Sum-frequency Spectra Research Articles

Orientation of benzoate ions bound to a monolayer of a cationic surfactant at the surface of water

Vibrational spectra of a monolayer of tetradecyltrimethylammonium bromide (C14TAB) on the surface of water in the presence of benzoate ions have been obtained by infrared–visible sum-frequency spectroscopy (SFS) with three different polarization combinations. The sum-frequency spectra of the benzoate ions arise solely from those ions bound to the monolayer and not from ions in the diffuse double layer or in the bulk solution. From an analysis of the intensity of the ν2 C—H stretching mode, values for the second-order nonlinear susceptibility are derived. Interpretation of these susceptibilities in terms of the molecular hyperpolarizability and the orientational distribution of the benzoate anions demonstrates that the ions are oriented nearly perpendicular to the air–water interface, with an upper limit of 28° for the mean tilt of the C2 axis from the surface normal.

Investigation of Surfactant Conformation and Order at the Liquid−Liquid Interface by Total Internal Reflection Sum-Frequency Vibrational Spectroscopy

The conformational order of sodium dodecyl sulfate (SDS) adsorbed at the D2O-CCl4 interface has been examined by total internal reflection sum-frequency vibrational spectroscopy. A change in conformation of the alkyl chain with increased surface coverage at the liquid−liquid interface is observed. A series of aqueous surfactant concentrations have been examined in order to determine the effect of surface coverage on the conformation of the alkyl chains at the interface. Polarization studies indicate that, for the concentration range examined, the symmetry axis of the terminal methyl group on the alkyl chain is oriented primarily along the surface normal. Identification of spectral features in the C−H region of the infrared region is facilitated by examination of the sum-frequency spectrum from an analogous deuterated compound.

Sum-frequency vibrational spectroscopy of soluble surfactants at the air/water interface

Vibrational spectra of seven surfactants adsorbed at the surface of aqueous solutions have been obtained by IR–VIS sum-frequency generation. From these spectra the degree of conformational disorder and the angle of the terminal methyl group are inferred. In general, the number of gauche conformations increases as the area per chain increases. The angle of the methyl group, which is an indicator of the tilt of the hydrocarbon chains, is not simply related to the area per chain. Comparison of surfactants with the same chain length and area per molecule shows that the structure of the chain region of the monolayer is sensitive to the nature of the head group and not just to the packing density. Quantitative models to explain peak intensities in sum-frequency spectra of surfactants are discussed critically.

A new one-dimensional correlation spectroscopy (1D COSY) for electron spin echo envelope modulation studies

A new one-dimensional correlation spectroscopy (COSY) method for electron spin echo envelope modulation (ESEEM) studies is developed based on introducing a general signal-acquisition scheme to detect both sum-frequency and difference-frequency 1D ESEEM spectra. Using a single crystal of Cu/Ni(bu 2dtc) 2 at 21 K, it is shown that the correlated nuclear transitions can be easily determined. To compare with existing 2D ESEEM schemes, it is experimentally demonstrated that higher sensitivity and resolution can be easily achieved by using this new 1D ESEEM COSY method. A graphical approach to using this spectroscopic method is also described.

A sum-frequency study of the two-dimensional phase transition in a monolayer of undecanol on water

Sum-frequency (SF) vibrational spectra are presented of monolayers of 1-undecanol on water in the vicinity of the first-order phase transition at 28°C. A combined analysis of SF and ellipsometric data permits the calculation of the molecular tilt and area per molecule in the high-temperature phase. The SF spectra demonstrate that the monolayer remains highly conformationally ordered above the phase transition.

Polymer-Surfactant Aggregates at a Hydrophobic Surface Studied Using Sum-Frequency Vibrational Spectroscopy

The structure of polymer-surfactant aggregates adsorbed at a hydrophobic surface has been investigated using sum-frequency spectroscopy (SFS). The surface vibrational spectra of a cationic surfactant (tetradecyltrimethylammonium bromide) show that the surfactant molecules form an oriented monolayer at the surface, with the polymer electrostatically binding to the charged head groups of the surfactant at the interface with the aqueous phase. The polymer-surfactant interaction, however, does not appear to increase the maximum packing density of the surfactant layer above that of a monolayer of pure surfactant at its cmc. The general structural features of the surfactant layer remain unaffected by the presence of polymer, but steric and hydrophobic interactions between the polymer and surfactant influence the detailed adsorbate structure. The sum-frequency spectra indicate that low concentrations of negatively charged polymers cause the adsorption of surfactant at lower concentrations than from a solution of pure surfactant. The effects of pH, polymer concentration, and added electrolyte on the polymer-surfactant adsorbate have been examined. The SF spectrum of the poly(acrylic acid) and deuterated cationic surfactant adsorbate provides evidence that this polymer also adopts a preferential orientation at the surface under certain conditions.

Characterisation of the solid-water interface using sum-frequency spectroscopy

The nonlinear optical effect known as sum-frequency generation is used to generate vibrational spectra of molecules adsorbed at interfaces. The characteristics of sum-frequency spectroscopy (SFS) are described and the unique features that make SFS well-suited to the study of the solid-water interface are explained. Examples of sum-frequency spectra of organised molecular systems are presented, with particular emphasis on the effect of interactions with other species in solution. Future developments of relevance to biosensor research are briefly addressed.

Sum-frequency spectroscopy of physisorbed and chemisorbed molecules at liquid and solid surfaces using band-width-limited picosecond pulses

We studied the vibrational Sum-Frequency (SF) spectra of long chain fatty alcohols and amines physisorbed at liquid/air interfaces and of OctadecylTrichlorSilan (OTS) chemisorbed on glass/air interfaces in situ, i. e., in normal laboratory environment. The intense, band-width-limited IR pulses generated by our laser system are tunable from 2600 to 4000 cm−1 with a constant pulse duration of 3 ps and a band width of 5 cm−1 (FWHM) over the entire tuning range, thus covering the CH-, NH-, and OH-stretching regions.

Vibrational sum-frequency spectroscopy of an adsorbed monolayer of hexadecanol on water. Destructive interference of adjacent lines

The vibrational spectrum of the long chain fatty alcohol hexadecanol adsorbed at a water/air interface is studied by sumfrequency (SF) spectroscopy, observing three vibrations of the terminal methyl group. A theoretical analysis is presented that predicts opposite phases of adjacent vibrational modes leading to destructive interference in the SF spectrum. This novel interference effect is reported with enhanced spectral resolution and signal-to-noise ratio. Application of this effect for the determination of the line amplitudes of the degenerate stretching mode yields more precise information on molecular orientation. The tilt angle of the molecular chain with respect to the surface normal in a spontaneously formed monolayer has been determined to be 8.2° ± 1.8°.

IR - visible sum frequency mixing in C 2H 4/Pd(111)

The C 2H 4/Pd(111) adsorbate system was investigated with picosecond infrared-visible sum frequency mixing. Sum frequency spectra have been taken with different polarizations of the involved laser beams. Due to symmetry selection rules the observed resonances in the spectra are tentatively assigned to the following C-H stretch vibrations: ν 11 at ▪ = 2980 cm −1, ν 9 at ▪ = 2998 cm −1, and ν 1 at ▪ = 2991 cm −1.

Effect of substrate photoexcitation on the dynamics of the Si-H stretch for Si(111)/H

The effect of carriers on the dynamics of the Si-H stretch is investigated. The carriers are generated with 8 ps pulses at 532 nm and the sum-frequency spectrum of the Si-H stretch is then monitored. Modelization of the evolution of the carrier density and subsurface temperature provides a quantitative agreement with the variation of intensities and frequencies of the spectrum. For carrier densities above 0.9×10 20 cm −3, a slight reduction of the lifetime of the Si-H stretch is observed. This is analyzed with a classical image-dipole coupling resulting in an effective distance of 9 Å to the image plane. It also appears that the thermalization occurs faster than 10 ps. This is analyzed in terms of a short lifetime (<10 ps) for the 200 cm −1 surface optical phonon.

Identification of strained silicon layers at Si-SiO2 interfaces and clean Si surfaces by nonlinear optical spectroscopy.

Optical second-harmonic and sum-frequency spectra of clean and oxidized Si(100) and Si(111) samples reveal a strong resonance band at 3.3 eV photon energy. It is concluded that the resonance arises from direct transitions between valence and conduction band states in a few monolayers of strained silicon at the Si-${\mathrm{SiO}}_{2}$ interface and at the selvedge of clean reconstructed silicon surfaces.

Sum Frequency Spectra in the C-H Stretch Region of Adsorbates on Iron

Infrared-visible sum frequency generation (SFG) was used to record vibrational spectra in the C-H stretch region of single monolayer Langmuir-Blodgett films of stearic acid, octadecylamine, and p-octadecylphenyl acetic acid on iron oxide, demonstrating the sensitivity of SFG to about 0.1 monolayer. Within our present signal-to-noise limitations, it was not possible to obtain SFG spectra of the aromatic C-H stretch in p-octadecylphenyl acetic acid or the N-H stretch in octadecylamine. Vibrational spectra in the C-H stretch region have also been obtained for chemically adsorbed monolayers of stearic acid and oleic imidazoline on iron oxide; however an undetermined fraction of these spectra result from hydrocarbon contamination. Finally, an SFG spectrum was obtained at a buried interface, that of p-octadecylphenyl acetic acid on iron oxide under water. The potential of SFG as a surface analytical tool is discussed.

Femtosecond coherent transient infrared spectroscopy of CO on Cu(111)

Femtosecond infrared coherent transients have been measured for the stretch vibration of CO on Cu(111). The free induction decay exhibits a dephasing time (T2) of 2±0.3 ps (and 2±0.1 ps assuming a single exponential decay between 2 and 3 ps). The decay was best fit by exponential relaxation, thereby suggesting that the CO vibrational band is almost entirely homogeneously broadened. The surface sum frequency spectrum was also measured at two coverages (0.10 and 0.45 L) using spectrally narrowed pulses. Interferences were observed leading to a determination of the relative phase and amplitude of the resonant and nonresonant second-order susceptibility in this system. The magnitude of the nonresonant susceptibility was only weakly dependent on coverage, suggesting that the nonresonant polarizability originates in the bulk Cu. Time and frequency domain results were in good agreement.