Dielectric spectroscopy and time dependent Stokes shift: two faces of the same coin?

Different types of spectroscopy capture different aspects of dynamics and different ranges of intermolecular contributions. In this article, we investigate the dielectric relaxation spectroscopy (DRS) of collective nature and the time-dependent Stokes shift (TDSS) of disputed nature. Our computational study of unconfined and confined water clearly demonstrates that the TDSS reflects local, non-collective dynamics. Surprisingly, we found that the reaction field continuum model (RFCM) used to estimate TDSS curves solely from collective DRS spectra correctly transforms collective dynamics to local ones even in cases when the relaxation time trends are quite different. This correct transformation is possible due to structural information available in the DRS amplitude in a Kivelsen-Madden like context.