Raman spectra and defect fluorescence of anthracene and naphthalene crystals at high pressures and low temperatures

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Abstract
Raman spectra of the lattice modes of crystalline anthracene and naphthalene have been measured at pressures to 36 kbar at several temperatures from 450 K to below 30 K. The frequencies of these lattice modes are very sensitive to pressure, and at room temperature, most modes double in frequency between 1 bar and 30 kbar. At pressures less than 10 kbar, the vibrational modes also are very sensitive to temperature at constant pressure. The Raman shifts of the lattice modes of anthracene increase significantly (10–20 cm−1) upon isobaric cooling from room to liquid helium temperatures at pressures below 10 kbar; at higher pressures, the Raman shifts of these modes change by less than 5 cm−1 upon cooling. For naphthalene, the smaller temperature dependence was observed even at 5 kbar, the lowest pressure studied. The naphthalene results agree well with observations to 10 kbar reported by Dows et al.; however, the anthracene results more closely correspond to the observations reported by Wong and Whalley, whose measurements also extend to pressures greater than 10 kbar. No evidence of a phase transformation to naphthalene II was observed even when a crystal had been maintained at 30 kbar and 425 K for more than 24 h. At pressures greater than 17 kbar for anthracene and 30 kbar for naphthalene, an excimerlike fluorescence of defects in the melt-grown crystals was directly excited by the argon laser at 514.5 nm and dominated the Raman spectra. The anthracene defect fluorescence could be excited with 457.9 nm radiation at all pressures and changed from a banded to a predominately broad structureless spectrum at less than 1 kbar; however, Raman spectra of anthracene at high pressures excited with 632.9 nm were characteristic of a compressed atmospheric-pressure structure. No additional features characteristic of a dimer defect could be detected in the Raman spectra of anthracene crystals that had been held at 30 kbar and irradiated with ∼100 mW of 363.8 nm radiation for as long as 8 h.