A. Hatta

Infrared absorption enhancement of monolayer species on thin evaporated Ag films by use of a Kretschmann configuration: Evidence for two types of enhanced surface electric fields

Infrared absorption enhancement of m- and p-nitrobenzoic acid deposited on thin-evaporated silver films has been investigated using the Kretschmanns ATR coupling method. The absorption spectra provide direct evidence that enhancement is prominent only for vibrations of the first monolayer adsorbed on the Ag surface. It is shown that all of the vibrational modes observed obey the normal dipole selection rule. Moreover, it was found that there exist two types of absorption enhancement; the first is enhanced (∼300) by both p- and s-polarized radiation, and the second is enhanced (∼60) by p-polarized radiation alone. The Ag film thickness optimum is about 50 Å in the former case and about 200 Å in the latter. The enhancement insensitive to the polarization state of radiation can well be explained by the excitation of the transverse collective electron resonance of the Ag islands, whereas that obtained by p-polarized radiation may be due to the excitation of delocalized surface plasmons modified by surface roughness. The image-dipole effect may also be significant in the mechanisms.

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.

Infrared absorption study of adsorbed species at metal/water interface by use of the Kretschmann configuration

Abstract An infrared ATR technique using the Kretschmann configuration for exciting the surface electromagnetic field has been applied for the in-situ observation of adsorbed species at metal/aqueous solution interface. When an evaporated Ag film of about 10 nm in thickness is in contact with an acidic solution of mercaptobenzothiazole (MBT), the vibrational bands of a surface complex, Ag(I)MBT, are enhanced in intensity irrespective of the polarization state of the radiation. The polarization independence of the absorption enhancement can well be explained in terms of the excitation of transverse collective electron resonance associated with Ag islands. When a Ag film of 19 nm in thickness is used, absorption of the Ag(I)MBT complex is enhanced by p-polarized radiation only. The absorption intensity is maximum at an angle of incidence of 70° at which the background reflectivity is minimum. This enhancement involves the excitation of surface plasmon polaritons. Nearly the same results were obtained when Cu was used in the place of Ag.

In-situ infrared measurement of thiocyanate at a silver/electrolyte interface by the excitation of surface plasmon polaritons

Abstract The Kretschmanns ATR coupling method to excite surface plasmon polaritons on thin metal films has been applied to the infrared absorption measurement of thiocyanate adsorbed at an Ag electrode/aqueous electrolyte interface. The variation of the thiocyanate CN stretching vibration upon change in the electrode potential is described and discussed.

Mechanism of absorption enhancement in infrared ATR spectra observed in the Kretschmann configuration

Abstract The infrared absorption of species on a metal film is enhanced remarkably in the Kretschmann ATR configuration. The change in IR intensity with experimental condition is in qualitative agreement with the electromagnetic (EM) field within a composite layer calculated by assuming a prism/metal-medium composite/medium model. The model calculation, however, cannot account for the observed remarkable IR enhancement. Furthermore, the enhancement is restricted to the species present in the extreme vicinity of the metal surface. Taking these facts into account, the short-ranged enhancement is attributed to the local strong EM field at the bumpy metal surface.

In situ photoluminescence spectral study of porous Si in HF aqueous solution

Time‐dependent change in the visible luminescence of porous Si immersed in HF aqueous solution was measured in a short acquisition time (5 s) with a charge coupled device detector. Under Ar+ laser light (514.5 nm) irradiation, the photoluminescence band of the porous Si shifted to shorter wavelength accompanying a reduction of the peak intensity. The result suggests that the size of the Si nanostructure is closely related to the gap energy of the porous Si.

Application of Infrared ATR Spectroscopy to Liquid Crystals. V. Orientation of 5CB Molecules in the Boundary Region of a Twisted Nematic Cell and Reorientation by a DC Electric Field

Abstract Infrared ATR measurements have been performed on a 90[ddot] twisted nematic 4-n-pentyl-4′-cyanobiphenyl (5CB) under an externally applied DC electric field. Analysis of polarized reflection absorbances for the C=N stretching band at 2225 cm-1 allows the characterization of the anisotropic structure of a boundary layer about 3000 A thick at the cell electrode surface. For instance, anisotropic absorption coefficients derived from the measured reflection absorbances at zero electric field confirm that molecular alignment at the boundary is substantially uniaxial. On the other hand, it is found from changes in the absorption coefficients with applied electric field that the threshold voltage for prodwing field-induced molecular reorientation is 5 V. The average direction of the cyano dipole with respect to the field direction is also obtained as a function of applied field strength.

Infrared absorption of polycyanoacrylate enhanced by Ag island films in the Kretschmann's ATR geometry: The coverage dependence

Abstract An infrared absorption enhancement is reported for films of polycyanoacrylate deposited on evaporated silver films of approximately 50 A in thickness in the Kretschmanns ATR geometry. The comparison between absorption intensities measured with and without the Ag underlayer film demonstrates that the absorption intensity is enhanced by the presence of the Ag film by roughly a factor of 10. The enhancement saturates at a distance of between 30 and 50 A from the Ag surface.

Change in luminescence properties of porous Si by F2 and D2O exposure: In situ photoluminescence, Raman, and Fourier-transform infrared spectral study

In situ photoluminescence (PL), Raman, and infrared (IR) spectra of porous Si (PS) during F2/D2O exposures were investigated. F2 exposure at 298 K resulted in a peak shift of PL band from 750 to 670 nm with an intensity reduction. IR spectra revealed that the surface hydrogenated Si of the PS was displaced by fluorinated one. By subsequent D2O exposure, the PL band further shifted to a shorter wavelength with a significant intensity increase: IR bands due to surface oxides as well as SiD and SiOD bonds were observed after the exposure. On the contrary, the average size of the Si crystallites in the PS evaluated from Raman spectra remained almost unchanged throughout the exposures. These results suggest that surface chemistry plays a crucial role in the PL of the PS.

Carbon monoxide adsorption on ultra-thin Fe film deposited on Cu(100)

This paper is mainly concerned with vibrational properties of CO adsorbed on fcc Fe(100) observed using IR reflection absorption spectroscopy. An epitaxial Fe film of eight monolayers has been grown at 300 K on Cu(100). At 0.1 L exposure only a single C-O stretch band appears at 1920 cm -1 which shifts to 1977 cm -1 at 0.55 L. This band further shifts to 1998 cm -1 and saturates in intensity at 0.88 L. In contrast, at 0.25 L a second C-O stretch band appears at 2020 cm - 1 which increases in intensity and shifts to 2048 cm -1 at 1.2 L. The initially observed band can be ascribed to the bridging CO and the second band to the on-top CO. Although the bridging CO was stable up to ca. 380 K, the band due to the on-top CO was less stable and disappeared above 313 K. The observed CO adsorption/desorption behavior on the epitaxial fcc Fe(100) surface is compared with that on a vacuum-evaporated bcc Fe polycrystalline film.