High Pressure Raman Investigations On Fermi Resonance

Fermi resonance is essentially a vibrational coupling and energy transfering phenomena which widely present intramolecular and intermolecular.The research of Fermi resonance has important theoretical significance in some physics field,such as the molecular vibration,the interaction of electron state,molecular structure and properties etc.The research of Fermi resonance is also deepened with the progress and development of optical instrument and quantum mechanics.The results of recent research show that Fermi resonance is rich in theory and application potential,so the new effect,new mechanism and new research method of Fermi resonance are the focus of research at home and abroad.The traditional research method of Fermi resonance is mainly the method of isotopic substitution and the method of solvent variation.Compared with the traditional research methods,high pressure is a new method for researching Fermi resonance.The pressure can continuously modulate the crystal structure of the molecule,the bond length and the bond angle of the functional group.The mutation of crystal structure?phase transition?that is induced by pressure can directly change the crystal structure of the molecule and the symmetry of molecular point group?an important parameter of Fermi resonance?;Pressure can continuous change the frequencies of coupling phonon?another important parameter of the Fermi resonance?because of pressure can modulate the bond length and the bond angle of the functional group.So it is scientific and effectively to research Fermi resonance by changing the environmental pressure.Some new phenomena and laws can be obtained under pressure which provides us a new way to further research the mechanism of Fermi resonance.In this paper,the following contents are studied by using in-situ Raman spectroscopy at high pressure:?.Fermi resonance between the fundamental mode:The Raman spectroscopy of TCNQ at high pressure.The molecule of TCNQ has a flat structure that containing? electron,it is one of the typical electron acceptor compounds.In-situ Raman spectroscopic measurements of TCNQ have been carried out at high pressure up to 10 GPa at room temperature.The first order phase transition occurs at the pressure of 2.3GPa according to the Raman shift-pressure curves of TCNQ.And we found the phenomenon of anharmonic coupling between the fundamental mode?1206cm-1 and 1186cm-1?when the pressure is higher than 2.3GPa,namely Fermi resonance between the fundamental mode.We can obtain the variation of each parameter of Fermi resonance with pressure by fitting the Raman bands and analyzing the relative strength of two Raman bands.The author argued that polymerization of TCNQ should have happened at external pressures higher than 7.6GPa.So the research of Fermi resonance of TCNQ is limited to 2.3-7.6GPa,we can obtain the relationship of frequency and intensity vary with the pressure of the two Raman bands by analyzing the regularity of the Fermi resonance between the fundamental mode:???Furthermore,the relationship of intensity ratio,coupling coefficient and frequency separation with pressure has been revealed.Ultimately,the author found out that the frequency separation dominates the decoupling of Fermi resonance between fundamental mode.?.Double Fermi resonance:High pressure Raman investigations on hexachloroethaneAt ambient condition,the solid state structure of hexachloroethane has a orthorhombic crystal Pnma?D2h 16?,there are three peaks are observed at 841,851 and 860cm-1 respectively.For the origin of these triple bands,the following three reasons can be excluded:Firstly,The changes of molecular symmetry;Secondly,The isotopic effects;Thirdly,The activation of other Raman silent modesSo we have sufficient reason to believe that the triple bands are due to the anharmonic coupling which present intramolecular?Fermi resonance?,and the triple bands located at 841,851 and 860cm-1 were assigned as Fermi resonance between v7 fundamental mode and the v2 overtone mode.The in-situ high pressure Raman spectra of hexachloroethane were carried out up to 20 GPa in order to further confirm that our identification of the triple bands.According to the Raman shift-pressure curves of hexachloroethane,there is no phase transition up to 20 GPa and this makes hexachloroethane an ideal model for Fermi resonance investigation.By analyzing the intensity ratio of the last two peaks?851 and 860cm-1?to 841 cm-1 Raman line with pressure,using a single exponential equation:IF= I₀ exp?-p/t?,We can obtain the decay coefficient is 1.06 and 1.08 for the 851 and 860cm-1 Raman bands respectively.From this point of view,the 851 and 860cm-Raman bands can be attributed to the v2 overtone mode,while the 841cm-1 Raman line can be assigned as v7 fundamental mode.So we first put forward the "Double Fermi resonance" between the degenerate fundamental mode and an overtone mode,and a scheme of this interaction is proposed.?.The enhancement of pressure-induced Fermi resonance:High pressure Raman investigation on Carbon Disulfide.Carbon Disulfide is a linear and center symmetric molecule that belongs to the D?h point group.The Raman spectroscopic measurements of Carbon Disulfide have been carried out at high pressure up to 10GPa at room temperature in order to explore the Fermi resonance of typical linear triatomic molecule.According to the Raman shift-pressure curves of Carbon Disulfide,We can obtain that Carbon Disulfide has respectively gone through the liquid->solid 1->solid 2->chemical reaction process within the pressure from 0 to lOGPa.We first found the phenomenon of pressure induced anharmonic coupling is enhanced by analyzing the relationship of the each parameter of Fermi resonance vary with pressure,and carries on the discussion.This new effect has enriched the research of Fermi resonance.