Three 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 solvation dynamics in acetonitrile (297 K), methanol (297 and 323 K), and ethylene glycol (297 and 397 K) utilizing the tricarbocyanine laser dye, IR144, as a probe. The spectral density, ρ(ω), governing the solute-solvent interaction was obtained for each solvent and temperature through numerical fitting of the three pulse photon echo peak shift relaxation using finite temporal-duration optical fields. An ultrafast three pulse photon echo peak shift relaxation, ascribed to the inertial component, was nearly identical for ethylene glycol at 297 and 397 K; this indicates the spectral density is essentially temperature independent from 10 to $\text{250\hspace{0.17em}}{\mathrm{cm}}^{\mathrm{-1}}$ over this temperature range. Conversely, the low-frequency spectral density $\text{(0–10\hspace{0.17em}}{\mathrm{cm}}^{\mathrm{-1}})$ obtained from three pulse photon echo peak shift relaxation of ethylene glycol at 297 and 397 K showed a strong temperature dependence which cannot be predicted using harmonic bath models. We calculated spectral densities for ethylene 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 experimental far-infrared absorption data and dielectric dispersion data. Additionally, we calculated spectral densities in terms of the extended reference interaction site model for methanol and acetonitrile. These calculated spectral densities describe our experimental methanol and acetonitrile photon echo better than all other solvation model spectral densities. Our results give insight into the domain of applicability of the harmonic model of liquid dynamics.