Thermal hole shifts in the optical spectra of a dye-doped amorphous polymer

Temperature and pressure effects were investigated on spectral holes burned in the inhomogeneous S1 absorption band of octaethylporphine in poly(methyl methacrylate) glass. The temperature-induced hole shift and broadening were recorded between 7 and 30 K. The dynamic, phonon-induced shift depends strongly on burning position. According to theory, negative quadratic electron–phonon coupling (QEPC) constant corresponds to a decrease of phonon frequencies in the excited state. This mode-softening behaviour prevails on the short-wavelength part of absorption, where the thermal shift is large and bathochromic. The absence of shift on the red edge cannot be explained with negligible absolute values of QEPC constants, since thermal hole broadening (that depends on the square of QEPC constant) is large and essentially independent on burning position.