Optical Studies of Proton Tunneling and Relaxation in Benzoic Acid

Abstract

The double proton transfer, leading to the tautomerization of carboxylie acid dimers, is an example of a translational tunneling system. It provides a model for the interplay of tunneling and relaxation in a condensed phase environment. At low temperatures the study of such systems was made possible by the discovery that the optical spectra of dilute mixed crystals of indigo dyes in benzoic acid reflect the proton structure and dynamics of neighboring benzoic acid dimers. Fluorescence line narrowing and time-resolved spectral measurements lead to the determination of the tunneling matrix element (0.16 cm⁻¹) and the rate of tautomerization by tunneling (1.7 – 4.5 × 10⁺⁸ sec⁻¹).