Some problems in spectroscopy of a single impurity molecule

Single-impurity-center spectroscopy (SCS) results are reported for molecules of pentacene in a p-terphenyl single crystal at 4.2 and 1.7 K. The shapes of about ten zero-phonon lines belonging to different single molecules are measured at both temperatures. A substantial dispersion of intensities and linewidths of the zero-phonon lines is observed. In addition, abrupt changes in the resonance frequencies of some of the lines are detected. Some theoretical problems of SCS are discussed. 18 refs., 6 figs.