Effect of pressure on the shift of zero-phonon lines of organic impurity crystals

Abstract

The object of our investigations was to determine the numerical values of the shift of zero-phonon lines which was due to thermal expansion of the lattice (AVla t) using high-pressure equipment. In the general case, the energy of the zero-phonon transition in an impurity crystal can be considered as a thermodynamic quantity depending on the temperature and volume of the crystal. From thermodynamics, we know ~p P = 1/V (SV/~T)p is the coefficient of volumetric thermal expansion; fiT = --1/V (SV/OP) T is the isothermal compressibility; (SE/DT)p is the overall temperature shift of the zero-phonoa lines (AVexp); (0E/ST)v is the temperature shift of the zero-phonon lines due to electron-phonon interaction (AVe_p), which can be calcttlated theoretically; and ~p/flT(SE/SP)T is the shift of the zero-phonon lines (A~lat) due to thermal expansion of the lattice. The latter was determined by independent experimental measurements of the baric shift of the zerophonon lines (SE/SP) T and the ratio ~P/flT. The baric shift of the zero-phonon lines was determined with a low-temperature high-pressure optical chamber [6, 7] at various fixed temperatures in the range 80-140~ The objects of the investigations were the spectra of solid solutions of coronene and 1,12-benzoperyle ne in n-hexane, n-heptane, and n-octane. The