Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session K11: Focus Session: Chemical and Spectroscopic Applications of Nonlinear Optics II |
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Sponsoring Units: DCP Chair: Alexander V. Benderskii, Wayne State University Room: Baltimore Convention Center 303 |
Tuesday, March 14, 2006 2:30PM - 3:06PM |
K11.00001: Sum-Frequency Spectroscopy as a Novel Probe for Molecular Chirality Invited Speaker: Conventional techniques to probe molecular chirality, such as circular dichroism (CD), suffer from poor sensitivity because they are electric-dipole forbidden. Sum-frequency generation (SFG) is electric-dipole allowed even in chiral liquids, and therefore could be a more sensitive spectroscopic technique for probing molecular chirality in both electronic and vibrational transitions. We describe here a series of experiments that explore such a possibility. Despite the finding that chiral SFG is intrinsically weak for various reasons, we have succeeded in observing chiral SFG at both electronic and vibrational transitions with monolayer sensitivity. This work was supported by the U.S. Department of Energy. [Preview Abstract] |
Tuesday, March 14, 2006 3:06PM - 3:42PM |
K11.00002: Chiral Phenomena in Nonlinear Optic Invited Speaker: Frequency doubling (second harmonic generation, SHG) and frequency mixing (sum frequency generation, SFG) are remarkably sensitive probes of interfacial structure. The application of these techniques in studies of biological systems and interfaces is particularly exciting, given their high sensitivity to chirality. Whereas absorbance measurements yield circular dichroic (CD) ratios of a few fractions of a percent, these same molecular systems can easily produce CD ratios approaching 100{\%} in thin films. In order for these emerging techniques to successfully undergo the transformation from academic curiosities to practical instrumental tools for biological characterization, experimental and theoretical advances are required. Novel ellipsometric approaches for polarization analysis have been developed to yield greater information content with simpler instrumentation. As a complement to these experimental studies, new and relatively simple theories have been proposed for interpreting the relationships between the detected signals and the molecular/macromolecular structures at the interfaces. Applications of these combined experimental and theoretical techniques include the demonstration of label-free methods for real-time biosensing, chiral-specific detection and analysis with ultra-high sensitivity, and the quantification of changes in protein secondary structure from polarization analysis. [Preview Abstract] |
Tuesday, March 14, 2006 3:42PM - 3:54PM |
K11.00003: Multiple-Sequence Two-Dimensional Infrared Spectroscopy of $N$-Acetyl-L-Prolinamide Soohwan Sul, Denis Karaiskaj, Ying Jiang, Nien-Hui Ge Femtosecond nonlinear 2D IR spectroscopy has been applied to study the conformational distribution of a model dipeptide, $N$-acetyl-L-prolinamide, in chloroform. Using rephasing, non-rephasing, and reverse photon echo pulse sequences at several polarization conditions, we resolved new spectral features associated with multiple conformers that were difficult to discern by linear IR spectroscopy. Ab initio calculations in vacuo suggested the existence of a major \textit{trans}-C$_{7}$ and a minor \textit{cis} structure. Calculated 2D spectra based on these structures agree reasonably well with experimental spectral features obtained at parallel polarizations, but better agreement between the calculated and experimental cross-peak patterns can be reached if the major conformer takes a structure deviated from the C$_{7}$ form. We will discuss our results in relationship with earlier findings from NMR studies and MD simulations. [Preview Abstract] |
Tuesday, March 14, 2006 3:54PM - 4:06PM |
K11.00004: Investigation of Surface Mounted Molecular Rotors Through Optical Second Harmonic Generation James D. Walker, Charles T. Rogers, Gregg S. Kottas, Josef Michl Exploiting the lack of inversion symmetry near a surface, optical surface second harmonic generation (SHG) can be a powerful tool for probing molecular monolayers (or multilayers) adhered to such a surface. In particular, spectroscopy by resonant SHG (Ti:Sapph laser pulses plus Optical Parametric Oscillators and frequency doubling) can provide insight into the orientation of the different sections of the molecular rotor (rotator and axle). The molecular population's orientation can then be studied in the presence of externally applied electric fields, given that the rotator has an intrinsic electric dipole moment, as well in variable temperature UHV environments (down to approx. 30 Kelvin). Concentrating on an anthracene based rotor molecule, synthesized in-house and utilizing silane attachment chemistry on fused silica substrates, we will describe progress made in probing the population's orientation through SHG as well as sample preparation and characterization (linear UV-VIS spectroscopy, FTIR, and XPS). [Preview Abstract] |
Tuesday, March 14, 2006 4:06PM - 4:18PM |
K11.00005: VSFS studies of soluble monolayers formed by linear and branched alcohols at the air/water interface S\"{u}leyman Can, Deesha Mago, Robert Walker Vibrational Sum Frequency Spectroscopy (VSFS) was used to examine the molecular structure and organization of linear and branched octanol and nonanol monolayers adsorbed to the air/water interface. Vibrational bands in the CH stretching region -- specifically the symmetric and antisymmetric normal modes of the methyl and methylene groups as well as bands resulting from Fermi Resonance interactions -- were used to characterize molecular orientation. Complementing these studies were a full series of surface tension measurements designed to measure surface excess concentrations at full monolayer coverage. VSFS data show that branched alcohols attached to an odd carbon (such as 3-nonanol) create monolayers having very similar surface structures and surface coverages, regardless of where the alcohol functional group is attached. Functional substitution at an even-carbon (2-octanol), however, leads to a monolayer having expanded surface coverage (44 sq. Angstroms/molecule) but a surprisingly high degree of conformational order. These results are examined in terms of a competition between hydrogen bonding of the alcohol with the water sub-phase and the tendency of alkyl chains to pack together in close registry. [Preview Abstract] |
Tuesday, March 14, 2006 4:18PM - 4:54PM |
K11.00006: Recent Progress in Molecular Dynamics Simulation of Vibrational Sum Frequency Generation Spectroscopy Invited Speaker: While the visible-IR Sum Frequency Generation (SFG) spectroscopy is a powerful probe of interfaces using a second-order optical process, understanding of the experiments involves many factors to be addressed, including surface density and thickness, molecular orientation, local environment, and higher-order bulk terms. Assignment of the spectra is also complicated by overlap or interference of vibrational components. Therefore, practical methods of analysis with the help of reliable theoretical calculations will greatly benefit the SFG experiments. We have developed computational methods of vibrational SFG spectroscopy in combination of ab initio molecular modeling and molecular dynamics simulation. These methods allow direct calculation of SFG spectra from a molecular model without resorting to empirical spectral fitting. An essential ingredient of the theoretical methods is calculation of frequency-dependent nonlinear susceptibility that may be dominated by vibrational effects. We have proposed two ways of modeling of the nonlinear susceptibility, based on the energy representation and the time-dependent representation. This talk summarizes the progress of these theoretical methods and application to aqueous interfaces. [Preview Abstract] |
Tuesday, March 14, 2006 4:54PM - 5:06PM |
K11.00007: Vibrational Spectroscopy and Symmetry Specific Vibrational Energy Relaxation of Ethanol Molecule: A Study with Sum Frequency Generation and Photoacoustic Stimulated Raman Spectroscopy Hong-fei Wang, Shi-lin Liu Orientational analysis of Sum Frequency Generation Vibrational Spectroscopy (SFG-VS) of un-deuterated ethanol indicated that the assignment of its C-H stretching vibrations in the literature was troublesome. The assignments were clarified using deuterated ethanols with their SFG-VS spectra at the vapor/liquid interface, and photoacoustic stimulated Raman spectra in the vapor phase. Comparison of SFG-VS spectra at vapor/liquid interface, infrared and Raman spectra in liquid phase, and photoacoustic stimulated Raman spectra in vapor phase of both un-deuterated and deuterated ethanol at room temperature indicated that the vibrational energy relaxation dynamics of ethanol molecule is significantly different for its symmetric and asymmetric C-H vibrational modes. These results showed that surface nonlinear spectroscopic methods can be used as effective tools for interrogating spectroscopy of molecules in bulk and vapor phases. [Preview Abstract] |
Tuesday, March 14, 2006 5:06PM - 5:18PM |
K11.00008: New forms of chiral spectroscopy: optical activity revisited Peer Fischer, Ambarish Ghosh Fresnel showed that an isotropic medium gives rise to optical rotation if its refractive indices for left- and right-circularly polarized light are unequal. Such is the case in optically active liquids, i.e. non-racemic solutions of chiral molecules. Nonlinear optical techniques have recently been developed that do not require the use of circularly polarized light to detect chirality. Here we present a new form of chiral spectroscopy which arises in linear optics and which yields information identical to that of conventional optical rotatory dispersion, yet without the need for a polarization analysis. The technique is independent of the optical path-length through the sample and permits optical activity to be imaged. [Preview Abstract] |
Tuesday, March 14, 2006 5:18PM - 5:30PM |
K11.00009: Structural properties and organization of hexadecanol isomers at the air/water interface Robert Walker, S\"{u}leyman Can, Deesha Mago A wealth of experimental data and theoretical modeling has led to well-honed intuition about the surface properties and structure of symmetric amphiphiles adsorbed to liquid surfaces. Less clear is how asymmetric amphiphiles organize in two dimensions at different surface coverages. We have studied the structure and two dimensional phase behavior of hexadecanol isomers adsorbed to the air/water interface. These isomers include the linear, n-hexadecanol as well structures with the alcohol functional group in the 2-, 3-, and 4- positions. Surface pressure methods are employed to study thermodynamic behavior of these insoluble monolayers, and vibrational sum frequency generation -- a vibrational spectroscopy with surface specificity -- is used to probe the molecular conformation and orientation of molecules within films. At their equilibrium spreading pressures, both 1- and 2- hexadecanol form very compact films having a high degree of conformational order and molecular areas of 19 and 28 sq. Angstroms/molecule in the tightly packed limit. In contrast, monolayers formed by 3-hexadecanol and 4-hexadecanol are much more disordered - but very similar to each other - and occupy much larger areas/molecule (75 sq. Angstroms/molec) in the tightly packed limit. [Preview Abstract] |
Tuesday, March 14, 2006 5:30PM - 5:42PM |
K11.00010: Interfaces of methanol:water mixtures with an OTS-coated substrate probed by sum-frequency vibrational spectroscopy Weitao Liu, Luning Zhang, Y. R. Shen Aqueous solutions of short chain alcohols are important reagents in organic chemistry, and their physical and chemical properties at hydrophobic interfaces play key roles in many applications. To study their interfacial structures at the molecular level, we applied sum-frequency vibrational spectroscopy (SFVS) to the interfaces of methanol-water mixtures with an octyltrichlorosilane (OTS)-covered silica as model systems. From the methanol CH3 symmetric stretching mode, we deduced the interfacial coverage and orientation of methanol molecules vs. its molar concentration, and studied the hydrogen-bonding property of methanol with neighboring molecules. It appeared that methanol molecules adsorbed at the interfaces with the same average orientation at all concentrations. From the dangling-OH stretching mode of water molecules, we observed the correlation between the methanol surface number density and the water free OH bonds. Ab initio calculations were used to help investigate the interfacial structures of hydrogen-bonded species. [Preview Abstract] |
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