Experimental Study of Non-Exponential Relaxation Processes in Random Organic Solids

We compare experimental results on (i) thermally induced merocyanine → spiropyran transformation of a dye molecule substitutionally doped into a polymer (PMMA) matrix; (ii) growth of a photophysical hole burnt into the inhomogeneously broadened S¹ ← S⁰ 0–0 absorption band of tetracenedoped into a MTHF ḡlass at low temperature; (iii) diffusion-controlled trapping of a triplet excitationin a benzophenone glass. A common feature is the time-dependence of the observed, i.e. averaged event rate. While in each case a stretched exponential {exp[–(t/t_o)^α]} provides a reasonably good fit to experimental data if restricted to a limited time domain, analysis of the short time behavior i dicates its failure for modelling the cases (i) and (ii). Instead, it isshown thatin these cases the concept of uncoupled parallel relaxations whose rate controlling parameter, an a tivation energy of atunnelling matrix element, is subject to a Gaussian distribution provides an appropriate description. Comparison with results of case (iii) demonstrates the possibility of distinguishing between parallel and serial relaxation mechanisms if experimental data span a sufficiently large time interval.