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FT Raman Microspectroscopy at High External Pressure : Crystalline Anthracene and trans-Cinnamic Acid

By increasing pressure inter- and intramolecular atom-atom distances are decreased and repulsive forces and anharmonicities become important. Anthracene is well known molecular crystal which serves as a model system in experimental and theoretical considerations. Recently Zhao et al. [1] have published a high-pressure Raman study of anthracene in which the use of liquid nitrogen as a hydrostatic pressure medium enabled them to record fluorescence-free Raman spectra excited with the 514.5 nm Ar+ laser line. Here we report on high-pressure and fluorescence-free Raman spectra of anthracene excited with the 1064 nm Nd:YAG laser line. The sample volume of diamond-anvil cell was filled with anthracene and sulphur as secondary pressure gauge [2]. The results are presented in Figs. 1 and Table 1. In order to complement our experiment we have applied quasi-harmonic approximation and recovered a part of the observed pressure-induced frequency shifts. Crystal structure at a given pressure P has been found by minimizing the potential U+PV with respect to lattice parameters and molecular translational and orientational coordinates. Using the same set of atom-atom interaction constants [3] the lattice dynamics at k = 0 was calculated taking into account all the molecular vibrational degrees of freedom. Assuming that the crystal is a set of independent anharmonic oscillators, a frequency shift of the ith oscillator can be shown [4] to be proportional to where  is the reduced mass, f    i and f   i are anharmonic and harmonic force constant, respectively, U is intermolecular interaction and derivatives are calculated at the free molecule equilibrium structure. Only the second contribution is accounted for in the quasi-harmonic approximation. The calculated frequency shifts are only minor part of the observed shifts (Table 1) meaning that the anharmonicity of the molecular oscillator coupled with the increased slope of the atom-atom potential at distances smaller 5-7 % than the equilibrium ones dominates the pressure-induced shifts. Exceptionally large slope obtained for the 750 cm-1 band might indicate that the first term need not always be dominant. Better agreement has been accordingly found for the phonons of crystalline anthracene (Fig. 2).

Raman; high pressure; anthracene; cinnamic acid

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