Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session H11: Focus Session: Chemical and Spectroscopic Applications of Nonlinear Optics I |
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Sponsoring Units: DCP Chair: Peer Fischer, Harvard University Room: Baltimore Convention Center 303 |
Tuesday, March 14, 2006 11:15AM - 11:51AM |
H11.00001: Second Harmonic and Sum Frequency Studies of Aqueous Interfaces Invited Speaker: Among the topics to be discussed are ultrafast electron transfer at aqueous/organic interface, method for measurement of the absolute orientation of molecules at interfaces, second harmonic imaging studies of live nerve cells, time permitting second harmonic studies of gold nanorods at the air/aqueous interface. [Preview Abstract] |
Tuesday, March 14, 2006 11:51AM - 12:03PM |
H11.00002: Ultrafast Vibrational Dynamics of Water Interfaces Probed by Sum-frequency Vibrational Spectroscopy John A. McGuire, Y.R. Shen Using sum-frequency generation (SFG) as a surface-specific probe, we measured the ultrafast population and dephasing dynamics of excitations of OH-stretch modes of water at fused-silica/water interfaces. A tunable femtosecond IR pulse was used to pump the vibrational mode. Another independently tunable femtosecond IR pulse, after a time delay, was used to probe the vibrational excitation and then up-converted by SFG for detection. Experiments on the hydrophilic fused-silica/water interface reveal population relaxation of the hydrogen-bonded OH modes on time scales of 200-500 fs, more rapid on the red side than the blue side. The width of transient spectral holes produced by the IR pump pulse correspond to homogeneous dephasing times of $\sim $ 50 fs. Measurements of the dangling OH bonds at the hydrophobic silica/$n$-octadecyltrichlorosilane/water interface reveal a population relaxation time of about 1.2 ps. [Preview Abstract] |
Tuesday, March 14, 2006 12:03PM - 12:15PM |
H11.00003: Phase-sensitive sum-frequency vibrational spectroscopy on water/vapor interfaces Victor Ostroverkhov, Na Ji, Chao-Yuan Chen, Yuen-Ron Shen Water/vapor interfaces are ideal hydrophobic interfaces relevant to many important applications. They have been studied extensively by sum-frequency vibrational spectroscopy (SFVS) in recent years. Unfortunately, different interpretations of the spectra from different research groups have led to a great deal of confusion. The problem arises because in conventional SFVS, only the magnitude of the spectrum is recorded. We present here a phase-sensitive (PS) SFVS study of water/vapor interfaces that captures both the magnitude and the phase of an interfacial vibrational spectrum. The results allow us to decompose unequivocally the various OH stretch peaks in the spectrum and the net polar orientations of the water species contributing to the peaks. [Preview Abstract] |
Tuesday, March 14, 2006 12:15PM - 12:27PM |
H11.00004: Measuring solvation dynamics using sum-frequency cross-correlation frequency resolved optical gating (SF XFROG) Nancy Levinger, Benjamin Langdon Solvation dynamics has been used to measure the dynamical response of a solvent to an electronic perturbation. The response is typically followed through sum-frequency conversion of sample emission with short light pulses to time-resolve spectral shifting of a fluorescent probe molecule. In contrast to traditional fluorescence upconversion techniques where the time resolved fluorescence signal is approximated from 10-15 different sum-frequency gated decays distributed across the fluorescence spectrum, sum-frequency cross-correlation frequency resolved optical gating offers the opportunity to determine the electric field of the sample fluorescence as a function of time. Dynamic quantities associated with changes in the sample emission, such as solvent relaxation, can then be obtained directly from the electric field without assumption of a particular spectral lineshape. We show that the time evolving phase of the SF XFROG signal corresponds to the solvent response function, $C(t)$, and that the technique also reveals details about vibrational modes associated with the relaxation. [Preview Abstract] |
Tuesday, March 14, 2006 12:27PM - 12:39PM |
H11.00005: Time- and Frequency-Domain Surface Spectroscopy of Polymers and Langmuir-Blodgett films Andrey Bordenyuk, Himali Jayathilake, Alexander Benderskii We combine frequency- and femtosecond time-domain IR+visible Sum Frequency Generation (SFG) measurements for spectroscopic characterization of material surfaces. Vibrational quantum beats observed in Langmuir-Blodgett monolayers are demonstrated to be sensitive to the molecular order/disorder in the film, depending on the two-dimensional phase. Nearly vertical alignment of all-trans alkyl chains is observed in films transferred at high surface pressure, while at lower surface pressure, changes in chain tilt angle and onset of gauche-defects are observed. The time- and frequency-domain data are analyzed simultaneously based on optical Bloch equations, allowing to utilize the information redundancy for more accurate determination of the spectral parameters. Polymer surfaces used as alignment layers in liquid crystal displays were investigated to elucidate how the mechanical rubbing affects the molecular orientation and conformation. Rubbing was found to induce pronounced azimuthal anisotropy of the alkyl side chain orientation, observed in the CH-stretch SFG spectra of the CH$_{3}$ and CH$_{2}$ groups. Orientational analysis of the SFG vibrational spectra as a function of azimuthal and tilt angles (in- and out-of-surface plane) of the polymer side chains shows their preferential tilt along the rubbing direction. [Preview Abstract] |
Tuesday, March 14, 2006 12:39PM - 1:15PM |
H11.00006: Small Molecules and Sum Frequency Generation Probes of Nanoparticulate TiO$_{2}$ Invited Speaker: Anatase TiO$_{2}$ is known to photo catalytically mineralize a wide variety of pollutants and pathogens, both airborne and in aqueous solution. One of the major benefits of basing water treatment systems on TiO$_{2}$ is that it is environmentally benign and so non toxic that it is used as a colorant in creamy salad dressing. The primary impediment to wide spread implementation of a TiO$_{2}$ based system for water decontamination is that the quantum efficiency in contact with condense phase water is less than 5{\%}. Since the quantum efficiency for destruction of airborne materials is greater than 80{\%}, the potential for increased efficiency is very real. To convert the potential to practice, the oxidation mechanism needs to be more fully understood. We will report on the results of using a nonlinear optical spectroscopy, sum frequency generation (SFG) as an \textit{in situ} probe of interactions at the TiO$_{2}$ surface. Results suggest that the dominant oxidation mechanism converts from a direct to an indirect mechanism as the water content (vapor pressure) increases. This presentation will discuss the probe technique as well as the results. [Preview Abstract] |
Tuesday, March 14, 2006 1:15PM - 1:27PM |
H11.00007: Molecular Adsorption on Nano Colloidal Particles Probed by Second Harmonic Generation Shih-Hui Jen, Hai-Lung Dai It is shown that second-harmonic generation (SHG), detected at 90 degree angle from the fundamental beam propagation direction, can be used to probe molecular adsorption on spherical nano colloidal particles with diameter as small as 50 nm. Measurements done with the malachite green dye adsorbed on polystyrene particles with diameters ranging from 50 to 250 nm show that the SHG signal from these surface adsorbed molecules tilts toward larger scattering angles when the particle size becomes smaller. This phenomenon can be rigorously described by the nonlinear Rayleigh-Gans-Debye theory and used for measuring the density and adsorption free energy of molecules adsorbed on nanometer size colloidal particles. [Preview Abstract] |
Tuesday, March 14, 2006 1:27PM - 1:39PM |
H11.00008: Spectrally- and Time-Resolved Sum Frequency Generation (STiR-SFG): a new tool for ultrafast hydrogen bond dynamics at interfaces. Alexander Benderskii, Andrey Bordenyuk, Champika Weeraman The recently developed spectrally- and time-resolved Sum Frequency Generation (STiR-SFG) is a surface-selective 3-wave mixing (IR+visible) spectroscopic technique capable of measuring ultrafast spectral evolution of vibrational coherences. A detailed description of this measurement will be presented, and a noniterative method or deconvolving the laser pulses will be introduced to obtain the molecular response function. STiR-SFG, combined with the frequency-domain SFG spectroscopy, was applied to study hydrogen bonding dynamics at aqueous interfaces (D$_{2}$O/CaF$_{2})$. Spectral dynamics of the OD-stretch on the 50-150 fs time scale provides real-time observation of ultrafast H-bond rearrangement. Tuning the IR wavelength to the blue or red side of the OD-stretch transition, we selectively monitor the dynamics of different sub-ensembles in the distribution of the H-bond structures. The blue-side excitation (weaker H-bonding) shows monotonic red-shift of the OD-frequency. In contrast, the red-side excitation (stronger H-bonding structures) produces a blue-shift and a recursion, which may indicate the presence of an underdamped intermolecular mode of interfacial water. Effect of electrolyte concentration on the H-bond dynamics will be discussed. [Preview Abstract] |
Tuesday, March 14, 2006 1:39PM - 1:51PM |
H11.00009: Non-linear vibrational spectroscopies for characterizing thin films of semi-conducting polymers Marc Gurau, Zachary Schultz, Dean DeLongchamp, Brandon Vogel, Joseph Kline, Youngsuk Jung, Eric Lin, Lee Richter The ongoing development of conjugated polymer electronic materials has shown that alterations of the structure of these materials and the means of processing can lead to large variations in the performance observed. Continued growth in this field requires the development of new metrologies suited to the examinations of thin polymeric films on a variety of substrates.~ Spectroscopic characterization of the vibrational structure of these systems provides a non-destructive means of monitoring such changes.~ The work presented in this talk demonstrates the use of nonlinear optical broadband coherent anti-stokes raman spectroscopy (CARS) and broadband sum frequency generation (SFG) to monitor both the bulk and interfaces of ultra-thin ($<$50 nm) semiconducting polymer films. SFG provides insight into order at the critical buried polymer/dielectric interface of thin-film transistors while CARS provides insight into chemical degradation of the films. [Preview Abstract] |
Tuesday, March 14, 2006 1:51PM - 2:03PM |
H11.00010: Ferroelectric domains in dipolar liquids observed by hyper-Rayleigh scattering. David Shelton Hyper-Rayleigh scattering (HRS) is a sensitive probe of polar modes in isotropic media. For the dipolar liquids water, acetonitrile, nitromethane and nitrobenzene the depolarized HRS spectrum is observed to contain a narrow spike at zero frequency shift due to a polar collective mode with a relaxation time greater than 30 ns. This spike in the HRS spectrum is attributed to ferroelectric domains in the liquid. The spike disappears when dilution of a dipolar liquid with a non-polar liquid reduces the dipole coupling strength of the solution below the predicted ferroelectric threshold. The size and dipole order parameter for the ferroelectric domains have been determined from HRS spectra for binary solutions of dipolar molecules. [Preview Abstract] |
Tuesday, March 14, 2006 2:03PM - 2:15PM |
H11.00011: Surfaces of alcohol-water mixtures studied by sum-frequency generation vibrational spectroscopy Jaeho Sung, Kyungsu Park, Doseok Kim Sum frequency generation vibrational spectroscopy in the CH$_{x}$ stretch vibration range used to investigate the surface of binary mixtures of water and short chain alcohols (methanol, ethanol, and propanol) at the air/liquid interface. It was found that the sum frequency signals from mixtures at low alcohol concentration are larger than that of pure alcohols. For example, the signal from a propanol mixture surface at 0.1 bulk mole fraction was $\sim $3 times larger than that from a pure propanol surface. On the other hand, the ratio between the sum-frequency signals taken at different polarization combinations was found to be constant within experimental errors as the bulk alcohol concentration was changed. This suggested that the orientation of surface alcohol molecules does not vary appreciably with the change of concentration, and that the origin of the signal enhancement is mainly due to the increase in the surface number density of alcohol molecules contributing to the sum-frequency signal for the alcohol/water mixture as compared to the pure alcohol surface. [Preview Abstract] |
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