Anisotropic dispersion of excitonic bands of the single-crystal pentacene (001) surface as measured by low-energy angle-resolved high-resolution electron energy-loss spectroscopy

Low-energy high-resolution electron energy-loss spectroscopy (HREELS) is a useful technique for the characterization of various excitation processes at solid surfaces. However, no successful work has been reported on molecular single-crystal samples yet. In the present study, low-energy angle-resolved HREELS measurements were conducted on single-crystal pentacene, an organic semiconductor. The results confirmed the excitonic bands exhibiting energy–momentum dispersion and anisotropy of these depending on the surface crystallographic directions, corroborating the occurrence of exciton delocalization, contrary to the ordinary notion of the Frenkel exciton for weakly interacting van der Waals molecular solids. The present results demonstrate that low-energy angle-resolved HREELS is applicable to the precise examination of the excitonic characteristics of solid-state surfaces, even for molecular semiconductor single crystals.