Fluids with State-Dependent Electronic Structure

Fluids with State-Dependent Electronic Structure... 

Percolation concepts are potentially important in the critical region of fluids with state-dependent electronic structure. Density fluctuations can create regions having very different electronic character depending on the local density. Also, as we shall discuss, mutually attracted electrons,... 

Knowledge of these unusual phenomena is fimdamental for any solution, but for metallic systems with state-dependent electronic structure and interatomic forces, these properties become especially interesting. Consider, for example, solutions in which the solvent is fluid mercury. In the critical region, the tem evolves from an assembly of neutral species (electrons bound to their parent atoms) to a partially ionized fluid. At the same time the compressibility increases rapidly due to its... 

A subsequent picosecond electronic absorption spectroscopic study of TPE excited with 266- or 355-nm, 30-ps laser pulses in cyclohexane found what was reported previously. However, in addition to the nonpolar solvent cyclohexane, more polar solvents such as THF, methylene chloride, acetonitrile, and methanol were employed. Importantly, the lifetime of S lp becomes shorter as the polarity is increased this was taken to be evidence of the zwitterionic, polar nature of TPE S lp and the stabilization of S lp relative to what is considered to be a nonpolar Sop, namely, the transition state structure for the thermal cis-trans isomerization. Although perhaps counterinmitive to the role of a solvent in the stabilization of a polar species, the decrease in the S lp lifetime with an increase in solvent polarity is understood in terms of internal conversion from to So, which should increase in rate as the S -So energy gap decreases with increasing solvent polarity. Along with the solvent-dependent hfetime of S lp, it was noted that the TPE 5ip absorption band near 425 nm is located where the two subchromophores— the diphenylmethyl cation and the diphenylmethyl anion—of a zwitterionic 5ip should be expected to absorb hght. A picosecond transient absorption study on TPE in supercritical fluids with cosolvents provided additional evidence for charge separation in 5ip. 

Strong dependence of the electronic structure on thermodynamic state implies a corresponding variation in the nature of the interparticle interactions in any fluid system. To understand radical changes in electronic structure and their implications for the interactions between particles in the fluid presents a challenging problem for theory. Together with the analogous issues raised by MNM transitions in the solid state, this represents one of the basic problems of modem condensed matter physics. 

To go beyond this qualitative observation to a treatment of the equation of state data over the whole liquid-vapor density range is far more dii cult for metals than for insulating fluids like argon. The radieal changes in electronic structure associated with the MNM transition must be taken into account in going from one phase to the other. The situation near the critical point is especially complicated because of the strong state-dependence of the electronic structure. But at least at temperatures far below the critical point, the situation is much simpler. Therefore, let us first focus on this region. 

Theories of electron mobility are intimately related to the state of the electron in the fluid. The latter not only depends on molecular and liquid structure, it is also circumstantially influenced by temperature, density, pressure, and so forth. Moreover, the electron can simultaneously exist in multiple states of quite different quantum character, between which equilibrium transitions are possible. Therefore, there is no unique theory that will explain electron mobilities in different substances under different conditions. Conversely, given a set of experimental parameters, it is usually possible to construct a theoretical model that will be consistent with known experiments. Rather different physical pictures have thus emerged for high-, intermediate- and low-mobility liquids. In this section, we will first describe some general theoretical concepts. Following that, a detailed discussion will be presented in the subsequent subsections of specific theoretical models that have been found to be useful in low- and intermediate-mobility hydrocarbon liquids. 

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