Time-resolved Vibrational Sum Frequency Research Articles

Vibrational Dynamics at Aqueous–Mineral Interfaces

The dynamics of water at mineral surfaces has attracted much attention due to the marked differences, compared to the bulk, in the ability of interfacial water populations to redistribute vibrational energy, largely due to perturbations in the local hydrogen-bonding environments at interfaces. However, many unanswered questions persist regarding these geochemically and technologically relevant systems. The evolution of our understanding and current state-of-the-art interpretation are reviewed for three important mineral/aqueous interfaces (Al2O3, SiO2, and CaF2). While we focus on time-resolved vibrational Sum Frequency Generation (vSFG), as it is inherently surface specific, we include complementary time-resolved techniques such as IR and THz spectroscopies, which combined can provide a broader picture of interfacial dynamics at mineral surfaces. We show that vibrational dynamics are uniquely positioned to inform on structure at interfaces, which could be missed using conventional static vibrational spectra. Insights presented here shine light on previous successes and suggest future avenues for transient vibrational spectroscopy at mineral/aqueous interfaces.

Energy relaxation path of excited free OH vibration at an air/water interface revealed by nonequilibrium ab initio molecular dynamics simulation.

Nonequilibrium ab initio molecular dynamics (NE-AIMD) simulations are conducted at an air/water interface to elucidate the vibrational energy relaxation path of excited non-hydrogen-bonded (free) OH. A recent time-resolved vibrational sum frequency generation (TR-VSFG) spectroscopy experiment revealed that the relaxation time scales of free OH at the surface of pure water and isotopically diluted water are very similar to each other. In the present study, the dynamics of free OH excited at the surface of pure water and deuterated water are examined with an NE-AIMD simulation, which reproduces the experimentally observed features. The relaxation paths are examined by introducing constraints for the bonds and angles of water molecules relevant to specific vibrational modes in NE-AIMD simulations. In the case of free OH relaxation at the pure water surface, stretching vibrational coupling with the conjugate bond makes a significant contribution to the relaxation path. In the case of the isotopically diluted water surface, the bend (HOD)-stretching (OD) combination band couples with the free OH vibration, generating a relaxation rate similar to that in the pure water case. It is also found that the reorientation of the free OH bond contributes substantially to the relaxation of the free OH vibrational frequency component measured by TR-VSFG spectroscopy.

Ions Tune Interfacial Water Structure and Modulate Hydrophobic Interactions at Silica Surfaces.

The structure and ultrafast dynamics of the electric double layer (EDL) are central to chemical reactivity and physical properties at solid/aqueous interfaces. While the Gouy-Chapman-Stern model is widely used to describe EDLs, it is solely based on the macroscopic electrostatic attraction of electrolytes for the charged surfaces. Structure and dynamics in the Stern layer are, however, more complex because of competing effects due to the localized surface charge distribution, surface-solvent-ion correlations, and the interfacial hydrogen bonding environment. Here, we report combined time-resolved vibrational sum frequency generation (TR-vSFG) spectroscopy with ab initio DFT-based molecular dynamics simulations (AIMD/DFT-MD) to get direct access to the molecular-level understanding of how ions change the structure and dynamics of the EDL. We show that innersphere adsorbed ions tune the hydrophobicity of the silica-aqueous interface by shifting the structural makeup in the Stern layer from dominant water-surface interactions to water-water interactions. This drives an initially inhomogeneous interfacial water coordination landscape observed at the neat interface toward a homogeneous, highly interconnected in-plane 2D hydrogen bonding (2D-HB) network at the ionic interface, reminiscent of the canonical, hydrophobic air-water interface. This ion-induced transformation results in a characteristic decrease of the vibrational lifetime (T1) of excited interfacial O-H stretching modes from T1 ∼ 600 fs to T1 ∼ 250 fs. Hence, we propose that the T1 determined by TR-vSFG in combination with DFT-MD simulations can be widely used for a quantitative spectroscopic probe of the ion kosmotropic/chaotropic effect at aqueous interfaces as well as of the ion-induced surface hydrophobicity.

Effect of Halide Anions on the Structure and Dynamics of Water Next to an Alumina (0001) Surface

While ions are known to perturb hydrogen bonding networks in bulk water, our understanding of such effects is less developed for interfaces. Alumina/water interfaces are highly ordered due to strong hydrogen bonding interactions between interfacial water molecules and adjacent aluminol groups. However, how ions alter this interaction is not yet known. Herein, to address the effect of sodium halide salts on the hydrogen bonding environment of interfacial water, we investigated charged alumina (0001) surfaces using steady-state and time-resolved vibrational sum frequency generation (vSFG) spectroscopy. Our results indicate that the effect of halide anions on the attenuation of the vSFG signal next to positively and negatively charged alumina surfaces followed the sequence F– ≫ Br– > Cl– > I– (slightly varied direct Hofmeister series) and Br– > I– ≈ Cl– > F– (slightly varied indirect Hofmeister series), respectively. Additionally, time-resolved vSFG reveals that only F– perturbs the vibrational lifetime of w...

Ultrafast Dynamics at Water Interfaces Studied by Vibrational Sum Frequency Generation Spectroscopy.

We present an overview of studies on the ultrafast dynamics of water at aqueous interfaces carried out by time-resolved vibrational sum frequency generation (VSFG) spectroscopies. This research field has been growing rapidly, stimulated by technical developments achieved recently. In this review, first, the principles and instrumentations are described for conventional VSFG, heterodyne-detected VSFG, and various IR-pump/VSFG-probe techniques, namely, time-resolved conventional VSFG, time-resolved heterodyne-detected VSFG, and their extension to two-dimensional spectroscopy. Second, the applications of these time-resolved VSFG techniques to the study of the femtosecond vibrational dynamics of water at various interfaces are discussed, in the order of silica/water, charged monolayer/water, and the air/water interfaces. These studies demonstrate that there exists water dynamics specific to the interfaces and that time-resolved VSFG spectroscopies can unambiguously detect such unique dynamics in an interface-selective manner. In particular, the most recent time-resolved heterodyne-detected VSFG and two-dimensional heterodyne-detected VSFG unveil the inhomogeneity of the hydrogen bond and relevant vibrational dynamics of interfacial water through unambiguous observation of hole-burning in the OH stretch band, as well as the subsequent spectral diffusion in the femtosecond time region. These time-resolved VSFG studies have also left several issues for discussion. We describe not only the obtained conclusive physical insights into interfacial water dynamics but also the points left unclear or controversial. A new type of experiment that utilizes UV excitation is also described briefly. Lastly, the summary and some future perspectives of time-resolved VSFG spectroscopies are given.

Insights on Interfacial Structure, Dynamics, and Proton Transfer from Ultrafast Vibrational Sum Frequency Generation Spectroscopy of the Alumina(0001)/Water Interface

Steady-state and time-resolved vibrational sum frequency generation (vSFG) were used to investigate the structure and dynamics of water at the α-Al2O3(0001) surface. The vSFG spectra of the O—H stretch of water next to the Al2O3(0001) surface are blue-shifted compared to the Al2O3(1120) surface, indicating its weaker hydrogen bonding network. Consequently, the vibrational dynamics of the O—H stretch of the neutral Al2O3(0001) surface is two times slower than the neutral Al2O3(1120) surface. Furthermore, the vibrational dynamics of the O—H stretch of water next to charged Al2O3 surfaces is observed to be faster than that in bulk water and at charged SiO2 surfaces, which could be due to (a) fast proton transfer dominating the vibrational relaxation and/or, (b) efficient coupling between the O—H stretch and the bend overtone via the presence of low frequency (∼3000 cm–1) O—H stretching modes. Lastly, the addition of excess ions (0.1 M NaCl) seems to have little to no effect on the time scale of vibrational...

Spectroscopy and Ultrafast Vibrational Dynamics of Strongly Hydrogen Bonded OH Species at the α-Al2O3(112̅0)/H2O Interface

Frequency and time-resolved vibrational sum frequency generation (vSFG) are used to investigate the behavior of water at the α-Al2O3(1120) surface. In addition to the typical water OH peaks (∼3200 and ∼3400 cm–1), the α-Al2O3(1120)/H2O interface shows an additional red-shifted feature at ∼3000 cm–1. Addition of ions (0.1 M NaCl) largely attenuates the water OH peaks but has little effect on the 3000 cm–1 peak. The 3000 cm–1 feature is assigned to the O–H stretch of surface aluminol groups and/or interfacial water molecules that are strongly hydrogen bonded to the alumina surface. Density functional theory calculations were performed to test this assignment, revealing the presence of both associated and dissociated H2O configurations (chemisorbed surface OH group) with frequencies at 3155 and 3190 cm–1, respectively, at a hydrated α-Al2O3(1120) surface. IR pump–vSFG probe measurements reveal that the interfacial OH species show very fast (<200 fs; bulk waterlike) vibrational relaxation dynamics, which i...

Quantitative Time-Resolved Vibrational Sum Frequency Generation Spectroscopy as a Tool for Thin Film Kinetic Studies: New Insights into Oleic Acid Monolayer Oxidation

Environmental air-water interfaces are often covered by thin films of surface-active organic substances that play an important role for air-sea gas exchange and aerosol aging. Surface-sensitive vibrational sum frequency generation (VSFG) spectroscopy has been widely used to study the static structure of organic monolayers serving as simple model systems of such films. Probably due to the difficulties to correlate the SFG signal intensity with the surface concentration, corresponding time-resolved studies of surface reactions are scarce. In this study, quantitative time-resolved measurements have been performed on the oleic acid monolayer ozonolysis, which is considered a benchmark system for investigating the reactivity and fate of unsaturated natural organics. Surface concentration calibration data have been obtained by combining the pressure-area isotherm and VSFG spectra acquisition such that the 2D phase behavior of the oleic acid film could be properly taken into account. In contrast to literature reports, surface-active oxidation products were found to be negligible and do not interfere with the VSFG measurements. A pseudo-first-order kinetic analysis of the time-resolved data yielded a bimolecular rate constant of k2(oleic acid + O3 → products) = (1.65 ± 0.64) × 10(-16) cm(3) molecules(-1) s(-1), corresponding to an uptake coefficient of γ = (4.7 ± 1.8) × 10(-6). This result is in very good agreement with most recent monolayer measurements based on alternative methods and underlines the reliability of the time-resolved VSFG approach.

Vibrational relaxation dynamics of catalysts on TiO2 Rutile (1 1 0) single crystal surfaces and anatase nanoporous thin films

Time-resolved vibrational sum frequency generation (VSFG) spectroscopy has been used to investigate the vibrational relaxation dynamics of the rhenium bipyridyl CO2-reduction catalyst Re(CO)3Cl(dcbpy) [dcbpy=4,4′-dicarboxy-2,2′-bipyridine] adsorbed onto the (110) surface of a Rutile TiO2 single crystal. IR pump-VSFG probe spectra of the a′(1) CO stretching mode indicate a ultrafast population equilibration between three CO stretching modes followed by their population relaxation via intramolecular vibrational energy transfer. Similar vibational relaxation dynamics was also observed for the same complex on anatase TiO2 nanocrystalline thin films measured by IR pump-IR probe transient absorption spectroscopy. The relaxation dynamics of ReCOA on TiO2, in DMF solution, and immobilized on Au through alkane thiol linkers were compared to examine possible effects of adsorbate-TiO2 interaction.

Ultrafast Vibrational Relaxation Dynamics of a Rhenium Bipyridyl CO2–Reduction Catalyst at a Au Electrode Surface Probed by Time-Resolved Vibrational Sum Frequency Generation Spectroscopy

The vibrational relaxation dynamics of a rhenium bipyridyl CO2-reduction catalyst chemisorbed to a model Au electrode surface have been investigated using broadband time-resolved vibrational sum frequency generation (VSFG) spectroscopy. IR pump-VSFG probe spectra indicate a biexponential decay of the excited vibrational population, consisting of an ultrafast initial relaxation followed by complete recovery of the ground vibrational state within tens of picoseconds. The ultrafast decay is assigned to rapid population equilibration between the strongly coupled CO stretching modes of the rhenium bipyridyl complex (τυ–υ = 0.26 ps), whereas the slower decay is assigned to population relaxation from these coupled modes to lower frequency modes within the molecule (T1 = 14.8 ps). This study demonstrates the ability of time-resolved VSFG to selectively monitor the vibrational energy relaxation processes at the electrode surface in relevant electrocatalytic systems.

Structural Inhomogeneity of Interfacial Water at Lipid Monolayers Revealed by Surface-Specific Vibrational Pump−Probe Spectroscopy

We report vibrational lifetime measurements of the OH stretch vibration of interfacial water in contact with lipid monolayers, using time-resolved vibrational sum frequency (VSF) spectroscopy. The dynamics of water in contact with four different lipids are reported and are characterized by vibrational relaxation rates measured at 3200, 3300, 3400, and 3500 cm(-1). We observe that the water molecules with an OH frequency ranging from 3300 to 3500 cm(-1) all show vibrational relaxation with a time constant of T(1) = 180 ± 35 fs, similar to what is found for bulk water. Water molecules with OH groups near 3200 cm(-1) show distinctly faster relaxation dynamics, with T(1) < 80 fs. We successfully model the data by describing the interfacial water containing two distinct subensembles in which spectral diffusion is, respectively, rapid (3300-3500 cm(-1)) and absent (3200 cm(-1)). We discuss the potential biological implications of the presence of the strongly hydrogen-bonded, rapidly relaxing water molecules at 3200 cm(-1) that are decoupled from the bulk water system.

Inducing non-adiabatic effects through coadsorption: CO + NO on iridium

We compare the response of the intramolecular C–O stretch of carbon monoxide alone and coadsorbed with nitric oxide on Ir{1 1 1} following femtosecond laser heating with the help of time-resolved vibrational sum frequency (SF) spectroscopy. The C–O stretch of a pure CO layer couples anharmonically to the CO frustrated translation and its frequency adiabatically follows the temperature of the iridium surface. In a mixed CO/NO layer, the C–O frequency exhibits non-adiabatic coupling to the hot iridium electrons with a friction coefficient that depends on the electron temperature and the CO:NO ratio. Two possible scenarios emerge: NO causes a static tilt of the CO with a tilt angle depending on the relative coverage. This increases the degree of bonding of the CO 2 π ∗ orbital to the iridium surface, which in turn increases the degree of non-adiabatic coupling. Alternatively, the C–O frequency reflects transient changes in the bonding configuration of the neighboring NO. The latter interaction could be the primary step in the direct reduction of NO by CO to form CO 2.

Ultrafast Surface Dynamics Studied with Femtosecond Sum Frequency Generation

We present an investigation into femtosecond time-resolved vibrational sum frequency spectroscopy as a tool for studying ultrafast surface dynamics. It is demonstrated that the presence of experimentally observed new spectral features does not necessarily mean that a novel chemical species has appeared on the surface. In addition, we present a simple model for calculating the sum frequency intensity as a chemical reaction takes place on the surface via a transition state or reaction intermediate. These calculations show that a transition state can be observed and its lifetime can be determined.