Dispersion of Lifetimes of Excited States of Single Molecules in Organic Matrices at Ultralow Temperatures

Fluorescence excitation spectra of single terrylene molecules in transparent naphthalene and polyethylene matrices at ultralow (30-100 mK) temperatures have been studied under the conditions where the widths of zero-phonon spectral lines are determined only by the lifetime T-1 of the excited electronic state. The experimentally observed dispersion of T-1 values for identical molecules is attributed to local field effects, which are responsible for the dependence of T-1 on the effective value of refractive index n of the matrix, which is characteristic of the localization region of each molecule. It has been shown that the dependence T-1(n) for ensembles of point emitters in organic matrices is satisfactorily explained within the model of a virtual cavity around the emitter inside a continuous medium, as well as within the developed quantum kinetic approach including various contributions of the local environment to the lifetime T-1. The recalculation of average T-1 values to the corresponding n values with the use of the expressions obtained for T-1(n) has shown that the difference of the calculated refractive indices of naphthalene and polyethylene from the well-known tabulated n values is less than 1%.