The chromatographic impulse response technique with the curve fitting method is a useful tool to measure binary diffusion coefficient under supercritical conditions. Based on our recent measurement data, the effectiveness of the simple predictive correlations such as the D₁₂-viscosity correlation and the Schmidt number correlation is presented. Moreover, the sensitivities of the parameters determined and sources of experimental error are discussed.

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In this article, recent advances in the measurement of the translational diffusion of the reaction intermediate radicals are reviewed. It was described how the diffusion of the reaction intermediates of short lifetimes were measured by the transient grating spectroscopy in supercritical fluids (SCFs). Transient ketyl radicals produced by the hydrogen abstraction reaction shows a relatively slower diffusion than that of the stable molecules of similar size in SCF. The relation of the slowness of the diffusion to the local density enhancement is discussed.

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Theories for diffusion-controlled reaction (DCR) dynamics based on the diffusion equation (DE) and the Fokker-Planck-Kramers equation (FPKE) have been reviewed, and the validity of the theories has been examined in the picosecond time region by comparing them with computer simulations in liquid Ar. At the initial a few picoseconds, the dynamics of DCR is not explained by the Smoluchowski theory based on DE. Although the Collins-Kimball treatment for DE works practically well, FPKE should be used for detailed discussion. The effect of the initial distribution of reactants is significant in this time range. At times longer than ten picoseconds, the effects of the inertia and the initial distribution are not important, and the dynamics of DCR is well explained by the Smoluchowski theory. A possibility of the application of the DE-based theories in interpreting the reaction dynamics in supercritical fluids has been discussed briefly.

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In this article, recent studies on the pressure dependence of the bimolecular fluorescence quenching with a nearly diffusion-controlled rate in liquid and supercritical fluid were reviewed. In liquid solution, the pressure dependence of the quenching rate constant, k_q, by heavy atom quenchers and oxygen was successfully analyzed by taking into account the pressure dependence of radial distribution function. The analysis was applied to the quenching systems in supercritical CO₂, and it was found that the treatment is applicable to the fluorescence quenching of 9,10-dimethylanthracene by oxygen and CBr₄ with high k_q, but not to that of 9-cyanoanthracene by oxygen with lower k_q.

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Density dependence of the rate constants of diffusion-controlled ionic recombination reactions in supercritical (SC) fluoroform was investigated to explore the effect of the local density enhancement on diffusion-controlled reactions. For the ionic recombination, the effect was clearly detected as strong retardation of reaction rate near the critical density. On the other hand, for the radical self-termination, the effect was not detected. This difference must be due to the strong solute-solvent interaction between ionic species and solvent molecules. The local density around the charged molecules should be drastically enhanced by Coulomb interactions.

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A fast kinetic technique using high energy electron beams was applied to the study of reactions of radical ions in supercritical water and carbon dioxide. The high energy electron beams can easily ionize water molecules and produce hydrated electrons, OH radicals and H atoms. The reaction rate between the hydrated electrons and negative ions decreased with increasing temperature because of the repulsive interaction. On the other hand, the reaction rate between the hydrated electron and hydronium ions increased with temperature because of the attractive interaction. In supercritical CO₂, dimer anions (CO₂)₂^- and dimer cations (CO₂)₂⁺ were produced by ionization of CO₂. Transient visible absorption spectra of these species were measured. Comparison with the photobleaching of CO₂ anion clusters in solid rare gas matrices and their reactivity with H₂ and O₂ confirms the assignment. It is well established that the adiabatic electron affinity of CO₂ is negative, but the adiabatic electron affinity of CO₂ dimer has been calculated to be 0.89 eV for D_2d symmetry (CO₂)₂^- in gas phase.

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Establishment of tissue cultures derived from deep-sea multicellular organisms has been extremely difficult, because of serious damage they sustain upon decompression and exposure to high temperature of surface seawater. We developed a novel pressure-stat aquarium system and a novel pressure chamber control system for the study of living deep-sea multicellular organisms and derived cells under the pressure. Using these devices, we successfully cultivated and freeze-stocked fin cells of deep-sea eel Simenchelys parasiticus collected at 1162m under atmospheric pressure condition.

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An essential and fundamental problem to determine a P-V-T thermal equation of state (EOS) of solid as a primary pressure standard is the fact that the pressure or volume dependence of thermal property of solid, that is necessary for the conversion of Hugoniot to isotherm, cannot be obtained experimentally. In this article, theory of P-V-T EOS of solid is reviewed first, and based on our recent ab initio results of gold, validity of previous P-V-T EOS models and especially volume dependence of Grüneisen parameter are discussed in detail.

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