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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 ν(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. [ABSTRACT FROM AUTHOR]

The thermally activated recombination reaction O+O2→O3 is observed in solid xenon matrices and in free-standing crystals of xenon at 14–25 K on the time scale 102–105 s. The reactants are prepared as spatially separated O...O2 pairs immobilized in solid Xe at 10 K by 266 nm photodissociation of O3 precursor molecules. The temperature dependence of the ozone recovery rate yields an activation energy for diffusion of O atoms in solid xenon of 2.0±0.5 kJ/mol. This value also represents an upper limit to the potential energy barrier to the O+O2 recombination reaction itself. In dilute samples (mole fraction of ozone less than 2×10-4) more than 90% of the initial O3 is recovered during the warming cycle. Only a small fraction of O atoms (<20%) escape geminate recombination with the partner oxygen molecule within the pair and react with other O2 molecules or O atoms. The experimental results are interpreted within the framework of a continuous diffusion model in which the initial spatial distribution of reactants is nonrandom. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The photochemistry of solid ozone and solid solutions of ozone in argon has been investigated at 10–30 K using 266 nm laser irradiation and detection by ultraviolet and infrared absorption spectroscopies. Pure ozone is converted to O2 with a quantum yield of 2.0±0.3 (independent of temperature) due to reaction of each photolytically generated O atom with a second ozone molecule. The quantum yield decreases when O3 is diluted in argon because geminate recombination of O+O2 becomes competitive with the O+O3 reaction. Photodissociation of ozone monomers is thought to occur by direct cage escape, which forms a separated O+O2 pair. This is the first experimental study in which the temperature dependence of the escape probability has been systematically investigated. The apparent activation energy of the process, 0.38±0.08 kJ/mol, is consistent with excitation of librations of ozone in the argon lattice, which brings it into orientations that are favorable for direct cage escape of the O atom. [ABSTRACT FROM AUTHOR]

Mixed anatase/rutile TiO2 exhibits high photocatalytic activity; however, the mechanism underlying the high performance of the mixed phases is not fully understood. We have performed time-domain ab initio calculations to study the exited state dynamics in mixed phase TiO2 and to investigate the impact of an oxygen vacancy on the dynamics. The anatase(100)/rutile(001) heterostructures with and without an oxygen vacancy used in this work exhibit type II band alignment with the conduction band of rutile residing above that of anatase. The oxygen vacancy introduces a hole trap state inside the bandgap. Owing to a strong coupling between the donor and acceptor states, the electron and hole transfers across the anatase/rutile interface occur on an ultrafast 100 fs timescale in both systems. The decoupling of electron and hole favors a long-lived charge separated state. The electron–hole recombination across the pristine anatase/rutile interface takes 6.6 ns and is significantly slower than that in the pure anatase and rutile phases, showing good agreement with experiments. The electron transfer dynamics is independent of the oxygen vacancy, which has some influence on the hole transfer and a strong effect on carrier recombination. By creating a hole trap state, the vacancy accelerates carrier losses by over an order of magnitude. The fast charge separation and the long lifetime of the charge separated state rationalize the enhanced photocatalytic performance of mixed phase TiO2 compared to the pure phases. [ABSTRACT FROM AUTHOR]

Wepresent a general response function formalism describing thecontribution of orientational dynamics of molecules at interfacesto spectroscopic line shapes in vibrational sum frequency generation(SFG). When reorientation occurs on the time scale comparable to vibrationaldephasing, its dynamics can be extracted from polarization-selectedSFG spectral line shapes. Unique features of orientational motionat interfaces are (1) the anisotropic case-specific equilibrium orientationaldistribution and (2) possible dynamic anisotropy (e.g., differentin-plane versus out-of-plane relaxation rates), both of which mustbe taken into account. Within the small-step rotational diffusionmodel, we present solutions for two cases, the weak-confinement model,applicable when the deviations from the isotropic case are not severe,and the wobbling-in-a-cone model, which considers a hard-wall orientingpotential. SFG line shapes are calculated for a rod-like chromophoreas a function of the rotational diffusion rate. For certain equilibriumorientational distributions, orientational dynamics may result inanomalous bi-Lorentzian line shapes (two Lorentzians of differentwidths centered and the same frequency). [ABSTRACT FROM AUTHOR]

We present a measurement of molecular orientation of water at charged surfactant aqueous interfaces as a function of surface charge density. The polarization dependent spectral line shapes of the water bend mode were measured by vibrational sum-frequency generation at the positively charged surfactant cetyltrimethylammonium bromide (CTAB)/water interface and negatively charged surfactant sodium dodecyl sulfate/water interface. Orientational analysis using the water bend mode as a vibrational probe, within the electric dipole approximation, reveals structural differences between these surfaces and quantifies how different hydrogen bonded species re-orient around the surfactant head groups as the surface charge density changes. As the concentration of the positively charged surfactant (CTAB) increases, the surface water molecules with free-OH groups reorient their hydrogen away from the bulk water and the C2v axis closer to the surface normal. This suggests that these free-OH molecules are in general located above the positively charged head groups of CTAB, and thus, the charge-dipole interaction pulls their oxygen "down" and pushes hydrogen "up." On the contrary, water molecules with two donor hydrogen bonds re-orient their hydrogen toward the bulk water, likely because most of these molecules are below the CTAB surfactant head groups. [ABSTRACT FROM AUTHOR]

The effect of a positively charged dodecyl trimethylammonium bromide (DTAB) surfactant on the aqueous solvation of coumarin 314 (C314), a solvatochromic molecule, at the air/water interface was investigated. Steady-state and femtosecond time-resolved second harmonic generation (SHG) spectroscopy was employed as a surface-selective tool to characterize the solvation of C314. The molecular orientation and the static and dynamic solvation of the probe C314 at the positively charged surfactant interface were found to be significantly different from that at the previously studied neat air/water interface and at a negatively charged surfactant air/water interface. Steady-state SHG spectra show a red shift and therefore increased local polarity at the positively charged interface (λmax = 436 nm) compared to the negatively charged interfaces (λmax = 431−432 nm) and the surfactant-free air/water interface (λmax = 423 nm), though less polar than bulk water (448 nm). Two diffusive dynamical time scales τ1 and τ2 of aqueous solvation were obtained at the DTAB interfaces, as is the case at air/water, at negatively charged surfactant interfaces, and in bulk water. Both the solvation times and their relative amplitudes change as a function of the positively charged surfactant surface coverage. As the DTAB surface density increases from 500 Å2/molecule to 100 Å2/molecule, the fast component τ1 slows from 380 to 550 fs, whereas the amplitude of the slower solvation component τ2 is reduced to almost zero. This is significantly different from the behavior observed at the negatively charged surfactant interfaces. [ABSTRACT FROM AUTHOR]

Sum frequency generation (SFG) is a surface-selective spectroscopy that provides a wealth of molecular-level information on the structure and dynamics at surfaces and interfaces. This paper addresses the general issue of spectral resolution and sensitivity of the broad-band (BB) SFG that involves a spectrally narrow nonresonant (usually visible) and a BB resonant (usually infrared) laser pulses. We examine how the spectral width and temporal shape of the two pulses, and the time delay between them, relate to the spectroscopic line shape and signal level in the BB-SFG measurement. By combining experimental and model calculations, we show that the best spectral resolution and highest signal level are simultaneously achieved when the nonresonant narrow-band upconversion pulse arrives with a nonzero time delay after the resonant BB pulse. The nonzero time delay partially avoids the linear trade-off of improving spectral resolution at the expense of decreasing signal intensity, which is common in BB-SFG schemes utilizing spectral filtering to produce narrow-band visible pulses. [ABSTRACT FROM AUTHOR]

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 CH3- and CH2-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. [ABSTRACT FROM AUTHOR]

Interactions with surfactant molecules can significantly alter the structure of interfacial water. We present a comparative study of water-surfactant interactions using two different spectroscopic approaches: water at planar surfactant monolayers by sum frequency generation (SFG) spectroscopy and interfacial water confined in reverse micelles formed by the same surfactants using IR absorption spectroscopy. We report spectral features in the OH-stretching region (3200-3700 cm-1) that are observed in both IR and SFG spectra, albeit with different relative amplitudes, for ionic surfactant sodium 1,4-bis-2-ethylhexylsulfosuccinate (AOT) and nonionic surfactant polyoxyethylene(4)lauryl ether (Brij L-4) reverse micelles in hexane and the corresponding monolayers at the air/water interface. A prominent feature in the SFG spectra of the OH stretch at 3560 cm-1 is attributed to water molecules that have a weak donor hydrogen bond to the surfactant headgroup. The same feature is observed in the IR spectra of reverse micelles after deconvoluting the interfacial versus bulk spectral contributions. We performed an orientational analysis of these water molecules utilizing the polarization-dependent SFG spectra, which shows an average tilt angle of the OH stretch of surfactant-bound water molecules of ~155° with respect to the surface normal. [ABSTRACT FROM AUTHOR]

Recently, we reported enhanced H 2 evolution on TiO 2 -passivated GaAs. Based on density functional theory (DFT) calculations, this enhancement was attributed to Ti 3+ states, which bind reactant species and increase charge transfer across the semiconductor–liquid interface. Here, we provide a quantitative correlation between Ti 3+ density, as measured by X-ray photoemission spectroscopy (XPS) and photoluminescence (PL) spectroscopy, and photocatalytic performance, which substantiates the hypothesis put forth previously. In the photo- I – V characteristics reported here, passivating GaAs with TiO 2 produces a shift in the onset potential of +0.35 V at 1 mA/cm 2 and enhances the photocurrent by 32-fold over bare GaAs (at 0 V vs. RHE), resulting in a peak photoconversion efficiency of 1.5% under AM1.5 G illumination. We find that just 1 nm of TiO 2 produces the best conditions for photocatalysis. XPS spectra show that thinner TiO 2 films (1 nm) have a higher density of Ti 3+ states than thicker films (5 nm), which have lower photocatalytic performance. PL spectroscopy provides further evidence for these Ti 3+ surface states, which cause increased surface recombination. While it is apparent that the TiO 2 films cause strong electron–hole recombination, the benefit that they provide by providing catalytically active sites outweighs their detriment associated with charge recombination. No enhancement is observed for TiO 2 thicknesses above 10 nm, which are crystalline and, therefore, considerably more insulating than thinner amorphous TiO 2 films. [ABSTRACT FROM AUTHOR]

Surface-bound azobenzenesexhibit reversible photoswitching viatrans–cis photoisomerization and have been proposed for a varietyof applications such as photowritable optical media, liquid crystaldisplays, molecular electronics, and smart wetting surfaces. We reporta novel synthetic route using simple protection chemistry to formazobenzene-functionalized SAMs on gold and present a mechanistic studyof the molecular order, orientation, and conformation in these self-assembledmonolayers (SAMs). We use vibrational sum-frequency generation (VSFG)to characterize their vibrational modes, molecular orientation, andphotoisomerization kinetics. Trans–cis conformational changeof azobenzene leads to the change in the orientation of the nitrilemarker group detected by VSFG. Mixed SAMs of azobenzene and alkanethiols are used to investigate the steric hindrance effects. While100% azobenzene SAMs do not exhibit photoisomerization due to tightpacking, we observe reversible switching (>10 cycles) in mixedSAMswith only 34% and 50% of alkane thiol spacers. [ABSTRACT FROM AUTHOR]

The air-water interface is perhaps the most common liquid interface. It covers more than 70 per cent of the Earth's surface and strongly affects atmospheric, aerosol and environmental chemistry. The air-water interface has also attracted much interest as a model system that allows rigorous tests of theory, with one fundamental question being just how thin it is. Theoretical studies have suggested a surprisingly short 'healing length' of about 3?ångströms (1?Å = 0.1?nm), with the bulk-phase properties of water recovered within the top few monolayers. However, direct experimental evidence has been elusive owing to the difficulty of depth-profiling the liquid surface on the ångström scale. Most physical, chemical and biological properties of water, such as viscosity, solvation, wetting and the hydrophobic effect, are determined by its hydrogen-bond network. This can be probed by observing the lineshape of the OH-stretch mode, the frequency shift of which is related to the hydrogen-bond strength. Here we report a combined experimental and theoretical study of the air-water interface using surface-selective heterodyne-detected vibrational sum frequency spectroscopy to focus on the 'free OD' transition found only in the topmost water layer. By using deuterated water and isotopic dilution to reveal the vibrational coupling mechanism, we find that the free OD stretch is affected only by intramolecular coupling to the stretching of the other OD group on the same molecule. The other OD stretch frequency indicates the strength of one of the first hydrogen bonds encountered at the surface; this is the donor hydrogen bond of the water molecule straddling the interface, which we find to be only slightly weaker than bulk-phase water hydrogen bonds. We infer from this observation a remarkably fast onset of bulk-phase behaviour on crossing from the air into the water phase. [ABSTRACT FROM AUTHOR]

We present a new technique of broad-band heterodyne-detected sum frequency generation (HD-SFG) spectroscopy and demonstrate its high sensitivity allowing surface-selective measurements of vibrational spectra at submonolayer surface coverage, as low as a few percent of a monolayer. This was achieved without the help of surface enhancement phenomena, on a transparent dielectric substrate (water), and without introducing fluorescent labels, in fact, without utilizing any electronic resonances. Only the intrinsic vibrational transitions were employed for the detection of the analyte molecules (1-octanol). Unlike conventional (homodyne-detected) SFG spectroscopy, where the signal intensity decreases quadratically with decreasing surface coverage, in HD-SFG, the scaling is linear, and the signal is amplified by interference with a reference beam, significantly improving sensitivity and detection limits. At the same time, HD-SFG provides the phase as well as the amplitude of the signal and thus allows accurate subtraction of the non-resonant background-a common problem for surfaces with low concentrations of analyte molecules (i.e., weak resonant signals). [ABSTRACT FROM AUTHOR]

The article offers information about a study that investigates the nanoscale geometric effects in a simple model system of approximately spherical gold nanoparticles of varying diameter approaching the size of the dodecanethiol adsorbate. Nonlinear surface-selective vibrational sum frequency generation spectroscopy is applied to characterize the conformation of dodecanethiol on gold nanoparticle surfaces.