Analysis of Multiphonon Absorption in Corundum

The frequency and temperature dependence of the infrared absorption coefficient of corundum over the range 1400 to 2500 cm−1 is analysed with an anharmonic model based on the concept of phonon self-energy. A renormalization accounts for important effects of thermal expansion and phonon frequency shifts. First, the analysis of the temperature dependence of the data allows to separate the contributions to the absorption of different w-phonon summation processes (n = 2 to 5). Results are found to be quite consistent with theoretical predictions. Then each n-phonon contribution can be investigated in detail, and it is shown that the use of simple forms for average anharmonic potentials is adequate to explain the frequency dependence of the w-phonon absorptions. The analysis of the results on the basis of a quite different theoretical model is also discussed. L'evolution du coefficient d'absorption infrarouge du corindon en fonction de la frequence et de la temperature entre 1400 et 2500 cm−1 est analysee sur la base d'un modele anharmonique utilisant le concept de self-energie de phonon. Ce modele est renormalise pour tenir compte des effets importants de la dilatation thermique et des glissements de frequence des phonons. D'abord, l'analyse en temperature des donnees permet de separer les contributions a l'absorption de processus somme multiphonon impliquant, selon les frequences, de 2 a 5 phonons; ces resultats se montrent tout a fait conformes aux prediotions theoriques. Alors chaque contribution n-phonon peut etre examinee en detail, et il est montre que l'utilisation de formes simples pour des potentiels anharmoniques moyens est suffisante pour interpreter l'evolution en frequence des termes d'absorption a n phonons. L'analyse de ces termes a partir d'un modele theorique tout a fait different est aussi discutee.

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