Thermal effects on zero-phonon holes in the optical spectra of molecular probes in polymer glasses

Abstract

Purely electronic zero-phonon transitions in impurity molecules are of interest as highly sensitive local probes of solids. Spectral holes were created at different positions in the inhomogeneous optical-absorption bands of tetrapyrrolic chromophores in polymer glasses, and the thermal shift and broadening were investigated between 5 and 50 K. In addition, hole shifts were determined under He gas pressure up to 200 bars with the aim to separate the phonon-induced line shift from the ``trivial'' component caused by thermal expansion of the matrix. A prominent frequency dependence of a pure thermal, phonon-induced shift was discovered. The shift becomes progressively more negative (bathochromic) with increasing transition energy in the inhomogeneous ensemble of chromophores. In theory the corresponding negative quadratic electron-phonon coupling constant W reflects phonon mode softening in the excited state. In several cases the holes burned on the low-frequency edge show practically no thermal shift. The line broadening that depends on the square of W remains almost constant over the inhomogeneous contour. The role of intermolecular repulsive and attractive potentials on local phonon frequencies is elucidated.