A prediction made when playing with neocube® is confirmed by impurity spectroscopy

Abstract

Earlier, we have used Lennard-Jones (L-J) model of glassy state to describe the inhomogeneous band shape of impurity spectra and its transformation under compression. Color effects on spectral holes burned at different frequencies could be rationalized for temperature and pressure changes (Lin et al. Chem Phys Lett 576:15–20, 2013; Renge, J Chem Phys 127:034504, 2007). Qualitatively, observation of a heap of balls allows one to draw important conclusions about the behavior of glassy state. For example, tight packed lattices, hexagonal or cubic, are not formed, because of friction, so that glasses contain ~ 10% of free space. The coordination number of a sphere is expected to be 8–11, instead of 12. Huge amount of potential energy must be locked in glassy matter. On the other hand, pair wise distances remain the same as in the “crystal”. Structurally, glass is not a frozen liquid. Moreover, glass formation cannot be regarded as a phase transition, since no phase boundary is created. Below glass temperature Tg the Boltzmann thermometer fails, and so does (equilibrium) thermodynamics (Lin et al. Chem Phys Lett 576:15–20, 2013; Renge, J Chem Phys 127:034504, 2007). The visualization of salient features of the glassy state is possible, notwithstanding the fact that isotropic, dispersive attraction of L-J 6–12 potential is replaced simply by gravitation. Depending on the size, nanoparticles are expected to show intermediate behavior between molecules and macroscopic bodies. “Crystallization” of uniform nanoparticles as a function of interaction type is a non-trivial phenomenon, and is of considerable interest. Collections of Nd magnets, typically 216 spherical beads of diameter 5mm have recently been commercialized as NeoCube®. Even without computer modeling, it becomes obvious that simple cubic lattice ofmagnetic dipoles is metastable and collapses easily into disorder. Also, close packed face centered cubic or hexagonal structures appear difficult to obtain. Instead, strings, loops, voids, and helixes tend to form. Such behavior of dipolar hard spheres is well known from Monte Carlo simulations of polar liquids, liquid crystals, polymers, Janus colloids, ferrofluids, nanoparticles, self-assembly of photonic materials, etc. (Weis and Levesque, Phys Rev Lett 71:2729–2732, 1993; Groh and Dietrich, Phys Rev E 63:021203, 2001). We wish to establish a closer connection of this active field of statistical physics with impurity spectroscopy. More realistic description of van der Waals glasses should be possible with Stockmayer potential comprising both the L-J and dipolar components. In a demonstration experiment, solvatochromism of a polar dye was considered. A greatly enhanced dipole moment in the excited state should increase average intermolecular distance. As a result, the matrix density dependence of absorption and fluorescence band shifts will be different. Indeed, the refractive index dependence of emission band positions is less steep than that of absorption for widely used laser dyes DCM, Nile Red, and Coumarins. In accordance with the Lorenz-Lorentz and London formulas, the solvent refractive index dependent solvent shifts, owing to dispersion and induction, have been assumed constant in literature for both absorption and fluorescence maxima. By contrary, it follows from Table 64.1 that emission bands are much less sensitive. Considerably higher dipole moment in the excited state causes an increase of effective distance between the solute and solvent molecules r, diminishing the absolute value of coefficient p. The dependence on ε′ is affected both by the change of r, and Onsager’s reaction field that is stronger in the zwitter-ionic excited state.