Broadband Vibrational Sum-frequency Generation Research Articles

Choose your own adventure: Picosecond or broadband vibrational sum-frequency generation spectroscopy.

Vibrational sum-frequency generation (VSFG) spectroscopy is a method capable of measuring chemical structure and dynamics within the interfacial region between two bulk phases. At the core of every experimental system is a laser source that influences the experimental capabilities of the VSFG spectrometer. In this article, we discuss the differences between VSFG spectrometers built with picosecond and broadband laser sources as it will impact everything from material costs, experimental build time, experimental capabilities, and more. A focus is placed on the accessibility of the two different SFG systems to newcomers in the SFG field and provides a resource for laboratories considering incorporating VSFG spectroscopy into their research programs. This Tutorial provides a model decision tree to aid newcomers when determining whether the picosecond or femtosecond laser system is sufficient for their research program and navigates through it for a few specific scenarios.

Considerations in upconversion: A practical guide to sum-frequency generation spectrometer design and implementation.

In this tutorial review, we discuss how the choice of upconversion pulse shape in broadband vibrational sum-frequency generation (SFG) spectrometer design impacts the chemical or physical insights one can obtain from a set of measurements. A time-domain picture of a vibrational coherence being mapped by a second optical field is described and the implications of how this mapping, or upconversion process, takes place are given in the context of several popular and emerging approaches found in the literature. Emphasis is placed on broadband frequency-domain measurements, where the choice of upconversion pulse enhances or limits the information contained in the SFG spectrum. We conclude with an outline for a flexible approach to SFG upconversion using pulse-shaping methods and a simple guide to design and optimize the associated instrumentation.

Tutorials in vibrational sum frequency generation spectroscopy. II. Designing a broadband vibrational sum frequency generation spectrometer.

In this Tutorial series, we aim to provide an accessible introduction to vibrational sum frequency generation (VSFG) spectroscopy, targeted toward people entering the VSFG world without a rigorous formal background in optical physics or nonlinear spectroscopy. In this article, we describe in depth how a broadband VSFG spectrometer is designed and constructed, using the instrument in SurfLab, Aarhus University, as an illustrative case. Detailed information about specific instrumentation (together with reasons why things are the way they are) is given throughout. This information is often omitted in other descriptions of such instrumentation and so will be invaluable to people new to the field.

Broadband vibrational sum-frequency generation spectrometer at 100 kHz in the 950-1750 cm-1 spectral range utilizing a LiGaS2 optical parametric amplifier.

We present a 100 kHz broadband vibrational sum-frequency generation (VSFG) spectrometer operating in the 5.7-10.5 µm (950-1750 cm-1) wavelength range. The mid-infrared beam of the system is obtained from a collinear, type-I LiGaS2-crystal-based optical parametric amplifier seeded by a supercontinuum and pumped directly by 180 fs, ~32 µJ, 1.03 µm pulses from an Yb:KGd(WO4)2 laser system. Up to 0.5 µJ mid-infrared pulses with durations below 100 fs were obtained after dispersion compensation utilizing bulk materials. We demonstrate the utility of the spectrometer by recording high-resolution, low-noise vibrational spectra of Langmuir-Blodgett supported lipid monolayers on CaF2. The presented VSFG spectrometer scheme offers superior signal-to-noise ratios and constitutes a high-efficiency, low-cost, easy-to-use alternative to traditional schemes relying on optical parametric amplification followed by difference frequency generation.

High-resolution and high-repetition-rate vibrational sum-frequency generation spectroscopy of one- and two-component phosphatidylcholine monolayers.

We present broadband vibrational sum-frequency generation (VSFG) spectra of Langmuir-Blodgett monolayers of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), and different mixtures of them as model systems of pulmonary surfactants. The systematic study explored the dependence of the vibrational spectra as a function of surface tension and mixture ratio in various polarization combinations. The extremely short acquisition time and the high spectral resolution of our recently developed spectrometer helped minimize sample degradation under ambient conditions throughout the duration of the measurement and allowed the detection of previously unseen vibrational bands with unprecedented signal-to-noise ratio. The dramatically improved capability to record reliable vibrational spectra together with the label-free nature of the VSFG method provides direct access to native lipid structure and dynamics directly in the monolayer. The resulting data deliver quantitative information for structural analysis of multi-component phospholipid monolayers and may aid in the development of new synthetic pulmonary surfactants.

Vibrational sum-frequency generation spectroscopy of lipid bilayers at repetition rates up to 100 kHz.

Broadband vibrational sum-frequency generation (BB-VSFG) spectroscopy has become a well-established surface analytical tool capable of identifying the orientation and structure of molecular layers. A straightforward way to boost the sensitivity of the technique could be to increase the laser repetition rate beyond that of standard BB-VSFG spectrometers, which rely on Ti:sapphire lasers operating at repetition rates of 1-5 kHz. Nevertheless, possible thermally induced artifacts in the vibrational spectra due to higher laser average powers are unexplored. Here, we discuss laser power induced temperature accumulation effects that distort the BB-VSFG spectra of 1,2-diacyl-sn-glycero-3-phosphocholine at an interface between two transparent phases at repetition rates of 5, 10, 50, and 100 kHz at constant pulse energy. No heat-induced distortions were found in the spectra, suggesting that the increase in the laser repetition rate provides a feasible route to an improved signal-to-noise ratio or shorter data acquisition times in BB-VSFG spectroscopy for thin films on transparent substrates. The results have implications for future BB-VSFG spectrometers pushing the detection limit for molecular layers with low surface coverage.

Effect of Temperature on the Organization of Acetonitrile at the Silica/Liquid Interface

Liquid acetonitrile at a silica surface is known to organize in a manner that, in some respects, resembles a supported lipid bilayer. Here we use broadband vibrational sum-frequency generation (VSFG) spectroscopy to study how this organization depends on temperature. We find that VSFG spectra in the methyl-stretching region of the spectrum decrease in magnitude and shift to the blue for all polarization combinations (SSP, SPS, and PPP) as the temperature is raised from 25 to 60 °C. The observed decrease in intensity with increasing temperature is unexpected and suggests that the dynamics of the second sublayer of acetonitrile molecules plays an important role in the spectral behavior of this system. We propose that the decrease in intensity with increasing temperature arises largely from the more static first sublayer becoming more disordered. This picture is supported by the observed blue shift at high temperature, which is consistent with faster reorientation-induced spectral diffusion. The rapid dynamics and average orientation of the second sublayer are biased by the first sublayer, but this bias is not influenced substantially by increasing the temperature.

Compact, high-repetition-rate source for broadband sum-frequency generation spectroscopy

We present a high-efficiency optical parametric source for broadband vibrational sum-frequency generation (BB-VSFG) for the chemically important mid-infrared spectral range at 2800–3600 cm−1 to study hydrogen bonding interactions affecting the structural organization of biomolecules at water interfaces. The source consists of a supercontinuum-seeded, dual-beam optical parametric amplifier with two broadband infrared output beams and a chirped sum-frequency mixing stage providing narrowband visible pulses with adjustable bandwidth. Utilizing a pulse energy of only 60 μJ from a turn-key, 1.03-μm pump laser operating at a repetition rate of 100 kHz, the source delivers 6-cycle infrared pulses at 1.5 and 3.2 μm with pulse energies of 4.6 and 1.8 μJ, respectively, and narrowband pulses at 0.515 μm with a pulse energy of 5.0 μJ. The 3.2-μm pulses are passively carrier envelope phase stabilized with fluctuations at the 180-mrad level over a 10-s time period. The 1.5-μm beamline can be exploited to deliver pump pulses for time-resolved studies after suitable frequency up-conversion. The high efficiency, stability, and two orders of magnitude higher repetition rate of the source compared to typically employed systems offer great potential for providing a boost in sensitivity in BB-VSFG experiments at a reduced cost.

Vibrational dephasing of self-assembling monolayer on gold surface

Ultrafast vibrational dephasing processes of ODT (1-Octadecanethiol) on Au surface had been investigated with time-resolved broadband vibrational sum frequency generation spectroscopy. Vibrational dephasing time constants obtained from different visible wavelength varying from 470nm to 810nm. The results showed that electronic structure of the gold substrate plays an important role in surface molecule dephasing dynamics.

Influence of Salt Purity on Na+ and Palmitic Acid Interactions

The influence of salt purity on the interactions between Na(+) ions and the carboxylate (COO(-)) head group of palmitic acid (PA) monolayers is studied in the COO(-) and OH stretching regions using broad-band vibrational sum frequency generation (VSFG) spectroscopy. Ultrapure (UP) and ACS grade NaCl salts are used for aqueous solution preparation after proper pretreatment. The time evolution of VSFG spectra of PA monolayers on solutions made from these two grades of salts is different, which reveals that the salt purity has a significant impact on the interactions between Na(+) ions and the COO(-) group of PA. The trace metal impurities in ACS grade salt, which are more abundant than those in UP grade salt, are responsible for this difference due to their stronger affinity for the carboxylate group relative to Na(+) and further affects the interfacial water structure. These results suggest that the alkali salt grade even after pretreatment is critical in the studies of alkali cation-carboxylate interactions and comparison of relative binding affinities of different cations.

Salty Glycerol versus Salty Water Surface Organization: Bromide and Iodide Surface Propensities

Salty NaBr and NaI glycerol solution interfaces are examined in the OH stretching region using broadband vibrational sum frequency generation (VSFG) spectroscopy. Raman and infrared (IR) spectroscopy are used to further understand the VSFG spectroscopic signature. The VSFG spectra of salty glycerol solutions reveal that bromide and iodide anions perturb the interfacial glycerol organization in a manner similar as that found in aqueous halide salt solutions, thus confirming the presence of bromide and iodide anions at the glycerol surface. Surface tension measurements are consistent with the surface propensity suggested by the VSFG data and also show that the surface excess increases with increasing salt concentration, similar to that of water. In addition, iodide is shown to have more surface prevalence than bromide, as has also been determined from aqueous solutions. These results suggest that glycerol behaves similarly to water with respect to surface activity and solvation of halide anions at its air/liquid interface.

Vibrational sum frequency generation spectroscopy using inverted visible pulses

We present a broadband vibrational sum frequency generation (BB-VSFG) scheme using a novel ps visible pulse shape. We generate the fs IR pulse via standard procedures and simultaneously generate an 'inverted' time-asymmetric narrowband ps visible pulse via second harmonic generation in the pump depletion regime using a very long nonlinear crystal which has high group velocity mismatch (LiNbO3). The 'inverted' ps pulse shape minimally samples the instantaneous nonresonant response but maximally samples the resonant response, maintaining high spectral resolution. We experimentally demonstrate this scheme, presenting SFG spectra of canonical organic monolayer systems in the C-H stretch region (2800-3000 cm(-1)).

Compact broadband vibrational sum-frequency generation spectrometer with nonresonant suppression

A compact broadband vibrational sum-frequency spectroscopy (SFG) apparatus is described to study molecules at surfaces and interfaces. Using an étalon as the frequency narrowing device, the visible pulse has a time-asymmetric profile that allows the user to deeply suppress nonresonant background signals that hinder detection of molecular vibrational resonances. Several features of the spectrometer that, in aggregate, improve signal-to-noise ratios by a large factor are described. The spectrometer features a series of interchangeable prealigned sample holders for different applications. Examples of applications are presented where nonresonant suppression greatly improves the ability to study adsorbates on single-crystal surfaces as a function of rotation about the azimuth, and where the rapid data acquisition abilities of the spectrometer are used to study electrochemical transformations on single-crystal electrodes.

Pentane, Hexane, Cyclopentane, Cyclohexane, 1-Hexene, 1-Pentene,cis-2-Pentene, Cyclohexene, and Cyclopentene at Vapor/α-Alumina and Liquid/α-Alumina Interfaces Studied by Broadband Sum Frequency Generation

The molecular orientation and structure of adsorbates at oxide interfaces is driven by surface−molecule and molecule−molecule interactions and is useful for predicting reactivity and product selectivity in heterogeneous chemical reactions, including those important in catalytic processes. Using broadband vibrational sum-frequency generation spectroscopy, we probed cyclic and acyclic alkanes and olefins at buried vapor/solid and liquid/solid interfaces of the α-alumina (0001) surface under ambient conditions. The spectroscopic signatures of the adsorbates were measured and compared with bulk phase spectra and spectra of model surfaces containing hydrocarbons covalently linked to glass slides using silane chemistry. By utilizing appropriate polarization combinations, the orientations of hydrocarbons adsorbed from the vapor were evaluated and compared to the hydrocarbon orientations at the liquid/solid interface. The data support a proposed orientation for n-alkanes and cycloalkanes at the liquid/solid interface in which the hydrocarbons lie with the planes of their carbon backbones parallel to the surface, whereas at the vapor/solid interface, the adsorbates are oriented with their carbon backbones in an orientation neither parallel nor perpendicular to the surface. The surface vibrational spectra of olefins at both types of interfaces indicate that their orientations differ from saturated counterparts, and their unique spectral signatures allow differentiation of adsorbed olefins from alkanes.

Ti:sapphire, broadband vibrational sum-frequency generation spectrometer with a counter-propagating geometry

We report the development of a counter-propagating, broadband vibrational sum-frequency generation spectrometer based on a Ti:sapphire regenerative amplifier. We present simple procedures for aligning the spectrometer and for setting the timing of the IR and visible pulses. We demonstrate that the use of this geometry offers a number of important advantages over a co-propagating geometry, including a high dynamic range, reduced nonresonant background signal at buried interfaces, and minimal beam deviation upon changing samples.

Two Reactivity Modes in the Heterogeneous Cyclohexene Ozonolysis under Tropospherically Relevant Ozone-Rich and Ozone-Limited Conditions

Important mechanistic differences regarding C=C double-bond oxidation processes under ozone-limited and ozone-rich reaction conditions for cyclohexene-functionalized fused silica substrates serving as model systems for studying heterogeneous C=C double bond oxidation chemistry in the troposphere are evaluated. By using broadband vibrational sum frequency generation (SFG), we track heterogeneous ozone reactions in real time. Ozone levels span three orders of magnitude and represent environments ranging from pristine remote continental regions to highly polluted urban centers, ranging from 30 ppb to 3 ppm (from 7 x 10(11) molecules cm(-3) to 7 x 10(13) molecules cm(-3)). We determine reaction rates and reactive uptake coefficients (gamma values). At these tropospherically relevant ozone levels, the heterogeneous reaction rates follow a Langmuir-Hinshelwood-type mechanism. The product formation rates, which we determine as a function of ozone concentrations, are found to be half of the olefin reaction rates. This ratio is consistent with the previously proposed reaction pathway involving the breaking of one C=C double bond containing two olefinic CH moieties to form a product containing only one methyl group and one polar carbonyl moiety. Contact angle histograms show that out of a total of 60 measurements, there are about 25 more measurements with contact angles up to ten degrees below the mean recorded prior to reaction when ozone levels resemble remote continental conditions (50 ppb) than when ozone levels resemble urban conditions (1 ppm). The implication of these results are that the methyl formation pathway in heterogeneous ozonolysis may be less favorable than the carboxylic acid- and secondary ozonide-production pathway for ozone-limited conditions (i.e., in the remote continental troposphere or during urban nighttime) as opposed to ozone-rich (i.e., polluted urban atmosphere) conditions.

Direct evidence for orientational flip-flop of water molecules at charged interfaces: A heterodyne-detected vibrational sum frequency generation study

Complex chi(2) spectra of air/water interfaces in the presence of charged surfactants were measured by heterodyne-detected broadband vibrational sum frequency generation spectroscopy for the first time. In contrast to the neat water surface, the signs of chi(2) for two broad OH bands are the same in the presence of the charged surfactants. The obtained chi(2) spectra clearly showed flip-flop of the interfacial water molecules which is induced by the opposite charge of the head group of the surfactants. With the sign of beta(2) theoretically obtained, the absolute orientation, i.e., up/down orientation, of water molecules at the charged aqueous surfaces was uniquely determined by the relation between the sign of chi(2) and the molecular orientation angle. Water molecules orient with their hydrogen up at the negatively charged aqueous interface whereas their oxygen up at the positively charged aqueous interface.

Chemically diverse environmental interfaces and their reactions with ozone studied by sum frequency generation

Using tailor-made organic compounds tethered to solid substrates through organo-silane chemistry, we present a reductionist model study aimed at understanding the mechanisms of heterogeneous organic oxidation reactions at solid/air interfaces. The surface vibrational spectra of glass slides functionalized with the tropospherically relevant olefins 1-pentene, 2-hexene, cyclopentene, cyclohexene, and a menthenol derivative via aniline-silane linkers have been obtained through polarization-resolved broadband vibrational sum frequency generation (SFG). The olefinic and aliphatic CH stretches located above and below 3000 cm −1, respectively, are clearly discernable and their spectral intensities are used to track, with 10 s time resolution, C C double bond oxidation reactions of surface-bound cyclohexene at room temperature and with low ppm amounts of ozone at 1 atm. The olefinic CH stretching mode disappears at a rate of 0.05(1) s −1, and the aliphatic asymmetric stretch modes increase at a rate of 0.04(1) s −1. Analogous experiments show the formation of methyl groups even for those olefins under investigation that do not originally possess methyl groups. The implications for heterogeneous organic oxidation chemistry involving tropospheric dust particles are discussed.

Theory of sum-frequency generation spectroscopy of adsorbed molecules using thedensity matrix method—broadband vibrational sum-frequency generation andapplications

A generalized theory of frequency- and time-resolved vibrational sum-frequency generation(SFG) spectroscopy of adsorbates at surfaces is presented using the density matrixformalism. Our theoretical treatment is specifically aimed at addressing issues thataccompany the relatively novel SFG approach using broadband infrared pulses. Theultrashort duration of these pulses makes them ideally suited for time-resolvedinvestigations, for which we present a complete theoretical treatment. A second keycharacteristic of these pulses is their large bandwidth and high intensity, which allow forhighly non-linear effects, including vibrational ladder climbing of surface vibrations. Wederive general expressions relating the density matrix to SFG spectra, and apply theseexpressions to specific experimental results by solving the coupled optical Blochequations of the density matrix elements. Thus, we can theoretically reproducerecent experimentally demonstrated hot band SFG spectra using femtosecondbroadband infrared excitation of carbon monoxide (CO) on a Ru(001) surface.

Frequency- and time-domain femtosecond vibrational sum frequency generation from CO adsorbed on Pt(111)

We have studied the effects of intermolecular and intramolecular coupling on the C-O stretching vibration of CO adsorbed on Platinum (111) by means of femtosecond broadband vibrational sum frequency generation (VSFG). Resonant intermolecular coupling is investigated through the coverage dependence of the VSFG signal. The experimental observations can be accurately modeled as lateral coupling of the molecular transition dipole moments; this coupling is invoked in the nonlinear optical response model as a local field correction. The linear polarizability, which appears in this model, is modified by both the dipole-dipole coupling and the population of bridged adsorption sites. By extending the formalism to include these effects, we deduce a vibrational polarizability of 0.32 A(3) from the data. Intramolecular coupling to the frustrated translational mode is observed as temperature dependence of the C-O stretch. The present data can be described either by pertubative or nonpertubative lineshape models from the literature. Measurements of the temperature dependence of the vibrational free induction decay indicate a population relaxation time T(1) of (0.8+/-0.1) ps, in agreement with the observed low-temperature linewidth. Moreover, the ability of this time-domain method to discriminate spectral inhomogeneity yields clear evidence of the order-disorder transition near 275 K. Above this temperature an inhomogeneous linewidth component of (12+/-3) cm(-1) is observed. This value allows us to estimate the structural heterogeneity of the disordered phase, which result agrees with published Monte Carlo simulations.