The structure of molecules at surfaces and interfaces plays an important role in determining the efficiency of solar cells. Our understanding of these interfaces next to electrodes has been limited due to lack of experimental and simulation tools. Because of it symmetry requirements, sum frequency generation (SFG) is uniquely sensitive to the molecular structure at these crucial buried interfaces. The combination of infrared and visible SFG wavelengths allows vibrational spectra of interfacial molecules to be obtained. In this collaborative project with the Dhinojwala, Tsige, and Gujrati groups, we develop methods for the theroeticial simulation of SFG spectra with the aim of providing the means for a confident interpretation of such spectra. The target systems are poly 3-alkyl thiophenes at semiconductor surfaces.

Methanol exhibits rapid tunneling between three equivalent torsional minima.  This large-amplitude torsional motion couples with other small-amplitude vibrations to rapid intramolecular energy redistribution (IVR) and to the formation of new kinds of vibrational motion at high levels of excitation.  Experimental methods include slit-jet cavity-ringdown spectroscopy (in Akron) and rotationally-selected infrared spectroscopy (IR-FTMW) in collaboration with Prof. Brooks Pate of the University of Virginia.  Supported by the U.S. Department of Energy.

Nitromethane has a low 6-fold barrier to internal rotation that enhances internal energy redistribution.  High-resolution slit-jet FTIR spectra, recorded at the Pacific Northwest National Laboratory by Robert L. Sams, have revealed several stages of intramolecular coupling.  The dependence of the interactions on the rotational quantum number has enabled the identification of the coupling mechanisms.   Supported by the U.S. Department of Energy.

The CH and NH vibrations of methylamine are coupled to the large-amplitude torsional and inversion motions.  This research uses high resolution cavity ringdown spectroscopy to explore that coupling and to test the validity of  the adiabatic separation of the large- and small-amplitude degrees of freedom.   Supported by the U.S. Department of Energy.

Layered fuel-air mixtures are produced for methane with an airfoil and methanol by evaporation from the liquid fuel.  Our wexperiment applies wave-length-modulated diode laser spectroscopy.  Measurements are made in 1-G on the laboratory benchtop and in 0-G in the 2-second drop tower at NASA Glenn Research Center.  Supported by NASA.

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