be rationalized by considering the
interaction between the oscillating electric field vector of the light beam and a changing
molecular dipole moment associated with the vibration. In an IR experiment, the light
interacts directly with an oscillating molecular dipole, so for a vibrational mode to be IR
active, it must be associated with a changing dipole moment. In general, asymmetric
vibrations tend to give stronger IR absorption than symmetric species, since they are
associated with larger dipole moment changes. Similarly, highly polar (“more ionic”)
molecules and crystals have stronger IR spectra than non-polar samples. In Raman
scattering, the light beam induces an instantaneous dipole moment in the molecule by
deforming its electronic wave function. The atomic nuclei tend to follow the deformed
electron positions, and if the nuclear displacement pattern corresponds to that of a
molecular vibration, the mode is Raman active. The magnitude of the induced dipole
moment is related to the ease with which the electron cloud may be deformed, described
by the molecular polarizability a.
The classification of the vibrational quantum states and the description of the
spectroscopic interaction are greatly simplified by exploiting the symmetry of the
vibrating atomic groups. The mathematical framework of the group theory is the basis of
the quantitative description of the symmetry relations possessed by the vibrating groups,
finally giving rise to the formulation of the symmetry-based selection rules. As the
symmetry of the atomic group increases, the number of different energy levels decreases.
The degeneracy, i.e. the number of vibrational states, which have the same energy
increases with increasing symmetry. The more symmetric the atomic group, the fewer
different energy levels it has, and the greater the degeneracy of those levels. The
symmetry must be compatible in order that the molecule may absorb light and the