H. Seki

Vibrational Raman and infrared spectra of chromatographically separated C60 and C70 fullerene clusters

Abstract We report vibrational Raman and infrared spectra for chemically separated C 60 and C 70 fullerenes. Thin film samples were prepared by subliming the chromatographically separated species onto appropriate substrates. The C 60 Raman spectrum shows eight clear lines and two weaker ones. If C 60 in fact that has the proposed buckminsterfullerene structure (as is strongly indicated by recent experiments), the present Raman measurements together with the four observed IR frequencies give a complete set of Raman and infrared active fundamental frequencies for this molecule. A comparison of this set with the calculated spectrum for buckminsterfullerene shows satisfactory agreement.

Electrode/electrolyte interphase study using polarization modulated ftir reflection-absorption spectroscopy

Abstract Polarization modulated Fourier transform infrared reflection-absorption spectroscopy (FT-IRRAS) is applied to the studies of adsorption and oxidation of CO on a platinum electrode in 0.5 M sulfuric acid and of adsorption of cyanide on gold and silver electrodes in 0.5 M potassium sulfate. The absorption intensity of the CO on platinum electrode is ~ 4–5% while that of the CN − on silver and gold is 0.2–0.5%. The potential dependence of the vibrational spectra was observed for both systems. Oxidation of the linearly adsorbed CO layer proceeds by different mechanisms depending on whether CO was adsorbed at a potential in the double-layer region or in the hydrogen region, i.e. at the edges of the CO islands in the former case and randomly in the latter case, in which the bridged CO species plays an important role. The vibrational frequency of the linearly adsorbed CO changes linearly with potential at a rate of 30 cm −1 /volt, which is independent of anion specific adsorption. The origin of the shift is most reasonably explained by the first-order Stark effect. For Ag/CN − and Au/CN − systems, the surface cyanide species is assigned as linearly adsorbed CN . The anodic reaction products in the solution from cyanide ions and the electrode metals are also observed in the vibrational spectra. The bands due to surface species and those due to solution species are distinguished by measuring the spectra with s- and p-polariz.ed light.

A Thin-Layer Electrochemical Cell for Infrared Spectroscopic Measurements of the Electrode/Electrolyte Interface:

A thin-layer electrochemical cell which allows for the convenient measurement of infrared reflection-absorption spectra (IRRAS) of the electrode/electrolyte interphase has been designed and built. In order to minimize the thickness of the electrolyte in front of the electrode surface, one mounts the working electrode on the end of a piston so that it can be pressed against an infrared window. Infrared radiation is then reflected from the electrode surface via the infrared window without experiencing severe attenuation due to the electrolyte. Contaminants are minimized by the machining of both the cell body and piston from an inert fluorocarbon plastic. Calcium fluoride was chosen as the window material because of its transparency, its low refractive index, and its low solubility in aqueous electrolyte. The use of this cell in a Fourier transform infrared reflection-absorption spectrometer utilizing polarization modulation is described. The IRRAS spectra obtained from this cell have been analyzed with the use of the generalized Fresnel equations for stratified media. Some of the computational results for the water/silver electrode interphase are used to interpret the measured spectra of water near the electrode. The discussion includes infrared intensity distributions in aqueous media and their influence on the IRRAS measurement.

In-situ FT-IR spectroscopic study of bisulfate and sulfate adsorption on platinum electrodes: Part 1. Sulfuric acid

Abstract The adsorption of bisulfate and sulfate ions on Pt in sulfuric acid has been studied by observing their asymmetric S-O stretching vibrations using potential difference Fourier transform infrared reflection absorption spectroscopy. It has been found that the two anions are coadsorbed over the potential range studied. A rather clear band of the bending mode of water was observed in the same spectra. This has been attributed to water molecules weakly bonded to the surface through H in the hydrogen region and O in the oxygen region. The band intensity ratio of sulfate against bisulfate ions is much larger than the calculated activity ratio in 0.5 M sulfuric acid solution, although the intensity ratio becomes closer to the calculated activity ratio in 0.05 M sulfuric acid. A large frequency shift with potential of 100 cm−1/V is observed for the adsorbed bisulfate ions in the double layer potential region, while the frequency shift is much smaller for sulfate ions. In the oxide formation region above 0.85 V (RHE), the sulfate ions appear to be replaced by bisulfate ions; this is interpreted as resulting from the formation of a hydrogen bond between HSO−4 and the negatively charged oxygen atoms which are present on the Pt surface.

Infrared photodesorption: Vibrational excitation and energy transfer processes on surfaces

Abstract Photon-stimulated desorption due to internal vibrational excitation of adsorbed molecules is a resonant phenomenon and has recently attracted considerable experimental and theoretical attention. n this paper, a brief review of prior studies is given. New results on NH3, ND3 and Xe in neatly adsorbed and co-adsorbed states excited by a tunable infrared laser and photodesorbed from Cu(100), NaCl and Ag films are presented to elucidate desorption mechanisms. Both single-photon and multiphoton excitation processes are examined and the velocity distributions of desorbed particles are analyzed. Various energy transfer and relaxation processes related to desorption are evaluated and a model for infrared photodesorption with particular emphasis on assessing the quantum and thermally-assisted effects is also discussed.

Atomic scale friction of a diamond tip on diamond (100) and (111) surfaces

The friction of a clean diamond tip on diamond (111) and (100) surfaces is studied using an ultrahigh vacuum force microscope that simultaneously measures forces parallel and perpendicular to the surface. The 30 nm radius diamond tip is fabricated by chemical vapor deposition. The attractive normal force curve between the tip and surface agrees well with calculated dispersion interactions. The frictional force exhibits periodic features, which on the (100) surface are tentatively associated with a 2×1 reconstructed surface convoluted over an asymmetric tip shape. The (111) surface shows features that cannot be simply related to the surface structure. As the tip is scanned back and forth along a line, the same features are observed in each direction, but offset, suggesting the presence of a conservative force independent of the direction of motion as well as a nonconservative force. The friction is approximately ≂3×10−9 N independent of loads up to 1×10−7 N.

The role of cavity sites in surface-enhanced Raman scattering

Abstract Thermal desorption spectra (TDS) of pyridine from silver films deposited in ultra high vacuum are reported. Marked differences in the TDS are seen depending on the deposition conditions and the thermal history of the films, which have been correlated with surface-enhanced Raman scattering (SERS). These results as well as some of the observations in electrochemical systems are discussed in light of the recent Xe probe analysis carried out by Albano et al.

Electromagnetic effect in enhanced infrared absorption of adsorbed molecules on thin metal films

Abstract The electric field strength within a homogeneous metal layer on a Ge hemi-cylindrical prism in the Kretschmann configuration has been calculated for infrared light using the Fresnel formula. To approximate the non-homogeneous island-like or porous nature of very thin metal films the dielectric constant of the metal layer was derived by the Bruggeman effective medium approximation. The dependence of the calculated intensity on the film thickness and the angle of incidence is in good agreement with the experimental observations of enhanced infrared absorption of adsorbed species. Combined with the observed short range effect of the enhancement this implies that the enhancement is due to actual high electric fields within the pores or between islands of the metal film which are proportional to the calculated effective field.

Polarization-modulated FT IR spectra of cyanide adsorbed on a silver electrode

Abstract Polarization-modulated Fourier transforms IR reflection-absorption spectroscopy (FT IRRAS) has been used to measure the IR spectra of cyanide species at the silver electrode-electrolyte interface. These spectra show absorption bands due to both surface and solution species and are presented as a function of the electrode potential.

Raman spectra of molecules on metal surfaces

Abstract Several studies of the Raman spectra of organic molecules on metal surfaces are described; included among these are: the enhancement of Raman signals by direct excitation of plasmon surface polaritons on holographic metal gratings and the wavelength dependence of the breathing mode vibrations of pyridine adsorbed on an anodized Ag electrode.