Specular Reflectivity Studies Research Articles

X-ray specular reflection studies on multilayer films of a ferroelectric liquid crystalline polymer

Abstract Results of our x-ray specular reflectivity studies on multilayer films of a ferroelectric liquid-crystal polymer are presented. The reflectivity curves are characterized by interference fringes (Kiessig fringes) that occur between successive Bragg peaks. Two types of intensity variations of the Kiessig fringes are observed a uniform variation and a modulated variation. The latter is shown to be due to a single-layer defect which percolates throughout the multilayer film. The electron density profiles are determined from Fourier analysis of the reflectivity.

IR specular reflectance studies on rough-surfaced solids and on layered systems with nonuniform interfaces: Optical simulations

Infrared specular reflection of strongly absorbing rough-surfaced samples was studied. An optical model was established which includes gradient layers on the interfaces. Damping of band intensity and absorption-like features within the reststrahlen region can be assigned to the excitation of surface electromagnetic waves. Specular reflectance spectroscopy is well suited to obtain optical data (dielectric function) of strongly absorbing samples, even with rough surfaces.

X-ray-reflectivity study of the copper-water interface in a transmission geometry under in situ electrochemical control.

Using an electrochemical cell that permits x-ray-scattering studies in a transmission geometry under {ital in} {ital situ} electrochemical control, we performed an x-ray specular-reflectivity study of the copper-water interface as a function of electrochemical potential. It is found that the reflectivity changes upon cyclic oxidation and reduction, indicating a phase transition between copper and copper oxide at the interface. We also find that the thickness of the pure copper and the roughness of the interfaces exhibit electrochemical irreversibility that is consistent with the potentials for the oxidation waves in the anodic cycle and the reduction waves in the cathodic cycle. A standard Fresnel expression for the x-ray specular reflectivity was applied in the data analysis. A smoothly varying Lorentzian interface profile was used for the individual rough interfaces. An incoherent average was additionally used to include the effect of {ital correlated} {ital roughness} between a pair of interfaces. The results of our data-fitting procedure are discussed in relation to the electrochemical information obtained in the corresponding cyclic {ital I}-{ital V} curves.

X-ray specular reflection studies of silicon coated by organic monolayers (alkylsiloxanes).

X-ray specular reflectivity has been used to characterize the structure of silicon--silicon-oxide surfaces coated with chemisorbed hydrocarbon monolayer films (alkylsiloxanes). Using synchrotron radiation the reflectivity was followed over 9 orders of magnitude, from grazing incidence to an incident angle of \ensuremath{\varphi}\ensuremath{\approxeq}6.5\ifmmode^\circ\else\textdegree\fi{}, or q=(4\ensuremath{\pi}/\ensuremath{\lambda})sin(\ensuremath{\varphi})=0.8 A${\r{}}^{\mathrm{\ensuremath{-}}1}$, allowing a spatial resolution of features approximately \ensuremath{\pi}/0.8\ensuremath{\approxeq}4.0 A\r{} along the surface normal. Analysis was performed by fitting the data to reflectivities calculated from models of the surface electron density and by calculating Patterson functions directly from the data. Although the measured reflectivities could be equally well described by different sets of model parameters, the electron densities calculated from these different parameters were remarkably alike. Inspection of the electron densities allowed identification of a layer of ${\mathrm{SiO}}_{2}$ (\ensuremath{\approxeq}17-A\r{} thick), a layer of head-group region where the alkylsiloxane adsorbs to the ${\mathrm{SiO}}_{2}$, and the hydrocarbon layer. Fitting the data also required that the various interfaces have different widths. The fact that the same local hydrocarbon density of 0.85 g/${\mathrm{cm}}^{3}$ was observed for both fully formed and partially formed monolayers with alkane chains of varying length excluded a model in which the partially formed monolayer was made up of separated islands of well-formed monolayers. Measurements before and after chemical reaction of a monolayer in which the alkyl chain was terminated by an olefinic group demonstrated the ability to use x-ray reflectivity to characterize chemical changes. The effects of radiation damage on these types of measurements are described.

Ellipsometric and X-ray specular reflection studies on naturally grown overlayers on aluminium thin films

The results of detailed investigations on the natural surface layer formed at room temperature on aluminium films exposed to air are presented. Aluminium films of high perfection, deposited onto very smooth glass substrates, have been studied over a 2 year period using ellipsometry. Soft X-ray specular reflection analysis revealed a composite surface layer structure composed by a thin ( d 2 = 0.8 nm) very compact alumina layer in contact with the aluminium substrate and by a thick ( d 3 = 3 nm) hydrated oxide layer. A new computer procedure was applied for this composite layer system, which evaluated 72 ellipsometric experimental data and achieved a best fit of the measured and calculated Ψ and Δ values. The resultant optical constants of the aluminium substrate were n = 1.09, 0.95, 0.535 and 0.370 for λ = 579 nm, 546 nm, 436 nm and 365 nm respectively, whereas k = 6.72, 6.40, 4.96 and 4.23 respectively for the same mercury lines. Among widely scattered data from the literature, these are in good agreement with results of Hass on the assumption of a similar surface layer structure, using n 2 = 1.77 (alumina) and n 3 = 1.58 (hydrated oxide). Our optical constants for aluminium were applied for evaluating ellipsometric experimental data obtained during the 2 year period. A slight systematic change in the d 2 and d 3 values of the samples was found, owing to hydration.

Specular reflectance studies of bromide adsorption on gold

Specular reflectance studies of bromide adsorption on gold

Specular reflectance studies of bromide adsorption on gold

Specular reflectance changes have been used to examine the specific adsorption of bromide on gold in the presence of a large excess of supporting electrolyte (NaF) which is not specifically adsorbed. A linear relation has been demonstrated between the reflectance changes and the surface excess of bromide through the examination of the time dependence of the reflectance under conditions where the rate of adsorption of the bromide is diffusion controlled and hence known. The adsorption isotherms have been found to follow Temkin behavior. The electrosorption valency has been evaluated from the charge and surface excess at constant potential and found to be −0.49 to −0.59, depending on the potential. Various mechanisms for the subtantial changes in reflectance attending the specific adsorption of anions are discussed. The observed effects cannot be explained on the basis of changes in the charge on the electrode and corresponding changes in the contribution of the conduction band to the surface optical properties. The principal mechanism is proposed to be modifications in the surface electronic states of the metal electrode through direct orbital interactions between the adsorbed anions and the metal.

Specular reflectivity studies of the adsorption and the oxidation of methanol on palladium in an alkaline solution

Adsorption and anodic oxidation of methanol on palladium electrode in 0·5 N NaOH was studied by measuring current—potential ( i-E) and reflectivity—potential ( R/R 0- E) curves. Presence of methanol retarded the formation of the oxide layer of Pd, giving rise to an anodic shift of the potential. A linear relation was found between the potential retarded and the logarithm of methanol concentration, the slope being about 30 mV/decade. The form of adsorbed entities was discussed based on the slope. The cathodic charge consumed by the reduction of Pd-oxide was smaller in the presence of methanol than in the absence. This was explained by three causes; the less amount of oxide layer formed during the anodic sweep owing to the adsorption of oxidation product of methanol, the reduction of formed oxide layer by a chemical reaction with methanol, and a compensation effect due to the anodic oxidation of methanol during the cathodic sweep. Plottings of the total charge in the anodic region vs — Δ R/ R 0 in the absence and the presence of methanol suggested that electro-oxidation of methanol proceeds only at the bare portion of Pd.

Complex plane analysis in electrode modulated specular reflectance studies

Complex plane analysis in electrode modulated specular reflectance studies

On Causes of Error in Multiple Specular Reflection Studies

not Available.

Specular reflectivity studies of the adsorption of halide anions on gold electrodes in 0.2 M HClO 4

The specular reflectivity of gold has been measured in 0.2 M HClO4 with and without halide ions present. The reflectivity potential curves depend on the halide concentration at potentials where halide ions are adsorbed. Increasing halide ion concentration decreases the reflectivity with a limiting value approached at higher halide ion concentrations. The decrease in reflectivity produced by anion adsorption also is potential dependent and approaches a limiting value at more anodic potentials. Hysteresis is evident in the reflectivity-potential curves recorded with cyclic linear potential sweeps when the sweep rates and halide ion concentrations are such that diffusion of halide ions to and from the electrode is expected to be rate controlling. Potential-step techniques and simultaneous reflectivity measurements have been used to demonstrate diffusion control of the adsorption kinetics at low halide concentrations.

Specular reflectivity studies of the adsorption of halide anions on gold electrodes in 0.2. M HClO4

The specular reflectivity of gold has been measured in 0.2 M HClO4 with and without halide ions present. The reflectivity potential curves depend on the halide concentration at potentials where halide ions are adsorbed. Increasing halide ion concentration decreases the reflectivity with a limiting value approached at higher halide ion concentrations. The decrease in reflectivity produced by anion adsorption also is potential dependent and approaches a limiting value at more anodic potentials. Hysteresis is evident in the reflectivity-potential curves recorded with cyclic linear potential sweeps when the sweep rates and halide ion concentrations are such that diffusion of halide ions to and from the electrode is expected to be rate controlling. Potential-step techniques and simultaneous reflectivity measurements have been used to demonstrate diffusion control of the adsorption kinetics at low halide concentrations. The factors involved in the change of the reflectivity of the gold-electrolyte solution interface with potential and anion adsorption are discussed. The change in the surface electron charge density appears to be particularly important. The reflectivity and voltammetry data have been used to determine the “adsorption potential” for the I−, Br−, and Cl− anions, (corresponding to the potential of the voltammetry peak or the maximum in the slope of the reflectivity-potential curve). These adsorption potentials have been found to fit a linear dependence on the third power of the ionic radius reasonably well as is to be expected if the principal factor responsible for the difference in adsorptivity of these three ions is their polarizability.

The Crystal Structure of Auramine Perchlorate: Correlation of the Results with Specular Reflection Spectroscopy

The x-ray analysis of auramine perchlorate has been performed in order to establish the orientation of the cationic chromophoric group in the crystal for comparison with the orientation that was proposed on the basis of specular reflection studies. The compound crystallizes with two molecules per unit cell in space group P1 and cell parameters a = 8.86 A, b = 9.69 A, c = 11.27 A, alpha = 97 degrees 40', beta = 99 degrees 34', and gamma = 106 degrees 16'. The orientation of the chromophore is in good agreement with the crystal model. Other features of the structure are the hydrogen bonding scheme of the perchlorate ion, the highly distorted chromophoric group, and the planar dimethylamine substituents.

Single Mirror Normal Incidence Reflectometer

An inexpensive single mirror reflectometer for specular reflectivity studies at nearly normal incidence is described. This design minimizes errors in the reflectivity which may occur as a result of spatial variations in both the detector sensitivity and the mirror reflectivity. The reflectometer has been successfully used for reflectivity measurements from 2 to 22 eV in a detector-sample chamber which is coupled to a McPherson vacuum ultraviolet monochromator.