We combine frequency- and femtosecond time-domain measurements of vibrational coherences for spectroscopic characterization of surface monolayer films, utilizing 3-wave mixing as the surface-selective technique. Frequency-domain spectra in the CH-stretch region are obtained by infrared + visible sum frequency generation (SFG). Time-domain coherences are measured using SFG free induction decay (SFG-FID), where a 75 fs IR pulse excites several vibrational modes and a delayed 40 fs visible pulse probes the oscillating surface polarization. A unified framework based on optical Bloch equations is used to simultaneously analyze the time- and frequency-domain data. We compare molecular organization of monolayers in different two-dimensional phases. Highly ordered films transferred at high surface pressure are dominated by two transitions in the frequency domain, CH3 symmetric stretch (2875 cm(-1)) and CH3-Fermi resonance with bend overtone (2935 cm(-1)), and a coherent quantum beat in the time-domain at the difference frequency (approximately 540 fs period). At lower surface pressure, relative amplitudes change and additional transitions emerge (CH3 asymmetric stretch and CH2 modes), indicating changes in molecular orientation and onset of disorder. Information redundancy in the combined frequency- and time-domain data allows more accurate determination of the spectral parameters than purely frequency- or time-domain techniques.

We report studies of metal nanoparticle surfaces using vibrational sum frequency generation (SFG) spectroscopy. SFG is demonstrated as a sensitive and information-rich probe of nanostructured surfaces. Detection of SFG spectra from a few percent of a monolayer of nanoparticles on a transparent substrate was achieved, corresponding to as few as 10 5 isolated particles within the laser beam spot. A new effect arises when the nanoparticle size approaches the molecular scale: the dependence of the molecular conformation on the geometry of the substrate. Conformation of the dodecane chain of the ligand shows systematic variation with the particle diameter in the 1.8-25 nm range. More gauche defects are observed on smaller particles, judged from the relative intensities of the CH 2 and CH 3 stretch transitions in SFG spectra. Similar behavior observed for both gold and silver particles suggests a nanoscale geometric packing effect, whence more volume is available to the chain on a curved surface than a flat surface. Drying-mediated aggregation of gold nanoparticles results in an enhancement of the SFG signal, enabling vibrational SFG spectra of single submicron size aggregates to be obtained. These are different from spectra of isolated particles, indicating a vibrational mode-selective enhancement mechanism. † Part of the special issue "Kenneth B. Eisenthal Festschrift".

Time- and frequency-domain three-wave mixing spectroscopy (IR+visible sum frequency generation) is developed as the lowest-order nonlinear technique that is both surface selective and capable of measuring spectral evolution of vibrational coherences. Using 70 fs infrared and 40 fs visible pulses, we observe ultrafast spectral dynamics of the OD stretch of D2O at the CaF2 surface. Spectral shifts indicative of the hydrogen-bond network rearrangement occur on the 100 fs time scale, within the observation time window determined by the vibrational dephasing. By tuning the IR pulse wavelength to the blue or red side of the OD-stretch transition, we selectively monitor the dynamics of different subensembles in the distribution of the H-bond structures. The blue-side excitation (weaker H-bonding structures) shows monotonic decay and nu(OD) frequency shift to the red on a 100 fs time scale, which is better described by a Gaussian than an exponential frequency correlation function. In contrast, the red-side excitation (stronger H-bonding structures) results in a blue spectral shift and a recursion in the signal at 125+/-10 fs, indicating the presence of an underdamped intermolecular mode of interfacial water.

Molecular organization at polyimide surfaces used as alignment layers in liquid crystal displays was investigated using vibrational sum frequency generation (SFG) spectroscopy. We focus on the orientation of the long alkyl side groups at the polymer surface using polarization-selected SFG spectra of the CH(3)- and CH(2)-stretch modes of the side chain. Mechanical rubbing and baking, an accepted industrial procedure used to produce pretilt of the liquid crystal, was found to induce pronounced azimuthal anisotropy in the orientational distribution of the alkyl side chains. Orientational analysis of the SFG vibrational spectra in terms of the azimuthal and tilt angles (in and out of plane, respectively) of the alkyl side chains shows their preferential tilt along the rubbing direction, with the azimuthal distribution narrower for stronger rubbed polymer samples.

Covalent surface attachment of carbohydrate moieties using maleimide-sulfhydril reaction was characterized by surface-selective vibrational sum-frequency generation (VSFG) spectroscopy. The comparative VSFG spectra of the precursor maleimide-terminated SAM and the product glucose adlayer reveal the high efficiency of the surface coupling reaction (>90%) and the details of the molecular organization of the formed carbohydrate adlayer. The glucose groups are orientationally well ordered, as judged by their sharp CsH stretch bands. The chemical structure of the linker can significantly affect the orientation of the carbohydrate moiety at the surface. Two alkanethiol linkers of different chain lengths (11 and 16 carbons) yield similar orientations of the glucose in the adlayer whereas the cysteinecontaining linker produces markedly different relative peak intensities of the glucose CsH stretch bands in the VSFG spectra, suggesting a significantly different orientation with respect to the surface plane.