3 PULSE STIMULATED PHOTON-ECHO EXPERIMENTS AS A PROBE OF POLAR SOLVATION DYNAMICS - UTILITY OF HARMONIC BATH MODELS

The three pulse photon echo peak shift technique was used to study sol vation dynamics in acetonitrile (297 K), methanol (297 and 323 K), and ethylene glycol (297 and 397 K) utilizing the tricarbocyanine laser d ye, IR144, as a probe. The spectral density, rho(omega), governing the solute-solvent interaction was obtained for each solvent and temperat ure through numerical fitting of the three pulse photon echo peak shif t relaxation using finite temporal-duration optical fields. An ultrafa st three pulse photon echo peak shift relaxation, ascribed to the iner tial component, was nearly identical for ethylene glycol at 297 and 39 7 K; this indicates the spectral density is essentially temperature in dependent from 10 to 250 cm(-1) over this temperature range. Conversel y, the low-frequency spectral density (0-10 cm(-1)) obtained from thre e pulse photon echo peak shift relaxation of ethylene glycol at 297 an d 397 K showed a strong temperature dependence which cannot be predict ed using harmonic bath models. We calculated spectral densities for et hylene glycol, acetonitrile, and methanol using the simple dielectric continuum model and the dynamical mean spherical approximation, using where possible, the relative permittivity constants calculated from ex perimental far-infrared absorption data and dielectric dispersion data . Additionally, we calculated spectral densities in terms of the exten ded reference interaction site model for methanol and acetonitrile. Th ese calculated spectral densities describe our experimental methanol a nd acetonitrile photon echo better than all other solvation model spec tral densities. Our results give insight into the domain of applicabil ity of the harmonic model of liquid dynamics. (C) 1997 American Instit ute of Physics.