Specular Angle Research Articles

Adjustable Aperture Stop to Control the Divergence and Orientation of the Beam Emerging from a Vacuum Ultraviolet Monochromator

An adjustable aperture stop for use in controlling the direction and angular divergence of the beam of radiant flux emerging from a MacPherson 225 VUV monochromator is described. Such a stop facilitates measurements of specular reflectance vs angle of incidence and measurements of diffraction grating efficiency. A discussion of the degree of control exercised by the aperture over the beam cross section is given that is applicable to any monochromator fitted with an aperture stop. For this application, the distance from the grating to the aperture stop is larger than the grating focal length. The result is an imaging effect that provides sharper definition of the beam cross section when the aperture stop is between the entrance slit and grating than if the stop is between the grating and exit slit. This sharp definition persists up to distances of several meters beyond the exit slit and is useful when measuring the efficiency of concave diffraction gratings. A fixed direction of the beam from the MacPherson i strument can only be obtained with the aperture between the grating and exit slit which results in some loss of definition of the beam cross section. However, as long as the pathlength beyond the exit slit is restricted to a few tens of cm, the definition is sufficiently good for measurements of reflectance vs angle of incidence.

Variable Energy Experiments on Atomic Beam Scattering from a NaF (001) Surface

Velocity selected hydrogen and deuterium atoms with kinetic energies from 20 meV to 140 meV have been scattered and diffracted from a NaF (001) cleavage plane. Highly resolved minima of selective adsorption yield binding energies and the gas-atom-surface interaction potential perpendicular to the surface. Specular intensity versus incident energy curves and specular intensity versus angle of incidence curves for different surface temperatures are compared to available theories. From these measurements the Debye-Waller-factor is extracted, which depends on the angle of atomic incidence and is characterised by a surface Debye temperature of θD⊥=370 K and the depth of the attractive potential D0=17.9 meV. The strength of the (first order) periodic part of the potential is found to be D1=0.025 D0.

Field emission microscopy of A IB V coated tungsten monocrystals : V G Kanev et al, Proc 5th Czech Conf Electron Vacuum Phys, Czech Acad Sci, Brno 1972, III b-3

Dissociation and inelastic scattering of ethanol molecular ions from self-assembled monolayer (SAM) surface of fluorinated alkyl thiol on an Au (1 1 1) monocrystal have been studied at 28.9 and 52.9 eV collision energies. A single dynamics mechanism for quasi-inelastic scattering was found at both energies. Ions recoil nearly parallel to the surface with very small kinetic energy losses of the order of ≤2 eV. Dissociation dynamics features for the main dissociation channel, loss of methyl radical, are dramatically different from that of inelastically scattered primary ions and are different at the two collision energies studied. At 28.9 eV two energetically and angularly resolved features are observed, one corresponding to the loss of very large amounts (nearly all) of ion’s translational energy and the other appearing to gain energy (superelastic scattering). This dynamics feature is interpreted as delayed dissociation of ions transmitted through the energy analyzer as molecular ions. This implies a lifetime of such excited ions of more than 5 μs. The same dynamics features are observed at 52.9 eV ion energy except that a second inelastic process begins to compete with the nearly fully inelastic process. Moreover, at this energy the delayed ion dissociation mechanism is the dominant mechanism. The hypothesis that collision of ethanol cations with the SAM surface initially involves collision of the ion with a single end group of the SAM polymer chain provides a useful rationale for the observed dynamics. Support for this hypothesis is provided by Newton diagrams, which summarize momentum conservation relationships in terms of a common center-of-mass, CMeff, which provides a basis for describing scattering mechanisms for this system. Observation of three energetically distinct scattering processes suggests uniquely different ion–surface interactions contributing to surface-induced dissociation of ethanol ions. Preliminary experiments with Ar+ scattered from the same surface exhibit very similar dynamics features to that observed for ethanol cations. Finally, we note that intensities of scattered primary or fragment ions never approach the specular angle at the energies investigated here.

Influence of radiation defects of crystal lattice produced by ion irradiation on angular and energy distribution of scattered ions: U A Arifov et al, Radiotekh Elektron, 17 (2), Feb 1972, 359–365 ( in Russian)

Dissociation and inelastic scattering of ethanol molecular ions from self-assembled monolayer (SAM) surface of fluorinated alkyl thiol on an Au (1 1 1) monocrystal have been studied at 28.9 and 52.9 eV collision energies. A single dynamics mechanism for quasi-inelastic scattering was found at both energies. Ions recoil nearly parallel to the surface with very small kinetic energy losses of the order of ≤2 eV. Dissociation dynamics features for the main dissociation channel, loss of methyl radical, are dramatically different from that of inelastically scattered primary ions and are different at the two collision energies studied. At 28.9 eV two energetically and angularly resolved features are observed, one corresponding to the loss of very large amounts (nearly all) of ion’s translational energy and the other appearing to gain energy (superelastic scattering). This dynamics feature is interpreted as delayed dissociation of ions transmitted through the energy analyzer as molecular ions. This implies a lifetime of such excited ions of more than 5 μs. The same dynamics features are observed at 52.9 eV ion energy except that a second inelastic process begins to compete with the nearly fully inelastic process. Moreover, at this energy the delayed ion dissociation mechanism is the dominant mechanism. The hypothesis that collision of ethanol cations with the SAM surface initially involves collision of the ion with a single end group of the SAM polymer chain provides a useful rationale for the observed dynamics. Support for this hypothesis is provided by Newton diagrams, which summarize momentum conservation relationships in terms of a common center-of-mass, CMeff, which provides a basis for describing scattering mechanisms for this system. Observation of three energetically distinct scattering processes suggests uniquely different ion–surface interactions contributing to surface-induced dissociation of ethanol ions. Preliminary experiments with Ar+ scattered from the same surface exhibit very similar dynamics features to that observed for ethanol cations. Finally, we note that intensities of scattered primary or fragment ions never approach the specular angle at the energies investigated here.

Scattering of Diatomic and Polyatomic Molecules from the (100) Crystal Face of Platinum

The angular distributions of thermal energy molecular beams of diatomic (CO, N2, O2, NO, H2, D2) and polyatomic (CO2, N2O, C2H2, NO2, NH3, and methylene cyclobutane) molecules were monitored following scattering from the (100) crystal face of platinum. All of the gases studied (with the exception of ammonia) yielded directed scattering patterns with the maximum intensity peaked at or near the specular angle for scattering from a clean platinum surface or from a platinum surface covered by a layer of ordered graphite. Broad, cosinelike angular distributions which were independent of incident angle were observed, however, for the cases of C2H2 scattered from an adsorbed layer of C2H2 and CO scattered from adsorbed CO. These results seem to indicate that efficient energy transfer between the surface atoms and gas molecules, as indicated by angular distributions, occurs only in specific instances. Apparently, the incident particles are reflected whenever the nonlocalized vibrational energy modes of surface atoms and the translational energy of the incident molecules are of similar magnitude. The adsorption of gases may lead to the formation of additional low frequency localized surface modes that can be excited by the incident molecules. The efficient transfer of energy via localized surface modes would increase the residence time of the incident molecules and could thus account for the cosinelike angular distributions observed for C2H2 and CO.

Effects of Surface Disorder, Various Surface Structures of Chemisorbed Gases and Carbon on Helium Atomic Beam Scattering from the (100) Surface of Platinum

Low-energy electron diffraction and helium atomic beam scattering have been combined to investigate on the (100) face of a platinum single crystal the effects of changing surface structure on the gas–solid interaction. The angular distribution of a helium beam scattered from the ordered and disordered crystal face has been monitored. The intensity of the beam scattered at the specular angle increases by nearly an order of magnitude upon atomic ordering of the metal surface. Ordered domains of graphitic carbon also yielded high intensity peaks in the specular direction. The magnitude of the intensity maxima for helium beams scattered from ordered surface structures of various chemisorbed gases (CO, C2H2) was sensitive to surface order but insensitive to the type of surface structure formed. For all the surface structures studied the scattered helium beam distribution was directed with the intensity maximum at the specular angle. The surface temperature dependence of the beam dispersion and intensity scattered at the specular angle was monitored in the temperature range 450–1300°K. Anomalous changes in the beam intensity could be associated with the adsorption of carbon monoxide on the crystal face.

Scattering of 0.05–5-eV Argon from the (111) Plane of Silver

The scattering of atomic beams of argon from the (111) plane of silver has been measured for incident energies in the range of 0.05–5 eV. Nearly monoenergetic beams were produced by the use of nozzle sources with pure argon and mixtures of argon with helium or hydrogen. Epitaxially grown crystal surfaces were obtained by vapor deposition of silver on mica. The observed scattering patterns were single lobes with the maxima lying near the specular angle at all energies. Results were similar to those of Smith, Saltsburg, and Palmer at energies below 0.3 eV with the maxima shifting toward the tangent to the surface with increasing energy. At higher energies the maxima were observed to shift back toward the normal as predicted by the theoretical calculations by Oman. Comparison of results with the simple hard- and soft-cube models for scattering indicates limitations of these models at hyperthermal energies.

Rotational transitions in the scattering of a beam of light diatomic molecules from a surface

The quantum mechanical theory of Jackson and Howarth for the interaction of diatomic molecules with solid surfaces is extended to allow the incident molecules to have a general orientation relative to the surface and to include an attractive potential well. The theory is used to calculate the outgoing translational and rotational states of a molecular beam of a light diatomic gas scattered from a solid surface. The analysis is applied specifically to the case of a beam of H2 incident with thermal energy on a cold surface. For this situation a separate scattering peak away from the specular angle and containing about 2 per cent of the incident molecules is predicted, consisting entirely of molecules in the same excited rotational energy state.

Reflection from a Periodic Dielectric Surface

The angular distribution of reflected radiation has been investigated using a model which considers both direct surface reflection and volume scattering of the refracted radiation with eventual emergence through the surface. The significant results are: (a) the off-specular peaks are composed almost entirely of locally specular radiation, (b) the magnitude of the off-specular peaks depends almost entirely on the index-of-refraction ratio, and (c) the angular position of the off-specular peak, relative to the specular angle, depends upon the radius of curvature of the surface near its peaks.

Off-Specular Peaks in the Directional Distribution of Reflected Thermal Radiation

Experiments have been performed which demonstrate that the directional distribution of reflected thermal radiation for moderate to large angles of incidence is not intermediate to the specular and diffuse limits as commonly assumed. Indeed, for such angles of incidence on roughened surfaces, maxima in the reflected intensity distribution occur at polar angles larger than the specular angle. In some cases, the intensity of this off-specular maximum is three or four times the intensity in the specular-ray direction. The materials studied were aluminum, nickel, copper, a nickel-copper alloy, and magnesium-oxide ceramic, thereby encompassing both metals and nonmetals. Controlled surface roughnesses of 0.23 to 5.8 microns were employed over a wavelength range extending from 0.5 to 6 microns. The angle of incidence was varied from 10 to 75 deg, while reflected radiation was collected in the plane of incidence at polar angles ranging from 0 to 89 deg. The experimental results are arranged on the basis of a surface-roughness-to-wavelength ratio, σm/λ. Certain trends of the off-specular peak with incident angle and σm/λ are noted.

Diffuse Reflection from a Plane Surface*

The angular dependence of the intensity of radiation scattered from plane diffuse surfaces when these surfaces are illuminated by wideband collimated light has been investigated. The angle between the incident and scattered radiation (scatter angle) was held constant, and the surface was rotated so that the normal to the surface remained in the plane defined by this angle. For constant scatter angle, measured intensities exhibit a systematic variation from those predicted by the use of Lambert’s law of diffuse reflection. Peak deviations of the order of 10% were found in the case of an aluminum oxide pressed powder surface. An empirical expression of diffuse reflection, based on a shape factor proposed by Hill, is introduced and found to agree with the experimental data to within 2%. As the scatter angle is varied, the maximum intensity of scattered radiation observed at the specular angle shows poor agreement with that predicted by the classical Fresnel Law of reflection from a plane surface.

Elementary Results for Scattering by Large Ellipsoids

Elementary high-frequency approximations for scattering by an arbitrary body are specialized to ellipsoids and applied in detail for source and observation points at distances large compared to the scatterer's size. It is shown, for example, that, if the direction of observation is varied in a fixed plane containing the direction of incidence, then except for the near-forward direction (“shadow region”) the first approximation for the far-distant field for a given ellipsoid is specified by a “universal curve.” In particular, if the fixed plane contains an axis of the ellipsoid, then there is a unique curve for the intensity versus the angle measured from the reflection of the direction of incidence in the contained axis. For the special case of the spheroid, the field on the “axially specular” cone (the cone defined by the angle of incidence with the axis of rotation) remains constant until one gets near the shadow region, and the fields on adjacent cones (i.e., with generators at other than the specular angle with the axis) vary slowly with the azimuthal angle; for many practical purposes, we may neglect the slowly varying effects and approximate the reflected intensity pattern in the vicinity of the axially specular cone by a conical lobe of revolution. Numerical examples are given for a family of spheroids; e.g., we determine contours of constant intensity on a plane containing the source as functions of the spheroid's distance from the plane, etc.

Atomic-Beam Scattering from Epitaxially Grown Gold Films

The spatial distribution of beams of He and Ar scattered by gold-target films was examined mass spectrometrically during and after the formation of these films by evaporative deposition. By controlling the target temperature during the deposition of the metal, varying degrees of crystallographic orientation and surface smoothness were obtained. The metallic deposition rate was maintained high as compared with the impingement rate of background gas particles, resulting in approximate clean-surface conditions during deposition. For rough polycrystalline films, only cosine scattering of He and Ar was observed. For smooth, well-oriented films, specular scattering of He was observed during deposition. In this latter case, a gradual time-dependent dispersion in He scattering was seen after deposition and is shown to result from the formation of adsorbed gas layers. For Ar scattered from a smooth gold film during deposition, a broad directed beam, displaced from the specular angle, is obtained, and very little dispersion occurs after deposition. This latter result is interpreted as being due to the greater interaction between Ar and gold, compared with that between He and gold, which masks the effects of adsorbed gases upon Ar scattering.

Reflection of a Plane Acoustic Wave from a Surface of Nonuniform Impedance

A theoretical analysis is made of the reflection of a plane wave of sound from a surface whose acoustic impedance is not uniform. The sound pressure received at a distant point consists of a specularly reflected wave plus scattered radiation. The magnitude of the specularly reflected wave is independent of wavelength and depends upon the impedance of the portion of the surface which may be viewed at the specular angle from the receiver. If the impedance differs only slightly from a constant value the directivity pattern of the scattered radiation is determined by the product of three factors. The condition for the scattered radiation to obey Lambert's cosine law is that the reflecting surface is one of approximately zero pressure and that there is no correlation in the impedance irregularities. The condition for perfectly diffuse scattered radiation is that the surface behave as approximately rigid with no correlation in the impedance irregularities. In the case of a surface whose impedance irregularities have a slow statistical variation with position the scattered radiation is confined to a beam whose axis lies along the direction of specular reflection but the energy within the beam is not necessarily closely confined to the specular angle.

On the Nonspecular Reflection of Electromagnetic Waves

The nonspecular reflection of plane electromagnetic waves of arbitrary polarization by certain perfectly conducting surfaces composed of either semicylindrical or hemispherical bosses on an infinite plane is analyzed. Solutions for the problem of the single boss on an infinite plane and a plane wave at an arbitrary angle of incidence are given and extended subject to the single-scattering hypothesis to obtain the far field solutions for certain small finite patterned distributions and both small finite and infinite uniform random distributions of bosses small compared with the wavelength. The results for the various cases are then compared in the plane of incidence and similarities between the analogous expressions for the distributions of semi-cylinders and hemispheres are noted. Expressions are obtained for the ratios of the components of the reflected intensity and radial energy flux polarized parallel and perpendicular to the plane of incidence as well as for the total intensity and radial energy flux for the case where the incident wave is unpolarized. It is found that for certain values of the parameters the reflected radiation may consist only of either the specular or the scattered contributions, while for other values one of the scattered contributions, either the parallel or perpendicular component, may vanish. The results also indicate the occurrence of an extremum in the reflected radiation in the vicinity of the specular angle of reflection, which for certain ranges of the parameters for the small finite distributions may be a minimum rather than a maximum. For these cases there is also some critical angle of incidence (not necessarily π/2 or grazing incidence) for which the reflection at the specular angle is completely specular. The analogous distributions of cylinders and spheres are also considered.

On the Nonspecular Reflection of Sound from Planes with Absorbent Bosses

The nonspecular reflection of plane waves of sound from certain surfaces composed of absorbent bosses (semicylinders or hemispheres of arbitrary impedance) on an infinite plane of ∞ or 0 impedance is considered. Exact solutions are obtained for the problem of the single boss and then extended, subject to the single-scattering hypothesis, to obtain far field solutions for certain planar distributions of bosses of radii small compared with the wavelength. The results are compared with those obtained previously for non-absorbent bosses, and it is shown that the effects of the finite impedance are most pronounced in the simple source terms of the scattered components and may lead to either a decrease or an increase in the radiation reflected at the specular angle. Another effect of the finite impedance (for the small finite distributions) is to shift the critical value of the angle of incidence for which the reflection at the specular angle consists only of the specular component—below this value the reflection at the specular angle being a minimum and above it a maximum. For the infinite uniform random distributions it is found that the effect of the bosses is essentially but to change the impedance of the plane—these effective impedances being functions of the angle of incidence and the parameters and distribution of the bosses. The effect of the finite impedance of the bosses is most pronounced for these distributions yielding terms much lager than those previously retained for the nonabsorbent bosses. The results for the analogous distributions of cylinders and spheres are also given.

Gloss Measurements on Fabrics

The gloss or luster of a surface is essentially a subjectively evaluated property which is re lated to the manner in which the surface reflects light falling on it. Although the impression or sensation of glossiness is a matter of common experience, measurement of this property in physical terms and correlation of the measurements with its psychological aspects are extremely difficult. The most satisfactory physical measurement involves the use of a goniophotometer, an instrument which measures the amount of light reflected from a surface at various angles. Curves showing intensity of reflected light vs. the scanning angle exhibit a peak at the so- called specular angle for surfaces rated as being of high gloss. Surfaces of more complex struc ture, such as those of fabrics, show a displacement of maximum reflectance from the specular angle; the displacement is related to the angle of the incident light. The curve shape is char acteristic for different fabrics. This article describes the experimental measurement of reflectance by means of a goniopho tometer for a group of fabrics. The results are presented in the form of curves, computed "gloss indexes" or "gloss numbers," and visual ratings by different observers.

Gloss Measurement of Papers: Application of the Barkas Analysis

The Barkas analysis is applied to a series of nine papers of known visual gloss. With some reservations, the analysis is found to hold approximately and the distribution of mirror facets so derived is found to accord well with the surface structure of the papers as found by microscopic examination. It is concluded that, whilst the analysis is of negligible value for practical gloss determinations, it appears to present a useful method of investigating the surface structure and "finish" of paper and similar delicate materials. The gloss of the papers examined appears to be assessed solely by the intensity of the light reflected at the specular angle and not by the contrast noted by examination at different angles nor by the ratio of specularly-reflected to diffusely-reflected light. Some suggestions for future research are given.

Analysis of light scattered from a surface of low gloss into its specular and diffuse components

It is assumed that surfaces showing low gloss consist of small elementary facets which may be set at any angle to the mean surface. These facets may be of two types, one diffusing a proportion of the incident flux according to Lambert's law, and the other reflecting, at the specular angle, a proportion s of the incident flux, where s is determined by Fresnel's equation and is dependent on the refractive index of the material. On these assumptions formulae are obtained whereby the emergent flux E can be resolved into its diffuse and specular components R and M and from which the proportional areas B of the mirror facets, set at different angles to the mean surface, can be calculated. The use of the equations is illustrated by analysing two families of curves obtained by means of an apparatus which is described and relating to light scattered from a surface of Bristol board and from a surface of magnesium-oxide smoke deposited on plane glass.

The Emission of Negative Ions under the Bombardment of Positive Ions

The results which Sawyer obtained from the reflection of lithium ions from reflectors of platinum foil and nickel crystal deposited on tungsten, indicated two distinct angles at which reflection occurs more strongly than at other angles, one coming nearly at the angle of specular reflection as found by Read and Gurney, and the other which appeared only at relatively high voltages, at an angle approximately normal to the reflecting surface. Experiments made by reflecting the lithium ions from a metal surface in a field free space gave the bundle at the specular angle as before, but no trace of a bundle at the normal to the surface could be found. When a field is applied so as to retard reflected positive ions there was evidence of negative emission coming from the target under the bombardment of lithium ions. A magnetic analysis system was set up to determine the nature of the negative charges. Negative fluorine, chlorine, oxygen and sulphur ions were obtained by bombarding NaF, Ca${\mathrm{F}}_{2}$; NaCl, CaO, PbS and oxide-coated vacuum tube filaments. Clean metal targets of platinum, gold, aluminum, tantalum, nickel, and tungsten were found to emit electrons and the negative ions $\mathrm{H}_{1}^{\ensuremath{-}}$, $\mathrm{H}_{2}^{\ensuremath{-}}$, O${\mathrm{H}}^{\ensuremath{-}}$, and ${\mathrm{Cl}}^{\ensuremath{-}}$ with traces of what is probably ${\mathrm{N}}^{\ensuremath{-}}$ and LiO${\mathrm{H}}^{\ensuremath{-}}$. The strongest bundles were the first four which indicate that water-vapor molecules were broken up into their components.