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Dr. Richard MacPhailAssociate Professor of Chemistryemail: ram@chem.duke.edu phone: (919) 660-1536 offices: 308 Gross Chem Lab: 314, 315 Gross Chemistry |
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B.A. '77, Oberlin College; Ph.D. '81, Univ. of California-Berkeley; Postdoctoral Fellow '82-'84, UCLA ; Asst. Prof. '84-'93, Assoc. Prof. '93-, Duke Univ., Director of Graduate Studies '95-'98. |
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Research InterestsPhysical ChemistryAn understanding of molecular dynamics in liquids and other disordered condensed phases is crucial to addressing fundamental problems in chemistry and materials science at the molecular level, problems such as controlling the rates of chemical reactions in solution and tailoring the properties of amorphous materials. In contrast to a dilute gas or crystalline phase, the dynamics of molecules in disordered condensed phases are complicated by the presence of strong, fluctuating intermolecular interactions and the lack of long-range symmetry. As a consequence, developing the theoretical framework to describe molecular dynamics in liquids has presented a special challenge, as has developing the experimental tools and methods of data analysis that can provide the molecular-level information needed to test these theories. Our research is focused on the latter challenge, that is on developing new experimental techniques and new ways to extract dynamical information from more traditional experiments. Our experimental tools of choice have been Raman spectroscopy and Brillouin spectroscopy, two laser light scattering techniques that probe the vibrational motions of molecules. These two techniques provide complementary information, the Raman spectrum revealing the dynamics of intramolecular vibrations, and the Brillouin spectrum revealing dynamics associated with intermolecular vibrations, or sound waves. In both cases the vibrations reflect dynamical interactions of molecules with their neighbors, and thus provide information that can be used to test and improve theories of such interactions. In the area of Raman spectroscopy our contribution has been a conceptual rather than a technological one. Here we have shown how information about ultrafast conformational dynamics of molecules in solution can be extracted from conventional Raman spectra, and we have applied this analysis to several simple molecules that provide excellent test cases for theory. Examples include studies of solvent effects on the torsional dynamics of n-butane and on the pseudorotation dynamics of cyclopentane. Current work is directed toward extending these studies to supercritical solvents, where the collision rate with the solvent can be changed continuously by varying the pressure. In the area of Brillouin spectroscopy we have used a new two-laser technique called stimulated Brillouin gain spectroscopy to study collective molecular motions in liquids. The high spectral resolution, accuracy, and precision of this technique have allowed us to investigate a range of dynamical phenomena, from the coupling between translational and rotational motions in carbon disulfide to the complex dynamics in pentylcyanobiphenyl that accompany the approach to a nematic liquid crystalline phase. We are presently focusing our efforts on supercooled liquids and glasses, which display a complex dynamical behavior whose origins are still poorly understood.
A high resolution stimulated Brillouin gain spectrum of liquid benzene at 51.9 oC. Trace a shows a series of frequency markers used to linearize the frequency scale, trace b shows the experimental Brillouin gain spectrum, trace c shows a least-squares fit to the experimental spectrum, and trace d shows the residuals from the fit. Selected Publications:
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