High pressure Raman investigation on the Fermi resonance of biphenyl

Raman spectra of biphenyl have been obtained under pressures up to 15 GPa. The results indicated that with the increase of pressure, the effect of inter- and intra-molecular π-πconjugation and delocation increases, accompanied by the intensity enhancement of the Raman bands, and blue shift of the frequency. The intensity ratio ( R f/ a ) of two Fermi resonance bands υ ' 6 + υ ' 1 and υ ' 8 decreases, and the frequency difference Δ increases with the pressure. The Fermi resonance phenomenon disappears when the pressure goos up to 8 GPa. Using J.F. Bertran's theory, we obtained the relationship of the inherent frequency difference Δ 0 and coupling coefficient ω with pressure. This phenomenon was explained by high-pressure phase transition. The mechanism of high pressure induced Fermi resonance weakening was also discussed.