Persistent spectral hole burning in the few-molecule limit: terrylene in p-terphenyl

Fluorescence excitation studies of persistent hole-burning spectra have been performed in p-terphenyl single crystals doped with terrylene molecules at low concentrations. They are characterized by weak inhomogeneous absorption lines with pronounced statistical fine structure (SFS). Laser irradiation in band X1 removes few molecular lines from the inhomogeneous ensemble, leading to spectral holes with depths comparable to, or even smaller than, the amplitude of the SFS; the spectral distribution of the missing molecules is no longer a smooth Lorentzian and the hole widths can be distinctly narrower than twice the homogeneous linewidth. Monte Carlo simulations confirm this finding and demonstrate the difference as compared to the “thermodynamic limit” of many molecules being present in the ensemble.