Pressure tuning spectroscopy is a general and powerful tool for studying the effects of atomic or molecular interaction on electronic phenomena-broadly defined. In this paper we present a review of two studies of pressure effects on the triplet state of organic molecules in solid media.
In the first study we demonstrate the effect of compression on the photophysics and photochemistry of representative aromatic carbonyls. There are dramatic increases in phosphorescence and fluorescence efficiencies as well as strong perturbations on the reactivity.
In the second study we demonstrate the powerful effect of compression on inhibiting intracomplex motions of ligands for ruthenium organometallic complexes. This inhibition strongly effects the phosphorescence efficiency of the molecule.

A short review is presented, which has been centred on results of measurements with a very extensive series of palladium/platinum alloys over a very wide range of hydrogen pressures extending up to 30 kbar (3 GPa). For the higher platinum content range of alloys, these studies have revealed that relationships at 25^oC between hydrogen content (n = H/(Pd+Pt) atomic ratio) and incremental changes , of electrical resistivity, can be represented to a good approximation by a common curve of similar form to those of Nordheim types of relationship representative of resistivity-substitutional alloy composition relationships. Consideration of these interrelationships has led to interpretations (inclusive of hysteresis effects) of phase relationships of the palladium-hydrogen and lower platinum content regions of the palladium-platinum-hydrogen systems. Over the highest range of hydrogen pressures, values of hydrogen contents, n, have necessarily had to be derived from electrical resistivity measurements. Use, however, could have be made of correlating possibilities of interrelationships between , n and equilibrium hydrogen pressure, p, up to approximately 20 bar that are available from results of an allied series of electrode potential based experiments.

Some results regarding regularities of the Rayleigh-Benard type convection initiated in electrochemical systems both with finite or infinite electrodes are presented and gene-ralized. The influence of the electrolysis regime has also been taken into account. Similarity and distinction between convection in the electrochemical system with infinite electrodes and close phenomenon of the "pure" thermal convection initiation are commented. Break of the "continuous convective layer" is described for the electrochemical system with finite electrodes. The influence of lateral walls on the hydrodynamic stability of the electrolyte has been described. Region of regular convection occurrence has been drawn up on the parameter plane. A theoretical model, leading to a satisfactory agreement with experiment, has been proposed for the description of the bottom margin of the "convection area".

The role of barrier fluctuations in the membrane enzymatic processes, among others in the active transport of ions through cell membranes is examined. Membrane electric potential plays the role of a barrier height (activation energy) for enzymes embedded in the cell membrane. This barrier height can be modulated either by internal fluctuations or by superimposed from without external electrical fields, either random or periodic. Measurements of the changes in reaction yield, or in ionic currents, governed by these enzymes (ionic pumps) can be used as tools for determining the parameters describing enzymatic action. Barrier fluctuations can change considerably the yield, including the inversion of the direction of transmembrane currents.

Transient periodic and chaotic oscillations observed during anodic dissolution of copper under potentiostatic conditions are described and analyzed.

A comparative examination of the friction coefficients, f_ik, coupling coefficients, q_ik, and mobility indices of water, RT/c_wr_ww, for the cation-exchange membranes, published from different laboratories, is presented. The counterion-coion-water and membrane network interactions have been discussed and general conclusions about the interrelat-
ions of the membrane morphology and properties have been drawn.

An expression relating the phenomenological coefficient L to the relaxation time tau and temperatures is derived within the formalism of non-equilibrium thermodynamics. For the model of hard spheres the coefficient L is expressed by number densities of components, the molecular masses, the diameters of spheres, the correlation function and temperatures. The values of tau and L are calculated for binary mixtures of noble gases. It is shown that L (similarly to tau) is time dependent, because the temperatures of components change during the relaxation process. Such changes are smaller than 10 percent even for relatively large temperature differences.

Research on the hydrogen solubilities in three different forms of Pd₃Mn are reviewed. These are the two ordered forms: L1₂, long-period superstructure (Al₃Zr-type), and the disordered form. Similar to the hydrogen solubilities in Pd₃Fe, studied earlier in Baranowski's laboratory, the ordered forms of Pd₃Mn dissolve significantly more hydrogen than its disordered form. Dissolved H induces metal atom diffusion so that the disordered form of Pd₃Mn orders at significantly lower temperatures than in vacuo. It orders in the presence of dissolved H to the L1₂ rather than the long-period superstructure (lps) form, which results from annealing in vacuo.
In the alloy system Pd-Rh, dissolved H is shown to enhance metal atom diffusion leading to phase separation in accord with the miscibility gap phase diagram. In the case of Pd-Pt alloys dissolved H can lead to phase separation via a ternary equilibrium; dissolved H is shown to enhance metal atom diffusion in this system.

The equilibrium pressure of hydrogen gas above solid face centered cubic (fcc) phase of chromium hydride has been determined at 150^oC. As compared to the hexagonal close packed (hcp), chromium hydride plateau for the alpha + ß phase region has been found to be lower by about 500 bars and equal to 0.25 +/- 0.01 GPa. As a consequence, lower value of standard free energy of formation of the fcc chromium hydride has been calculated and a lower equilibrium pressure at normal temperature as well. One can also assume a lower hydrogen pressure necessary for a direct synthesis of the fcc hydride, as compared to the hcp phase. The search for possible pressure induced phase transition from the fcc to the hcp structure has been unsuccessful up to 37 GPa. The compressibility of the fcc hydride is similar to that of the hcp phase, showing pressure independence of the hydrogen volume in the metal lattice, and confirming the general rule observed in transition metal hydrides.

In this paper high-pressure differential thermal analysis (DTA) and differential scanning calorimetry (DSC) measurements up to 300 MPa are presented on a selected liquid crystal (4-n-decyloxy-4'-cyanobiphenyl, 10OCB) and triglyceride (1,2,3-tris-eicosano-yloxypropane, Triarachidin, AAA), respectively. In particular T(p) phase equilibrium data are given as well as transition enthalpy values as a function of pressure along the transition lines. For 10OCB a stable pressure-induced nematic phase has been detected at pressures above 34 MPa; for this substance the T(p) phase diagram was also determined by high-pressure polarization microscopy. For pure AAA a metastable phase is found at low pressures that transforms to the stable ß modification with increasing temperatures; at higher pressures only the melting of a solid phase could be detected, whereas for the substance saturated with helium both transitions could be observed in the whole experimental pressure range up to about 100 MPa by DTA and 200 MPa by DSC.

The intermolecular hetero Diels-Alder reaction of enamino ketone 1 with isopropenyl methyl ether 2 leading to the diastereomeric products 3 and 4 is studied under high pressure up to 5 kbar and at temperatures between 30^oC and 120^oC. The kinetics are measured by on-line FT-IR spectroscopy up to 3 kbar. The ratio of the diastereomeric products is determined by HPLC-analysis. In order to gauge the influence of solvent properties on the kinetics and diastereoselectivity, the cycloaddition is carried out in dichloromethane, in acetonitrile and in a mixture of heptane/isodurene (2:1). Clear effects of solvent polarity on the rate coefficients as well as on the activation parameters are observed. To quantify the solvent influence on the activation volume, an estimate of the intrinsic (solvent-independent) contribution to the activation volume, V _intr, is made

High pressure self-diffusion coefficients and densities are reported for the binary system tetradecane/methane in a wide range of temperature at pressures up to 200 MPa. The experimental diffusion data of these mixtures are analyzed in the frame work of the rough hard sphere (RHS) model under application of hard sphere (HS) molecular dynamics simulations for binary mixtures of hard spheres obtained recently in our group. The rotation-translation coupling parameter A shows a weak temperature dependence, falling slightly when the temperature is lowered. The RHS-diameter for the lighter compound is identical to the value found for neat methane. The value for the RHS-diameter of tetradecane in the mixture is found slightly larger than in the neat tetradecane. Both diameter show a slight increase with falling temperature..

Thermoelectric power of Ni₉₅Co₅, Ni₉₀Co₁₀ and Ni_90.7Mn_9.3, Ni_82.5Mn_17.5 alloys was measured as a function of hydrogen pressure up to 1.7 GPa for nickel-cobalt and up to 1.0 GPa for nickel-manganese alloys. For the last alloys the change of thermopower during the hydride formation is positive and approaches the value characteristic for pure copper. The behavior of nickel-cobalt alloys was, unexpectedly, different from that of all other nickel alloys investigated so far. When 5 at.% of cobalt is dissolved in nickel the hydride forms at 25^oC almost without change of thermopower. When the concentrat-
ion of cobalt increases to 10 at.% the hydride formation is accompanied by a large negative change of thermopower. A qualitative interpretation is proposed.

The phase diagram of the Tl₂Te-Cu₂Te system has been established by thermal analysis (cooling curve method) and differential thermal analysis. The results obtained are compared with those of electrochemical studies on the same system. In the system three compounds are formed (CuTl₉Te₅, Cu₆Tl₄Te₅ and Cu₉TlTe₅).

The optic phonon modes of hydrogen in dhcp iron hydride (FeH_x with x close to 1) were investigated by neutron spectroscopy. The optic phonons have energies between ~70 and ~140 meV. The average energy of the optic phonons is ~105 meV.

Thermal analysis and X-ray diffraction were used to study both the dynamic process of dehydration and the high-temperature phase transition at T_p in KH₂PO₄ and RbH₂PO₄. Our results show strong evidence that at T_p an onset of partial polymerization takes place rather than a polymorphic phase transition.

Specific heats and magnetic properties as a function of temperature and magnetic field for U₆MH₁₇ (M = Fe, Co, Mn and Ni) hydrides have been measured. These hydrides crystallize in the cubic Pm3n space group, like ß-UH₃. The compounds are ferromagnets. The essential thermodynamic functions of the entropy S and energy E for magnetic heat contribution were evaluated. It was established, that 3d-metals change the magnitude of the magnetic anisotropy having, however a negligible influence on the magnetic transition temperature. The spin-reorientation transitions, suggested previously, were not confirmed.

The electrochemical behaviour of 3-nitrophtalic (3-NPA) and nitrobenzoic acids on mercury electrodes has been investigated. By using chronovoltammetry and chronocoulometry with pulse time ranging from 4 to 100 msec, it was found that above pH 8 in the case of 3-nitrophtalic acid anion one 4-electron wave separates into 1-electron and 3-electron wave. Such behaviour was not observed in other nitrobenzene derivatives such as nitrobenzoic acids. The influence of pH, pulse time, reactant concentration and Lewis acids on the 3-NPA wave separation has been studied. Semi-empirical calculations of charge distribution for 3-NPA, nitrobenzoic acids and nitrobenzene, as well as their free radicals and dianions, have shown significantly higher electron density on nitro group of 3-NPA radical than on other species studied. This difference in electron density distribution may generate the non-typical electrochemical behaviour of 3-nitrophtalic acid.

The impedance spectra of the hydrogen electrode reaction (HER) as a function of potential at several temperatures in pure molten ethylammonium chloride (EACl) were recorded. The recorded impedance spectra were worked out using an equivalent circuit approach and the theory of impedance of a quasi reversible electrode process preceded by a chemical (dissociation) reaction. This allows for the determination of the exchange current density, its activation energy, diffusion coefficient of the hydrogen cation and the forward and backward rate constants of the preceding dissociation of the ethylammonium cation.

A number of alkylsilanes and alkylcarbosilanes of widely different molecular structure are characterized in terms of their ability to the formation of amorphous hydrogenated silicon-carbon (a-Si:C:H) film in atomic hydrogen-induced chemical vapour deposition (AHCVD). The compounds, containing only the Si-C bonds or four-membered carbosilane rings appear to be inactive, while those with the Si-Si or Si-H bonds are capable of the a-Si:C:H film-formation. The reactivity of the latter group of compounds in the AHCVD is characterized by the deposition yield parameter, determined at constant and variable feeding rates. Basing upon the reactivity data, the mechanism of the initiation step is proposed.

Extreme changes of electron dynamical observables in the hydrogen atom are examined. It is found that these changes calculated for the fundamental electron observables (position, momentum, time period and energy) can be combined into products whose values are not much different than the Planck constant h. A similar property is demonstrated for the rotation angle and orbital angular momentum of an electron enclosed in a spherical cavity. Extreme changes calculated for the observables being a function of electron position and momentum can be also combined into products, which are formally similar to expressions obtained for the commutators of the observables.

Crystal structure of antimony sulfoiodide SbSI has been determined in all three phases: ferroelectric, antiferroelectric and paraelectric. In the ferroelectric phase III crystals of SbSI are polar in the space group Pna2₁ in agreement with previous reports. Above the Curie point at 298 K SbSI in phase II becomes antiferroelectric in the space group P2₁2₁2₁ and alternating arrangement of polar double chains parallel to [001]. Only above the high temperature phase transition at 410 K, the crystals of SbSI become centrosymmetric in the space group Pnam.

Results of phase studies of Tl₂Te-M_xTe_y systems are presented, obtained by DTA, TA, concentration cell EMF and X-ray diffraction methods. Factors influencing the formation of ternary compounds are discussed.

Correct diffusion measurements using electrochemical time-lag methods must be carried out under careful consideration of the experimental boundary conditions. Additionally, the interpretation of the measured data requires an exact knowledge of the sample composition, structure and the hydrogen concentration.
Considering these basic requirements, the current pulse technique yields reliable data for the hydrogen diffusion in metals and alloys with high solubilities, from extremely low to high hydrogen concentrations, as well as with low solubilities. This is mainly demonstrated on measurements on the Pd_xAg_1-x system.