Specular Reflection Spectra Research Articles

ChaRacterization of Carbon-Filled Polymers by Specular Reflectance

Carbon-filled polymers, due to their opacity, are normally difficult to characterize by IR spectroscopy. We describe a method for obtaining spectra based on a specular reflectance measurement at normal incidence using the IR microscope. Peaks in the specular reflectance spectrum have a derivative-like shape due to refractive index dispersion. After application of the Kramers-Kronig transformation, an absorption index spectrum is obtained; this has a more conventional appearance and is thus much easier to interpret. The technique was found to be applicable so long as certain criteria were met, such as optical flatness and no contributing reflectance effects from the subsurface. Various polymeric materials were examined which contained carbon in the range 2 to 25% w/w. This approach was found unsuitable for carbon-fiber/polymer composites which show predominantly diffuse reflectance due to scattering at the surface features.

Infrared reflectance spectra of Sb-doped SnO2 ceramics

Infrared specular reflectance spectra of Sn1 –xSbxO2 ceramics have been measured over a range of bulk doping levels extending up to x= 0.03 At low doping levels conduction electrons screen out coupling of the infrared radiation to bulk phonon modes in a way expected from a model in which reflectivity is calculated from the bulk dielectric function. However, at higher doping levels a reflectivity minimum develops at ca. 800 cm–1. This is interpreted in terms of a model where a surface layer with low carrier concentration sits on top of an undepleted bulk.

A sol-gel derived yellow-transmitting coating on glass

Bright yellow, transparent coatings were deposited on sheet glass by the sol-gel dip coating process. Sols were prepared from titanium tetraethoxide, ferric nitrate nonahydrate and a mixture of 1-propanol and 2-butanol. Advantage was taken of the absorption of Fe 3+ and Fe 2+ (a part of the ferric ion being reduced to ferrous ion during baking of the film, as confirmed by Mössbauer and UV-VIS-NIR spectroscopy) in the UV and near-IR regions, respectively, and of the absorption of Ti 4+ in the UV range. The coatings show a distinct transmission maximum which shifted in position depending on the physical thickness; this indicated the role of interference in monitoring the exact yellow shade of the transmitting color. Specular reflectance and transmission spectra of the coatings were examined to elucidate their optical behavior. The coating material was used in a multilayer design to obtain selective transmission behavior.

Optically induced restructuring of a hydrogenated amorphous silicon thin-film surface

Hydrogenated amorphous silicon thin films have been characterized using specular reflectance spectra. An ultraviolet light-induced irreversible increase in reflectance has been attributed to changes in surface microstructure. These changes have been explained as being caused by phonons emitted during photocarrier thermalization. An irreversible increase of photoconductivity substantiates these observations.

Mapping mineralogy distribution and abundance in rock samples by scanning Fourier transform infrared microscopy

AbstractScanning Fourier transform infrared microscopy (SFTIRM) provides a nondestructive, automated technique that gives both spatial and quantitative description of mineralogy in rocks on a microscopic scale. SFTIRM is essentially a combination of Fourier transform infrared spectroscopy and scanning microscopy. This report describes and demonstrates the principle and operation of the technique. Mineral identification is based on specular reflectance spectra collected in the midinfrared. Mineral abundances are extracted by two different spectral fitting algorithms: least‐mean‐squares (LMS) and linear area fitting (LAF). A sedimentary rock composed mostly of dolomite, some quartz, and finely layered illite is imaged. The SFTIRM images clearly show the spatial distribution and abundance variations of the minerals.

Characterization of silica gels by infrared reflection spectroscopy

The FTIR specular reflection spectra of porous silica gels, either dried at room temperature or subjected to partial densification at 400°C, are significantly different from the spectrum of bulk vitreous silica. Namely, the high frequency SiOSi stretching vibrational peak near 1100 cm −1 exhibits shifts in the gels of up to 40 cm −1 towards lower frequencies, whereas the relative intensity of the companion high frequency shoulder simultaneously increases. The origin and possible structural meaning of the observed changes were investigated by Kramers-Kronig analysis of the experimental near-normal infrared reflectivity spectra of a series of silica xerogels, porous Vycor and bulk vitreous silica. Major differences were found for the energy loss function Im(−1/ϵ ∗), the peaks of which have been associated with the LO components of the dominant vibrational modes, and the largest red shifts corresponded to the highest frequency mode. Simple calculations show that the observed behavior of the infrared spectra could in part be due to a certain degree of strain in the SiOSi bridging bonds at the surface of the gel pores.

Optical properties of chemical-vapor-deposited diamond films

Results of room-temperature optical studies on ∼10 micron thick, free-standing diamond films are reported. The films were grown on Si(100) substrates by hot filament-assisted chemical vapor deposition (CVD) from a methane/hydrogen mixture. The as-grown, free surface of the films exhibited a surface roughness of scale σ ∼ 0.2 to 5 microns, depending on the methane/hydrogen mixture, which introduces significant optical scattering loss for frequencies greater than 0.5 eV. Specular reflection and transmission spectra in the range 0.01–10 eV were collected. Below the threshold for interband adsorption near ∼5 eV, the films studied behaved approximately as thin parallel plates of refractive index 2.4, with the rough free surface leading to increasingly larger loss of specular transmission/reflection with decreasing wavelength. Structure in the mid-infrared transmission spectra was observed and attributed to disorder-induced one-phonon absorption, intrinsic multi-phonon absorption, and infrared active –C–H2 stretching modes. The strength of the C–H band was observed to increase with increasing methane pressure in the growth chamber. At 5.3 eV, the onset of interband absorption was observed, in good agreement with the value of the indirect bandgap in type IIa (intrinsic) diamond.

Measurement of Infrared Spectra of Dense Ceramics by Specular Reflectance Spectroscopy

It is shown that high‐quality IR specular reflectance spectra can be obtained from polished surfaces of dense ceramics. The reflectance spectra can be deconvoluted by the Kramers‐Kronig transformation to yield absorption spectra or the real and imaginary parts of the dielectric function. IR line shapes and peak wavenumbers are comparable to single‐crystal data providing that the particle size is small compared with the IR wavelengths of interest. For noncubic structures, the spectra are a superposition of polarization components. Modes separated by less than a bandwidth are merged into a single band, thus limiting the resolution of the measurement procedure. Grain size effects were modeled using the spectra of single‐crystal and polycrystalline quartz as an example.

Some Novel Applications of an Infrared Microscope

In recent years there has been considerable growth in the use of infrared microscopy as an analytical tool. These applications have focused on the spatial resolving power of the microscope for analysis of small samples or small contaminants or inhomogeneities within a material. The infrared microscope can also serve as a more versatile sampling accessory. Depending on the nature of the material under investigation, diffuse reflectance or true specular reflectance spectra can be recorded from bulk polymeric materials. In either case, useful chemical or functional group information can be obtained in a noninvasive and nondestructive manner. As a further example of the versatility of the infrared microscope, some initial results are presented illustrating its potential use in the study of monomolecular films on aqueous substrates.

Polarized specular reflectance spectra of the partially oxidized metallophthalocyanines Ni(pc)I and Co(pc)I: charge-transfer transitions in one-dimensional conductors

Polarized specular reflectance spectra of the partially oxidized metallophthalocyanines Ni(pc)I and Co(pc)I: charge-transfer transitions in one-dimensional conductors

Transient behaviors in thermal radiation characteristics of heat-resisting metals and alloys in oxidation processes

A high-speed spectrophotometer system is developed to study radiation characteristics of materials. The system allows measurements of the spectra at wavelengths of 0.35–10μm repeatedly with a period of less than 1 s. It is applied to the study of transient behaviors in reflection characteristics of heat-resisting alloys and the constituent transition metals in air-oxidation processes at high temperatures. An interference phenomenon due to the multiple reflection at the upper and lower boundaries of the oxide film is observed in the diffuse reflection spectra of oxidizing rough-finished surfaces as well as in the specular reflection spectra of oxidizing specular-finished surfaces. The phenomenon is found to be fairly reproducible and consistent over all the materials investigated. It is attributed to the interference and diffraction of radiation at three-dimensional nonparallel film elements of the polycrystalline oxide grains. A possibility is suggested for the theoretical modeling of radiation characteristics of real surfaces in the actual environments of industry.

Reflectivity and Roughness of X-ray Multilayer Mirrors. Specular Reflection and Angular Spectrum of Scattered Radiation

Abstract A simple and accurate analytical model for the reflectivity of an X-ray multilayer mirror has been developed. This model can be used in the determination of interfacial roughness over large ranges of wavelength and roughness. There are no restrictions such as the roughness being less than the scale of the wavelength such as are present in theories based on the multiplication of the sharp boundary reflectivity coefficient by an exponential factor. The model is also applied to scattered radiation. Angular spectra are calculated and the use of scattered radiation in determining interfacial roughness is demonstrated.

Polarized reflection spectrum and molecular orientation in uniaxially drawn poly(ethylene terephthalate)

AbstractThe polarized electronic spectrum of oriented poly(ethylene terephthalate) (PET) sheets was obtained from the specular reflection spectrum using the Kramers‐Kronig relationship. The surface orientation function of drawn and drawn/annealed PET sheets was determined from the dichroic ratio of the second π* ← π transition observed at 41,000 cm‐1. The bulk orientation functions in the crystalline and amorphous regions were evaluated from wide‐angle X‐ray diffraction and sonic modulus measurements. On annealing of drawn PET sheets, the crystalline orientation and crystallinity were much improved, but the amorphous orientation function showed a minor decrease. The overall molecular orientation in the surface of the drawn PET sheet was shown to be approximately equivalent to the molecular orientation in the bulk.

Glass Structure and Glass Durability

ABSTRACTThe vibrational spectra of silicate glasses provide some clues on the structure and stability of the glass-forming network. Wavenumbers of Raman bands are related to the types of linkages within the network and Raman intensities to the “speciation” within the network. Absolute infrared intensities which can be obtained from deconvolution of specular reflectance spectra can be used to obtain dipole derivatives which in turn can be correlated with effective charges on bridging and non-bridging oxygens. IR and Raman spectra measured on a variety of silicate glasses are used to extract network polymerization and bond effective charges which in turn are related to the durability of the glasses.

Polarization-Modulation FT-IR Reflection Spectroscopy Using a Polarizing Michelson Interferometer

We report the first results obtained on a mid-IR FT spectrometer equipped with a polarizing Michelson interferometer (PMI), and the application examples of polarization spectroscopy are illustrated. A relatively simple method for conversion of a conventional instrument to PMI operation is designed. Specular reflection spectra of poly(vinyl acetate) film on copper and ATR spectra of a Langmuir-Blodgett film on silicon prism are presented.

Polarized single-crystal specular reflectance spectra of (TMTSF)2ClO4 in the visible and ultraviolet

Abstract Single-crystal specular reflectance spectra of three polarizations of (TMTSF) 2 ClO 4 in the visible and ultraviolet regions and their Kramers-Kronig-transformed absorption spectra are reported. Earlier reports of visible transitions are confirmed with the addition of integrated intensities, and several new transitions are reported quantitatively in the ultraviolet.

Observation of the J-band in the crystal spectra of pseudoisocyanine (PIC)

The specular reflection spectra of crystalline PIC iodide measured at 11 K yield structure essentially identical with the aggregate J-band of PIC. This establishes the aggregate's proposed brickwork pattern. The bandwidth of the crystal J-band shows a rapid decrease from 70 to 40 K. Vibrational structure is associated with the band in the crystal.

Hydrogen peroxide determination by I.R. specular reflectance spectroscopy during uncatalyzed and copper (oxides)‐catalyzed oxidation of isotactic polypropylene

AbstractIR specular reflectance spectra with respect to hydrogen peroxide formation during uncatalyzed and Cu (oxide)‐catalyzed oxidation of isotactic Polypropylene films have been measured as function of time and temperature (90–130°C). Energies of activation for the various cases have been obtained. The originally proposed kinetic scheme based on oxygen‐absorption measurements has been modified in order to accommodate the spectroscopical results. The amount of ROOH groups present at any time on the polymer is very small, indicating relatively slow rates of ROOH formation and fast rates of their decomposition. The kinetic scheme fits well the experimental data. However, the reasons for the variations of the relevant energies of activation obtained for the catalyzed oxidation in absence and presence of the main volatile reaction products, H2O and CO2, are not yet understood, i.e., the mechanism needs further investigations.

Complete Elimination of Specular Reflectance in Infrared Diffuse Reflectance Measurements

The Kubelka-Munk relationship is the equation most commonly used to describe the reflectance from a scattering medium. This equation breaks down, however, in several important situations in the mid-infrared. Most notably, the relationship does not account for specular reflectance, which is simply the energy reflected from the front surface of the sample. When a sample reflects diffusely and specularly, as do all real-world samples, there is a specular reflectance spectrum superimposed on the diffuse reflectance spectrum, and deviations from this equation occur. In the case of organic samples, this causes a curved line (concave downward) for the plot of f(R∞) or Kubelka-Munk units vs. sample weight percent in a nonabsorbing matrix. For inorganic samples, the effect can be more severe. Since the specular reflectance of inorganic bands can be much greater than organics, often complete band inversions (reststrahlen bands) or derivative shaped peaks result. In either case, quantitative analysis is not straightforward, and in the latter case, even qualitative interpretation is difficult.

Routine determination of the absorption and dispersion spectra of solids with a Fourier-transform infrared spectrometer

A method is given for the routine determination of the absorption and refractive index spectra of solids from their Fourier-transform infrared (F.t.i.r.) specular reflection spectra. The innovation responsible for making these determinations routine and rapid is the application of the fast Fourier-transform software of the F.t.i.r. spectrometer in the calculation of the optical constants. For strongly absorbing materials, accurate values of the resonance absorption frequencies and corresponding absorptivities cannot be obtained from either the reflectance or transmittance spectra. The absorption and refractive index spectra of fused silica are determined here by this method. Starting with a reflection spectrum, the entire procedure requires only 20 min at a resolution of 2 cm−1. These calculations are impossible with the existing software of most small F.t.i.r. spectrometers; minor software modifications reduce the time required to a few minutes.