Direct vibrational energy transfer in zeolites

Abstract

With two-color picosecond infrared laser spectroscopy the dynamics of O–H and O–D stretch vibrations in zeolites are investigated. Zeolites appear to be good model systems to study transfer of vibrational energy in a solid. For the O–D vibrations, transient spectral holes are burnt in the inhomogeneously broadened absorption bands by saturating the absorption with a strong pump pulse. From the spectral hole widths the homogeneous absorption linewidths are obtained. The excited population lifetimes are determined using a time-resolved pump–probe technique, and in combination with the homogeneous linewidth the pure dephasing time is revealed as well. For high concentrations of O–H oscillators the vibrational stretch excitations are found to diffuse spectrally through the inhomogeneous absorption band. This spectral diffusion process is explained by direct site-to-site transfer of the excitations due to dipole–dipole coupling (Förster transfer). The dependences of the transient spectral signals on oscillator concentration and the results of one-color polarization resolved experiments confirm this explanation. The spectral transients are satisfactorily described by simulations in which the site-to-site transfer by dipole–dipole coupling is taken into account.