Resolving the ambiguity in the relation between Stokes shift and Huang-Rhys parameter

Electronic transitions in luminescent molecules or centers in crystals couple to vibrations. This results in broadening of absorption and emission bands, as well as in the occurence of a Stokes shift E-Stokes. In principle, one can derive from E-Stokes the Huang-Rhys parameter S, which describes the microscopic details of the vibrational coupling and can be related to the equilibrium position offset Delta Q(e) between the ground state and excited state. The commonly used textbook relations E-Stokes = (2S - 1)(h) over bar omega and E-Stokes = 2S (h) over bar omega are only approximately valid. In this paper we investigate how E-Stokes is related to S, taking into account the effects of a finite temperature. We show that in different ranges of temperature, different approximate relations between E-Stokes and S are appropriate. Moreover, we demonstrate that the difference between the barycenters of absorption and emission bands can be used to determine S in an unambiguous way. The position of the barycenter is, contrary to the Stokes shift, unaffected by temperature.