Highest-energy optical phonons

If we further assume that there is only one dominant mode of coupling, the symbol Ijgj can be replaced by L gm, where is an effective degeneracy factor, and the subscript m denotes the effective phonon mode responsible for the radiationless process. Of the modes responsible for the decay, the highest energy optical phonon modes can be expected to make the greatest contribution. We further note that, for radiationless transitions in rare earth ions in crystals, the inequality... 

The highest frequency or energy of an external phonon is that of an optical phonon at K = 0. It has the value 3.9 THz or 16.1 meV. Compared to the optical phonon of Si at K = 0, these values are about a factor of 4 smaller. The width of the whole spectrum of the optical phonons is around 2.5 THz. Above the maximum frequency of 3.9 THz, the spectrum exhibits a gap which is about half as wide as the overall width of the spectrum of the optical phonons. The frequencies of the internal molecular vibrations are observed only above this gap. Their dispersion is weaker than that of the external modes. The gap in the spectrum, the number of phonon branches below... 

The highest phonon frequency measured so far has been reported for NaF(lOO). By using primary beam energies of 90meV, Brusdeylins et al. have detected optical surface modes with frequencies of 40meV. The intensities, however, were rather low and the multiphonon background dominated the energy loss spectra. 

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