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| Professor Winn received SB degrees in Chemistry and Physics from M.I.T. in 1969 and a Ph.D. in Chemistry from the University of California at Berkeley in 1973 working in the area of ion-molecule scattering. Following two years of post-doctoral research at Harvard on van der Waals molecules, he returned to Berkeley to join the faculty. He joined the Dartmouth faculty in 1982. |
Position: Professor of Chemistry
Our research centers around the information on molecular structure and dynamics that can be attained from high resolution spectroscopy using matrix isolation spectroscopy. We have recently completed the design and construction of a new apparatus for studying species trapped in a solid hydrogen matrix at temperatures as low as 2 K. Solid hydrogen is a novel matrix material (a so-called "quantum solid") that has attracted much attention recently. The low mass and weak attraction hydrogen has makes its solid phase rather special: it has a very low density and very open structure. Moreover, the inability of hydrogen to quickly relax its first excited rotational state to its ground state (because such a relaxation requires a nuclear spin flip as well as a rotational motion energy loss) means there are effectively two types of hydrogens: those in the lowest rotational state (J = 0 or para-hydrogen, pH2) and those in the first-excited (J = 1 or ortho-hydrogen, oH2) state. (At these low temperatures, these are the only two states that matter.) We can control the relative amounts of these two states through a pre-cooler that lowers the temperature of the hydrogen feed gas to just above the vaporization temperature (about 15 K) and passes the cold gas through a paramagnetic catalyst that speeds the gas-phase rotational state equilibration. We have recently completed an extensive study of OCS in such matricies (and we have also used D2 as well as H2). These studies were motivated by our earlier studies of OCS in solid rare gas (Ar, Kr, Xe) matrices in which we used a new theoretical model of the infrared lineshape of species isolated in simple cryogenic solids to understand the packing of the matrix around a simple linear solute. In the case of solvated OCS, comparisons of the theoretical lineshapes to the experimental data show two major matrix sites. In one, the site is described by the relaxation of a perfect rare gas crystal with a double substitutional site for OCS. In the other, the site is more akin to the sequential addition of gas atom after gas atom to the OCS. The site structures are very similar in gross appearance, but they have dramatically different effects--a factor of ten--on the linewidth. We have extended this method to the study of pure CO in Ar as well as to CO fragments produced by photolysis in the matrix. We have shown, for example, that the CO fragment from OCS is in a very different environment from that around pure CO in an Ar matrix. Interestingly, however, we find that OCS cannot be photolysed in a hydrogen matrix. In addition to these studies of OCS monomer, we have observed features at high OCS concentrations that we have assigned to (OCS)2 and (OCS)3. The analysis of these features has been greatly aided by ab initio structure studies done in collaboration with our colleague here, Professor Robert Ditchfield.
Shown below are some of our spectra of OCS in very pure pH2 illustrating an interesting temperature effect on two monomer spectral features. We believe these features differ by their local hydrogen packing coupled to smaller satellite features that may be due to irreversable weak association of OCS to one or more oH2 impurities.
