Conclusion and prospects for further investigations

Raman scattering spectra for 1BNa 14Ni cH4Br crystals in the region of W4(k) + 0J4(—k) transitions. The heavy line distinguishes the region of two-parti-cle transitions of unbound phonons. The peak at 2788 cm-1 corresponds to the excitation of local biphonons. Local phonons do not exist. Concentrations are (a) x = 0.0037 (6) (1) x = 0.0037 and (3) x = 0.3. 

The investigations of biphonons in one- and two-dimensional crystals may also be of special interest. In such crystals, as well as in three-dimensional crystals in which certain phonons can be assumed to be quasi-one-dimensional or quasi-two-dimensional, the conditions for the formation of biphonons should be more favorable, all other things being equal, than in ordinary three-dimensional crystals. 

Speaking of biphonons in one-dimensional crystals, we should also like to draw attention to their possible role in the transfer of energy along protein molecules (81). In such molecules there are vibrations with energy ftfl ss 0.2 eV. Hence, the formation of stable and mobile complexes, consisting of two or three vibration quanta, may prove to be an essential factor in the transfer of the 

Also worthy of further development is the theory of surface biphonons. The conditions required for the formation of these states are different from those of the formation of surface states for the spectral region of the fundamental vibrations. It was demonstrated on the model of a one-dimensional crystal (26) that situations may exist, in general, in which the surface state of the phonon is not formed and the spectrum of surface states begins only in the frequency region of the overtones or combination tones of the vibrations. 

When the frequency of the surface biphonon lies within the band of the surface polariton, Fermi resonance occurs and the dispersion curve of the po-lariton is subject to a number of essential changes (gaps appear, etc. (86)). Consequently, experimental research of surface polariton dispersion under these conditions could yield, like similar investigations of bulk polaritons, a great deal of interesting information, not only about the surface biphonons themselves, but about the density of states of surface phonons and the magnitude of their anharmonicity constants as well. 

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