Heterodyne-detected Sum Frequency Generation Spectroscopy Research Articles

Molecular Orientations of Myristic Acid Derivatives with Different Perfluoroalkyl Chain Lengths at the Air/Water Interface Evaluated by Heterodyne-Detected Sum Frequency Generation Spectroscopy

Surface-related properties of perfluoroalkyl compounds such as hydrophobicity and adhesion force can have a close relationship with the orientational order of the molecules at the surface. Myristic acid (MA) derivatives with perfluoroalkyl (Rf) groups have been known to form a well-ordered monolayer at the air/gold substrate interface. In this study, molecular orientations of the MA derivative monolayers at the air/water interface were investigated by heterodyne-detected vibrational sum frequency generation (HD-VSFG) spectroscopy. We estimated the molecular orientation of MA derivative monolayers with a marker band observed at around 1370 cm–1. The Rf chain length and the surface pressure dependence of the molecular orientation of four MA derivatives were examined. The marker bands were observed except for the derivative with the shortest Rf chain at the higher surface pressure, whereas the marker bands were observed only for the two derivatives with relatively longer Rf chain lengths at the lower surface pressure. The observation indicates that the longer the Rf chain is and the higher the surface pressure is, MA derivatives tend to adopt a standing orientation closer to perpendicular to the interface.

Experimental and Theoretical Heterodyne-Detected Sum Frequency Generation Spectroscopy of Isotopically Pure and Diluted Water Surfaces.

The χ(2) (second-order nonlinear optical susceptibility) spectrum of the water surface has been a matter of debate for a few decades. Here, we report that we experimentally measured the isotopic dilution dependence of the χ(2) spectrum and theoretically reproduced it by employing the quantum/classical mixed approach with a new idea to subtract an artifact. The present theoretical framework allows for clarifying the effects of the intramolecular, intermolecular, and Fermi resonance couplings on the OH-stretch vibrational spectra of water at the surface as well as in the bulk.

Effect of Head Group on Low-Level Ozone Oxidation of Unsaturated Phospholipids on a Water Surface

Abstract The effect of the head group on the oxidation of unsaturated phospholipids under low-level ozone was investigated. The phospholipids with head groups of glycerol and choline show different surface-pressure profiles but similar degradation kinetics of the C=C moieties was observed by heterodyne-detected sum frequency generation spectroscopy.

Enhanced structural disorder at a nanocrystalline ice surface.

Detailed knowledge of the structure and dynamics of the surface of ice particles is of considerable importance for understanding catalytic reactions in the upper atmosphere. Here we report the enhanced structural disorder specific at a nanoscale ice island studied by using heterodyne-detected vibrational sum-frequency generation spectroscopy under ultrahigh vacuum. Ultrathin films of isotopically diluted HOD crystalline ice are grown on Rh(111), whose average height (≥1.4 nm) is controlled by varying the nominal film thickness. The Im χ(2) spectrum of the hydrogen-bonded O-H stretching band shows a bipolar line shape reflecting the orientation-dependent hydrogen bond length alternation in the subsurface of the ice island. The peak splitting and the bandwidth of the bipolar spectrum increase with a decrease in the nominal film thickness. This is ascribed to the significant enhancement of structural disorder at the surface of the ice island as the terrace size is decreased. Temperature dependence of the Im χ(2) spectra of the hydrogen bonded O-H stretching band indicates that the thermal expansivity of the top layer increases upon decreasing the island size. In addition, the stretching frequency of the dangling OD band at the island surface with the average height less than 18 nm shows a systematic blue shift with increasing temperature from 100 to 145 K: this is in stark contrast to thick ice films and bulk ice showing a negligible peak shift at a temperature lower than 180 K. These findings indicate that the anharmonicity of the intermolecular potential at the top layer of the ice island is strongly enhanced upon decreasing its terrace size, providing valuable insights for understanding the properties of ice particles in the outer atmosphere including polar mesospheric clouds.

The Surface of Ice Is Like Supercooled Liquid Water.

The surface of ice has been reported to be disordered at temperatures well below the bulk melting point. However, the precise nature of this disorder has been a topic of intense debate. Herein, we study the molecular properties of the surface of ice as a function of temperatures using heterodyne-detected sum-frequency generation spectroscopy. We observe that, down to 245 K, the spectral response of the surface of ice contains a component that is indistinguishable from supercooled liquid water.

Proton Order toward the Surface of Ice Ih Revealed by Heterodyne-Detected Sum Frequency Generation Spectroscopy.

Using heterodyne-detected sum frequency generation (HD-SFG) spectroscopy, we investigated surface proton order at the basal, primary prism, and secondary prism faces of single-crystalline ice Ih at ca. 130 K. The complex phase of the obtained spectra clearly indicates that second-order nonlinear polarization from which the HD-SFG signal arises is generated exclusively at the surfaces. This suggests surface proton ordering along the normal, whereas the bulk remains proton-disordered, as is well known for ice Ih. A strong positive peak observed in the HD-SFG spectra enables us to determine the "direction" of the surface proton order as "H-up", that is, the hydrogen atom of the OH group pointing away from the bulk, irrespective of the ice faces. Reliable HD-SFG measurements carried out in the present study have greatly advanced our understanding of surface structure of ice Ih.

Vibrational Coupling at the Topmost Surface of Water Revealed by Heterodyne-Detected Sum Frequency Generation Spectroscopy.

Unraveling vibrational coupling is the key to consistently interpret vibrational spectra of complex molecular systems. The vibrational spectrum of the water surface heavily suffers from vibrational coupling, which hinders complete understanding of the molecular structure and dynamics of the water surface. Here we apply heterodyne-detected sum frequency generation spectroscopy to the water surface and accomplish the assignment of a weak vibrational band located at the lower energy side of the free OH stretch. We find that this band is due to a combination mode of the hydrogen-bonded OH stretch and a low-frequency intermolecular vibration, and this combination band appears in the surface vibrational spectrum through anharmonic vibrational coupling that takes place exclusively at the topmost surface.

Weakly Hydrogen-Bonded Water Inside Charged Lipid Monolayer Observed with Heterodyne-Detected Vibrational Sum Frequency Generation Spectroscopy

Many biological processes such as photosynthesis and signal transduction proceed at lipid/water interfaces. Although water molecules inside lipid membranes are considered to have a critical role in these processes, it has been difficult to experimentally observe those water molecules with high selectivity. Here, we measure the Im χ(2) (imaginary part of second-order nonlinear optical susceptibility) spectra of water at anionic lipid (1,2-dipalmitoyl-sn-glycero-3-phosphoglycerol, DPPG) and cationic lipid (1,2-dipalmitoyl-3-trimethylammonium-propane, DPTAP) interfaces with single-channel heterodyne-detected sum frequency generation spectroscopy and investigate the structure of water molecules at the lipid interfaces. The presence of weakly hydrogen-bonded water molecules located above the lipid head group is clearly identified. The Im χ(2) spectra indicate that water molecules above the head group are oriented oppositely to those below the head group and they donate hydrogen bonds to the carbonyl and glycer...

PH Dependence of Interfacial Water in the Presence of Amino Acid Side Chains Revealed by Heterodyne-Detected Sum-Frequency Generation Spectroscopy

Proteins are built of amino acid residues that differ from each other only by their side chain. The pH-dependent response of these side chains on protein surface is vital for various biological processes. Here we have investigated the aqueous interface in the presence of different amino acid side chains at varying pH (bulk) using heterodyne-detected vibrational sum-frequency generation (HD-VSFG) spectroscopy. It is observed that amine/imidazolic (e.g., lysine/histidine) and alcoholic (e.g., serine and threonine) side chains preferentially orient the interfacial water as “H-down” (i.e., the water hydrogens are pointed toward the aqueous bulk) in acidic solution (pH ∼2). At physiological pH (7.4), the interfacial water takes “H-up” orientation (i.e., the water hydrogens are pointed away from the aqueous bulk) for the alcoholic/imidazolic side chains but remains “H-down” for the amine containing side chain. On further increasing the pH (up to 12.0), the interfacial water becomes increasingly H-up oriented, r...

Heterodyne-detected sum frequency generation spectroscopy of polyacrylic acid at the air/water-interface.

The conformational and hydration behavior of polyacrylic acid (PAA) at the air/water interface is studied using heterodyne-detected vibrational sum frequency generation spectroscopy (HD-VSFG). The amount of adsorption at the water-air interface and the influence of PAA on the water structure at the interface are found to be highly dependent on the degree of ionization (acid dissociation) of the carboxylic acid groups. At a low degree of ionization PAA is surface active and remains in a dense packing arrangement with intrachain hydrogen bonds. The interfacial water molecules show a net orientation with their OH groups pointing to the surface. Ionization of the carboxylic acid groups leads to an increase of the negative charge and to a break-up of the intrachain hydrogen bonds. The increase in negative charge is accompanied by an increase of the VSFG signal of the interfacial water molecules, as a result of their enhanced orientation. At ionization degrees α > 0.203 PAA loses its enhanced surface propensity and is well dissolved in the bulk. Addition of NaCl to the solution is observed to lead to a recurrence of PAA at the interface.

Development of single-channel heterodyne-detected sum frequency generation spectroscopy and its application to the water/vapor interface.

Single-channel heterodyne-detected sum frequency generation (HD-SFG) spectroscopy for selectively measuring vibrational spectra of liquid interfaces is presented. This new methodology is based on optical interference between sum frequency signal light from a sample interface and phase-controlled local oscillator light. In single-channel HD-SFG, interferometric and spectrometric measurements are simultaneously carried out with an input IR laser scanned in a certain wavenumber range, which results in a less task than existing phase-sensitive sum frequency spectroscopy. The real and imaginary parts of second-order nonlinear optical susceptibility (χ((2))) of interfaces are separately obtained with spectral resolution as high as 4 cm(-1) that is approximately six times better than existing multiplex HD-SFG. In this paper, the experimental procedure and theoretical background of single-channel HD-SFG are explicated, and its application to the water/vapor interface is demonstrated, putting emphasis on the importance of a standard for the complex phase of χ((2)).

Unified Molecular View of the Air/Water Interface Based on Experimental and Theoretical χ(2) Spectra of an Isotopically Diluted Water Surface

The energetically unfavorable termination of the hydrogen-bonded network of water molecules at the air/water interface causes molecular rearrangement to minimize the free energy. The long-standing question is how water minimizes the surface free energy. The combination of advanced, surface-specific nonlinear spectroscopy and theoretical simulation provides new insights. The complex χ((2)) spectra of isotopically diluted water surfaces obtained by heterodyne-detected sum frequency generation spectroscopy and molecular dynamics simulation show excellent agreement, assuring the validity of the microscopic picture given in the simulation. The present study indicates that there is no ice-like structure at the surface--in other words, there is no increase of tetrahedrally coordinated structure compared to the bulk--but that there are water pairs interacting with a strong hydrogen bond at the outermost surface. Intuitively, this can be considered a consequence of the lack of a hydrogen bond toward the upper gas phase, enhancing the lateral interaction at the boundary. This study also confirms that the major source of the isotope effect on the water χ((2)) spectra is the intramolecular anharmonic coupling, i.e., Fermi resonance.