- Alexander Van Humboldt Fellow, University of Dortmund (1982-83, 84)
- NIH Postdoctoral Fellow (1967-1968), University of South Carolina (James R. Durig)
- Ph.D. (1967), Washington State University (Edward L. Wagner)
- B.S. (1963), Michigan State University
Generally, our interests focus on the electro-optical properties of supracritical dense gases. Because of the supracritical property we can vary the density with complete freedom without condensation taking place. This allows us to probe the intermolecular potential of the system via interaction induced (ii) Raman light scattering. The Raman spectral intensity, I, may be written as
I = 2 N2 <m12>2 + 4 N3 <m12m13> + N4 <m12m34>
where N is the number density and the mijs are the induced spectral moments. The N3 term’s moments are negative so at high enough density values the spectral intensity will begin to fall off sharply. Thus, the Raman ii signal may be thought of as arising from local density fluctuations giving rise to transient local field gradients.
We have investigated neat methane and methane solution spectra at supracritical conditions. We have seen that the Raman depolarization ratios (RDR) track the ultra-strong rotation-vibration coupling (coriolis constant) in the methane molecule. The RDR changes very rapidly at elevated densities (pressure) indicating changes in the intermolecular potential function. Depending upon the molecules surrounding the methane, the position of the sigmoidal curves will shift reflecting the inter-body potential change. In the figure below, frequency shifts are denoted by triangles and the RDR data by squares.
THE ROLE OF FLUCTUATIONS IN THERMODYNAMICS AND LIGHT SCATTERING
Intermolecular Raman light scattering depends upon the electron polarizabilty between molecules. As the molecules move the polarizability must change. Thus, as the molecular motion fluctuates so does the polarizabilty. As a result, the polarizability tracks the molecular positional fluctuations. We now ask how energy and entropy may be considered functions of position.
If the molecules fluctuate in position, then E and S must fluctuate as well. Let's ask the following question, "Is there a volume for which equilibrium thermodynamics holds and another volume that is too small for equilibrium thermodynamics to hold?" *
The answer is yes to both... suppose we have some thermodynamic variable Y, for equilibrium thermodynamics to be applicable the rms value of Y, dY, must be something like 1x10-6 the magnitude of Y or smaller.* If a box has a side of 0.03 mm, we find that dY/Y = 4x10-7, clearly within equilibrium behavior. However, if the box is 100 Å on a side (i.e., 10-15 molecular diameters), then dY/Y = 4 x10-2 and equilibrium thermodynamics will fail and number density can increase and ΔS is (-) and ΔE is (+). Such configurations can form for femtoseconds.
* Modern Thermodynamics by Kondepudi and Prigogine.
- Palmer, T.; Stanbery, W.; Baglin, F.G. An interpretation of the solute-solvent interactions in supercritical binary fluids as monitored by interaction-induced Raman light scattering. J. Mol. Liq. 2000, 85, 153.
- Baglin, F.G.; Murray, S.K.; Daugherty, J.E.; Palmer, T.E.; Stanbery, W. Interaction-induced Raman light scattering as a probe of the local density of binary supercritical solutions. Mol. Phys. 2000, 98, 409.
- Baglin, F.G.; Sweitzer, S.; Friend, D.G. Interaction induced Raman light scattering studies of CH4/H2 mixtures as a function of density. J. Phys. Chem. B 1997, 101, 8816-8822.
- Baglin, F.G.; Sweitzer, S.; Stanbery, W.J. Raman light scattering from supracritical binary fluid mixtures: CH4/CF4. Chem. Phys. 1996, 105, 7285.
- Baglin, F.G.; Rose, E.J.; Sweitzer, S. Identification of 1, 2 and 3 body Raman scattering by the field gradient induced dipole A tensor in methane. Mol. Phys. 1995, 84, 115.