Formation and hopping motion of molecular polarons

Abstract

This paper addresses polaron formation and intermolecular hopping of a carrier confined to molecules immersed within a polar medium. Three types of polaron state are possible: (i) The carrier can encompass all equivalent sites of the occupied molecule. (ii) The carrier can be localized at a single atomistic unit of the molecule. (iii) If the carrier's interactions with the medium's ions are strong enough, the self-trapped carrier can be localized among a subset of the molecule's equivalent sites. High-temperature intermolecular hopping requires the formation of a coincidence configuration in which the carrier is shared between the jump's two molecules. The hop's activation energy depends on the expansion of a molecule's carrier as it approaches a coincidence from its minimum-energy configuration. The carrier's intramolecular motion in response to atoms' motions also reduces these atoms' vibrational frequencies thereby lowering their vibrational free energy. This carrier-induced vibrational softening affects the temperature-independent coefficient of the Arrhenius jump rate. Thus carrier's intrasite motion, ignored in Holstein's model, can significantly affect small-polaron formation and hopping.