Experimental investigations of biphonons and Fermi resonance with polariton

7 Experimental investigations of biphonons and Fermi resonance with polariton 

As has been pointed out previously, in many crystals biphonons were observed both in infrared absorption spectra and in RSL spectra. 

In discussing infrared absorption spectra, we refer, first of all, to the papers by Dows and Schettino (54) and of Schettino and Salvi (55). Dows and Schettino (54) investigated the CO2 crystal spectrum in the frequency region corresponding to the combination tone of the intramolecular vibrations v and 1/3 u /y3 Ri 3720 cm-1). Schettino and Salvi (55) measured the infrared (IR) spectra of N2O and OCS crystals. The CO2 and N2O molecules are linear, have no permanent dipole moments, and form a simple cubic lattice upon crystallization. This lattice has four molecules per unit cell, which are oriented along the axes of a tetrahedron. The OCS molecule is also linear, but it forms a crystal of the trigonal system with one molecule per unit cell. 

Since the CO2 molecule is symmetrical, its stretching vibration v is IR inactive and has practically no dispersion. Hence, the Van Kranendonk model can be employed to interpret the experimenatl results obtained in the region of the combination frequency v + 1/3. This has actually been taken into account by Bogani (10). Shown in Fig. 6.9 is the transmission spectrum, measured by Dows and Schettino (54), of a crystal of 1.8 /xm thickness. Calculations carried out by Bogani (10) indicate that the sharp absorption peak obtained in this case corresponds to the excitation of a biphonon. 

Similar results were obtained (55) for the N20 crystal in the frequency region that corresponds to the combination frequency i/ + 1/3. The N2O molecule is not symmetrical and therefore all of its three intramolecular vibrations, i/, 1/2 and 1/3, are IR active. Its flexural vibration v2 is two-fold degenerate and, owing to 

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