Density crystallization effects

The LIDA concept, discussed above, has demonstrated the potential of organic dyes for use in high density optical recording systems. However, it is apparent that the contrast density is marginal, particularly in the reflection mode. Furthermore, vacuum deposited dyes are also subject to crystallization effects which lead to focussing and archival problems. 

J. Overgaard, M. P. Waller, J. A. Platts and D. E. Hibbs, Influence of crystal effects on molecular densities in a study of 9-ethynyl-9-fluorenol, /. Phys. Ghent. A107, 11201-11208 (2003). 

A laser-induced change in the temperature of an isotropic liquid crystal can modify its refractive index in two ways, very much as in the nematic phase. One is the change in density dp due to thermal expansion. This is the thermal absorptive component discussed before [Eq. (9.18) for p ] this term may be written as (0n/0p) p. The other is the so-called internal temperature change dT which modifies the spectral dependence of the molecular absorption-emission process we may express this contribn-tion as (0n/07)p dT. A pnie density change effect arises from the electrostrictive component p, which contribntes a change in the refractive index by (0u/0p) p . 

In general, the phonon density of states g(cn), doi is a complicated fimction which can be directly measured from experiments, or can be computed from the results from computer simulations of a crystal. The explicit analytic expression of g(oi) for the Debye model is a consequence of the two assumptions that were made above for the frequency and velocity of the elastic waves. An even simpler assumption about g(oi) leads to the Einstein model, which first showed how quantum effects lead to deviations from the classical equipartition result as seen experimentally. In the Einstein model, one assumes that only one level at frequency oig is appreciably populated by phonons so that g(oi) = 5(oi-cog) and, for each of the Einstein modes. is... 

Equation (Bl.8.6) assumes that all unit cells really are identical and that the atoms are fixed hi their equilibrium positions. In real crystals at finite temperatures, however, atoms oscillate about their mean positions and also may be displaced from their average positions because of, for example, chemical inlioniogeneity. The effect of this is, to a first approximation, to modify the atomic scattering factor by a convolution of p(r) with a trivariate Gaussian density function, resulting in the multiplication ofy ([Pg.1366]

Cortona embedded a DFT calculation in an orbital-free DFT background for ionic crystals [183], which necessitates evaluation of kinetic energy density fiinctionals (KEDFs). Wesolowski and Warshel [184] had similar ideas to Cortona, except they used a frozen density background to examine a solute in solution and examined the effect of varying the KEDF. Stefanovich and Truong also implemented Cortona s method with a frozen density background and applied it to, for example, water adsorption on NaCl(OOl) [185]. 

The term electrochromism was apparently coined to describe absorption line shifts induced in dyes by strong electric fields (1). This definition of electrocbromism does not, however, fit within the modem sense of the word. Electrochromism is a reversible and visible change in transmittance and/or reflectance that is associated with an electrochemicaHy induced oxidation—reduction reaction. This optical change is effected by a small electric current at low d-c potential. The potential is usually on the order of 1 V, and the electrochromic material sometimes exhibits good open-circuit memory. Unlike the well-known electrolytic coloration in alkaU haUde crystals, the electrochromic optical density change is often appreciable at ordinary temperatures. 

The effects of a solvent on growth rates have been attributed to two sets of factors (28) one has to do with the effects of solvent on mass transfer of the solute through adjustments in viscosity, density, and diffusivity the second is concerned with the stmcture of the interface between crystal and solvent. The analysis (28) concludes that a solute-solvent system that has a high solubiUty is likely to produce a rough interface and, concomitandy, large crystal growth rates. 

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