Lattice vibrations multiphonon excitations

In the i.r. range usually investigated (0.5-0.1 eV), besides the vibrational contributions of adsorbed species and multiphonon modes, plasmonic contributions associated with free electrons in the conduction band and defect excitations can also contribute, whose relative intensity is strongly influenced by the pretreatment conditions. In particular when the conduction band is extensively populated, the plasmonic modes couple with the vibrational ones (due to both adsorbed species and multiphonon lattice vibrations) causing a dramatic spectral modification and loss of any vibrational detail. 

Most of the earlier theoretical studies dealt with the simplest relaxation mechanism where the internal vibrational energy of the guest is dissipated directly into the delocalized and harmonic lattice phonons. The common results of these works " were, as we mentioned above, predictions of a strong temperature dependence for the relaxation and an exponential decrease in the rates with the size of the vibrational frequency. The former result has its origin in stimulated phonon emission the conversion of vibrational energy into lattice phonons is greatly facilitated if some excited phonon states are thermally populated. The energy-gap law is due to the fact that the order of the multiphonon relaxation increases with the size of... 

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