Dynamics in supercritical fluids studied using ultrafast pulse laser spectroscopy

Solvation dynamics and rotational relaxation in supercritical fluids have been examined using the techniques of the subpicosecond laser spectroscopy. The fluorescence up-conversion technique has been employed to detect the fluorescence anisotropy decay of coumarin 153 in supercritical CO ₂ (310 K) and CHF ₃ (302, 310 K) over a wide density range. The rotational correlation times in supercritical CHF ₃ exhibit the remarkable increase at 0.6 0.7 times ρ _c, which implies that the local solvent density augmentation slows down the rotational dynamics in supercritical fluids. This conclusion is supported by computer simulations based on a 2-site model of fluoroform. The solvation dynamics of the probe 4-dimethylamino-4′-cyanostilbene have been measured in supercritical CHF ₃ at 310 K using the optical Kerrgated emission technique, which appears to have captured most (∼90%) of the dynamics occurring over the accessible density range between 1.4-2.0ρ _c. The measured spectral response exhibits a bimodal decay with ∼0.6 ps and ∼9 ps components and only a modest density dependence.