Polarization energies of organic solids determined by ultraviolet photoelectron spectroscopy
The polarization energies of 44 organic solids were determined by ultraviolet photoelectron spectroscopy in the gaseous and solid states. Condensed polycyclic aromatic hydrocarbons with planar molecular structures were found to have a common value, 1.7 eV, independent of their molecular sizes and also their crystal structures. The common value is approximately interpreted by the first-order expression for the polarization energy. A large variation of values in the range 0.9–3.0 eV was obtained for several compounds. Among them, molecules with intricate structures have smaller values and those with large molecular polarizabilities have larger values than the common value. These results indicate that the polarization energy of an organic solid is mainly determined by two factors: the molecular polarizability and the molecular packing in the solid. Intermolecular interactions in the solid, other than the van der Waals force, also contribute to the value.