Vibrational dynamics in supercritical fluids and the collisionless gas phase

Solute vibrational relaxation is studied experimentally and theoretically in several different supercritical fluids. Supercritical fluids are gases or liquids which have been heated above their critical temperature and no longer undergo condensation or evaporation. While many properties of these fluids are in-between those of gases and liquids, some very interesting phenomena occur near the critical point. The currently accepted physical models of supercritical fluids make use of the idea of attractive solute-solvent interactions, i.e. solute-solvent clustering, around the critical point. Studying ultrafast processes like vibrational energy transfer gives insight into the solute-solvent interactions. We find that the vibrational relaxation of the ν ₆ asymmetric CO stretch of W(CO)₆ has similar density dependences in three supercritical solvents; the vibrational lifetime gets shorter with an increase in density. However, we observe markedly different temperature trends based on the solvent and isochore. In particular, we observe that vibrational relaxation actually slows down with increasing temperature along some isochores in ethane, an effect never seen before at a fixed solvent density. In the other cases, the vibrational lifetime gets faster with increasing temperature. We have introduced a new theory which can predict both the temperature and density trends. The theory is based on the thermodynamics/hydrodynamics of the solvent. By incorporating the large changes in the bulk thermodynamics associated with proximity to the critical point, we show that the experimental data is well modeled without invoking solute-solvent clustering.; Relaxation dynamics have also been followed in the gas phase, i.e., in the absence of a solvent. We observe three different regimes in the energy transfer of the ν₆ W(CO)₆ vibration. A long component derives from a heating-induced frequency shift of the mode. An intermediate region is the actual vibrational lifetime at zero density, i.e., the intramolecular vibrational redistribution (IVR) rate. A fast decay is also observed, which is not measured in liquids. This fast component arises from spectral diffusion-like processes involving the evolution of the W(CO)₆ molecular structure due to the motion of other internal vibrations. Experiments with argon show that the fastest and slowest decays disappear when the solute has substantial collisions with a solvent.