Inhomogeneous broadening and pressure shifts of the optical spectra in organic glasses at low temperatures

Abstract

Inhomogeneous bandwidths were measured at 6 K in glassy solutions of poly(methyl methacrylate) and ethanol for chromophores with comparatively weak electron–phonon coupling: polymethine dyes, polycyclic hydrocarbons and tetrapyrrolic compounds. The contributions of dispersive attraction, the internal electric field effects and the repulsive forces to the band broadening are considered. Spectral holes were burned in the inhomogeneous S 1←S 0 absorption bands and the pressure shift coefficients d ν/d P were determined using gaseous He as pressure transmitter. Pressure shift coefficients show a linear dependence on hole-burning frequency. The slope of this dependence generally differs from the value of 2 β T ( β T is the isothermal compressibility of the matrix), predicted from the dispersive solvent shift equation. The slopes smaller than 2 β T are accounted for in terms of other solvent shift mechanisms, such as linear and quadratic Stark effects in the cavity field. The slopes larger than 2 β T are ascribed to repulsive interactions.