Microscopic vibrations involving the motion of the atomic nuclei (which are much heavier than the electrons) usually have their resonance frequencies in the infrared region. These characteristic frequencies depend on the oscillating masses and the strength of the bonding between them and hence can be used for material identification (which is the reason for the dominant role IR spectroscopy plays in analytical chemistry). In the case of silicon important impurities like carbon and oxygen are detected by their characteristic vibrational modes. Susceptibilities describing microscopic vibrations can be modeled by harmonic oscillator terms:   .                                                 ΩP gives the oscillator strength, Ωτ  the damping and ΩΤΟ the resonance position. The index TO is used here since with the same oscillator model transverse optical phonons (lattice vibrations, TO-modes) are described.

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