Infrared Optical Constants from Pressed Pellets of Powders: I. Improved n and k Values of (NH4)2SO4 from Single-Angle Reflectance

2020 ◽  
Vol 74 (8) ◽  
pp. 851-867
Author(s):  
Timothy J. Johnson ◽  
Emmanuela Diaz ◽  
Kendall D. Hughey ◽  
Tanya L. Myers ◽  
Thomas A. Blake ◽  
...  
Keyword(s):  
Optical Constants ◽  
Complex Refractive Index ◽  
Optical Quality ◽  
Substantial Improvement ◽  
Index Formula ◽  
Pressing Pressure ◽  
Void Space ◽  
Specular Reflectance ◽  
Pellet Formation ◽  
Infrared Spectrometer

In combination with other parameters, the real, n([Formula: see text]), and imaginary, k([Formula: see text]), components of the complex refractive index, [Formula: see text] =  n + i k, can be used to simulate the optical properties of a material in different forms, e.g., its infrared spectra. Ultimately, such n/k values can be used to generate a database of synthetic reflectance spectra for the different morphologies to which experimental data can be compared. But obtaining reliable values of the optical constants n/k for solid materials is challenging due to the lack of optical quality specimens, usually crystals, large enough to measure. An alternative to crystals is to press the powder into a uniform disk. We have produced pellets from ammonium sulfate, (NH4)2SO4, powder and derived the pellets' n and k values via single-angle reflectance using a specular reflectance device in combination with a Fourier transform infrared spectrometer. The single-angle technique measures amplitude of light reflected from the material as a function of wavelength over a wide spectral domain; the optical constants are determined from the reflectance data using the Kramers–Kronig relationship. We investigate several parameters associated with the pellets and pellet formation and their effects upon delivering the most reliable n/k values. Parameters studied include pellet diameter, mass, and density (void space), drying, grinding, sieving, and particle size in the pellet formation, as well as pressing pressure and duration. Of these parameters, using size-selected mixtures of dried, small (<50 µm) particles and pressing at ≥10 tons for at least 30 min were found key to forming highly reflective samples. Comparison of two sets of previous literature n([Formula: see text]) and k([Formula: see text]) values obtained from crystalline (NH4)2SO4 both as crystal reflectance as well as extinction spectra of aerosols measured in a flow tube shows reasonable agreement, but suggests the present values, as confirmed from two independent techniques, represent a substantial improvement for n/k values for (NH4)2SO4, also demonstrating promise to measure the optical constants of other materials.

Improved Infrared Optical Constants from Pressed Pellets: II. Ellipsometric n and k Values for Ammonium Sulfate with Variability Analysis

2020 ◽  
Vol 74 (8) ◽  
pp. 868-882
Author(s):  
Tanya L. Myers ◽  
Thomas A. Blake ◽  
Michael O. Yokosuk ◽  
Gilles Fortin ◽  
Timothy J. Johnson
Keyword(s):  
Ammonium Sulfate ◽  
Optical Constants ◽  
Preparation Method ◽  
Complex Refractive Index ◽  
Index Formula ◽  
Oscillator Model ◽  
Angle Of Incidence ◽  
Infrared Reflectance ◽  
Data Set ◽  
Fitting Procedure

Infrared reflectance analysis is facilitated via the comparison of spectra recorded in situ to a databank of actual or synthetic infrared reflectance spectra. It has recently been shown that reference spectra corresponding to the many different morphological forms of the same chemical can be generated synthetically using the imaginary, k, and real, n, components of the complex refractive index, [Formula: see text] =  n + i k. One method to obtain the n and k vectors is infrared ellipsometry, which measures the changes in amplitude, tan Ψ, and phase, Δ, of polarized light reflected from the sample both as a function of wavenumber and angle of incidence. The method requires specularly reflected light, so best results are usually obtained with polished planar samples of large surface area. Due to the difficulties of obtaining such samples, however, we investigate the possibility of pressing powders of neat materials and obtaining the corresponding optical constants from the pellets. In this paper, variability in the sample pellet and preparation method is investigated, as is variability in the fitting procedure for the derived optical constants. The n/k vectors are derived from the measured ellipsometric parameters, tan ψ and Δ, as they are fit by an oscillator model which yield n([Formula: see text]) and k([Formula: see text]) vectors as a function of wavenumber, [Formula: see text]. Construction of the oscillator model is not automatic and depends on significant input from the analyst as well as the sample’s physical characteristics. For pellet pressing, the experimental variability was found to be minimized for size-selected powdered samples as gauged by the minimal variance in ψ and Δ for three different pellets; similarly, the analytical precision for multiple measurements of the same pellet was also quite good, suggesting that a pressed pellet is a viable sample preparation method. Experimental variabilities were comparatively small; the greatest variability came in the analytic fitting procedure with differences in the k-peak values up to 10% for only the sharpest bands arising from four different fits to the same data set. The final ellipsometric n/k data are compared to literature values obtained from crystalline ammonium sulfate ((NH4)2SO4) samples as well as single-angle reflectance measurements that also used pressed pellets. Comparison with the previous literature values shows generally good agreement, although larger k-values are observed for the independent sets of data derived from pressed pellets. These data are suggested as an improved set of optical constants for (NH4)2SO4.

Soft X-ray refractive index by reconciling total electron yield with specular reflection: experimental determination of the optical constants of graphite

2018 ◽  
Vol 25 (5) ◽  
pp. 1433-1443
Author(s):  
C. Jansing ◽  
H. Wahab ◽  
H. Timmers ◽  
A. Gaupp ◽  
H.-C. Mertins
Keyword(s):  
Refractive Index ◽  
Optical Constants ◽  
Specular Reflection ◽  
Complex Refractive Index ◽  
Reflection Spectra ◽  
Total Electron Yield ◽  
Total Electron ◽  
X Ray ◽  
Electron Yield ◽  
Saturation Effects

The complex refractive index of many materials is poorly known in the soft X-ray range across absorption edges. This is due to saturation effects that occur there in total-electron-yield and fluorescence-yield spectroscopy and that are strongest at resonance energies. Aiming to obtain reliable optical constants, a procedure that reconciles electron-yield measurements and reflection spectroscopy by correcting these saturation effects is presented. The procedure takes into account the energy- and polarization-dependence of the photon penetration depth as well as the creation efficiency for secondary electrons and their escape length. From corrected electron-yield spectra the absorption constants and the imaginary parts of the refractive index of the material are determined. The real parts of the index are subsequently obtained through a Kramers–Kronig transformation. These preliminary optical constants are refined by simulating reflection spectra and adapting them, so that measured reflection spectra are reproduced best. The efficacy of the new procedure is demonstrated for graphite. The optical constants that have been determined for linearly polarized synchrotron light incident with p- and s-geometry provide a detailed and reliable representation of the complex refractive index of the material near π- and σ-resonances. They are also suitable for allotropes of graphite such as graphene.

scholarly journals High-resolution optical constants of crystalline ammonium nitrate for infrared remote sensing of the Asian Tropopause Aerosol Layer

2020 ◽  
Author(s):  
Robert Wagner ◽  
Baptiste Testa ◽  
Michael Höpfner ◽  
Alexei Kiselev ◽  
Ottmar Möhler ◽  
...  
Keyword(s):  
Ammonium Nitrate ◽  
Optical Constants ◽  
Asian Summer Monsoon ◽  
Complex Refractive Index ◽  
Extinction Spectrum ◽  
Stable Phase ◽  
Aerosol Layer ◽  
Data Set ◽  
Tropospheric Aerosol ◽  
Infrared Measurements

Abstract. Infrared spectroscopic observations have shown that crystalline ammonium nitrate (AN) particles are an abundant constituent of the upper tropospheric aerosol layer which is formed during the Asian summer monsoon period, the so-called Asian Tropopause Aerosol Layer (ATAL). At upper tropospheric temperatures, the thermodynamically stable phase of AN is different from that at 298 K, meaning that presently available room-temperature optical constants of AN, that is, the real and imaginary parts of the complex refractive index, cannot be applied for the quantitative analysis of these infrared measurements. In this work, we have retrieved the first low-temperature data set of optical constants for crystalline AN in the 800–6000 cm−1 wavenumber range with a spectral resolution of 0.5 cm−1. The optical constants were iteratively derived from an infrared extinction spectrum of 1 micrometer-sized AN particles suspended in a cloud chamber at 223 K. The uncertainties of the new data set were carefully assessed in a comprehensive sensitivity analysis. We show that our data accurately fit aircraft-borne infrared measurements of ammonium nitrate particles in the ATAL.

Calculation of Spectral Optical Constants Using Combined Ellipsometric and Reflectance Methods for Smooth and Rough Bulk Samples

2021 ◽  
pp. 000370282110478
Author(s):  
Gilles Fortin
Keyword(s):  
Rough Surface ◽  
Optical Constants ◽  
Sample Surface ◽  
Infrared Region ◽  
Homogeneous Sample ◽  
Specular Reflectance ◽  
Scattering Effects ◽  
Spectral Ranges ◽  
Reflectance Measurements ◽  
Polarized Reflectance

Spectra of the optical constants n and k of a substance are often deduced from spectroscopic measurements, performed on a thick and homogeneous sample, and from a model used to simulate these measurements. Spectra obtained for n and k using the ellipsometric method generally produce polarized reflectance simulations in strong agreement with the experimental measurements, but they sometimes introduce significant discrepancies over limited spectral ranges, whereas spectra of n and k obtained with the single-angle reflectance method require a perfectly smooth sample surface to be viable. This paper presents an alternative method to calculate n and k. The method exploits both ellipsometric measurements and s-polarized specular reflectance measurements, and compensates for potential surface scattering effects with the introduction of a specularity factor. It is applicable to bulk samples having either a smooth or a rough surface. It provides spectral optical constants that are consistent with s-polarized reflectance measurements. Demonstrations are performed in the infrared region using a glass slide (smooth surface) and a pellet of compressed ammonium sulfate powder (rough surface).

scholarly journals Optical Constants Of Kerogen From 0.15 To 40 μm: Comparison with Meteoritic Organics

1991 ◽  
Vol 126 ◽  
pp. 99-101 ◽  
Author(s):  
B.N. Khare ◽  
W.R. Thompson ◽  
C. Sagan ◽  
E.T. Arakawa ◽  
C. Meisse ◽  
...  
Keyword(s):  
Infrared Absorption ◽  
Optical Constants ◽  
Optical Quality ◽  
Vacuum Evaporation ◽  
Type Ii ◽  
Organic Residue ◽  
Absorption Properties ◽  
Murchison Meteorite ◽  
Evaporation Technique ◽  
Vacuum Evaporation Technique

AbstractA vacuum evaporation technique has been used to produce thin, optical quality films of samples of Type II kerogen and of insoluble organic residue from the Murchison meteorite. Using these films, optical constants have been measured from 0.15 to 40 μm for kerogen, and from 2.5 to 40 μm for the Murchison residue. The infrared absorption properties of these materials show many similarities, although Murchison residue is more opaque throughout the infrared than is kerogen, and shows no distinct aliphatic absorptions.

Infrared Reflectance Spectrometer for Liquids at High Temperatures

1969 ◽  
Vol 23 (3) ◽  
pp. 230-234 ◽  
Author(s):  
John R. Sweet ◽  
William B. White
Keyword(s):  
Absorption Spectrum ◽  
Computer Program ◽  
High Temperatures ◽  
Vibrational Spectra ◽  
Liquid Surface ◽  
Reflectance Spectrum ◽  
Infrared Reflectance ◽  
Specular Reflectance ◽  
Infrared Spectrometer

A modification of an infrared spectrometer is described which permits the measurement of vibrational spectra of glasses and liquids at high temperatures. Spectra are measured by specular reflectance from the liquid surface. A computer program is described which transforms the specular reflectance spectrum to an absorption spectrum by means of a Kramers—Kronig analysis.

Infrared Refractive Index of Thin YBa2Cu307 Superconducting Films

1992 ◽  
Vol 114 (3) ◽  
pp. 644-652 ◽  
Author(s):  
Z. M. Zhang ◽  
B. I. Choi ◽  
T. A. Le ◽  
M. I. Flik ◽  
M. P. Siegal ◽  
...  
Keyword(s):  
Refractive Index ◽  
Complex Refractive Index ◽  
Wavelength Region ◽  
Wave Theory ◽  
Superconducting Films ◽  
High Tc ◽  
Thin Film Optics ◽  
Infrared Spectrometer ◽  
Thickness Independent ◽  
Constant Refractive Index

This work investigates whether thin-film optics with a constant refractive index can be applied to high-Tc superconducting thin films. The reflectance and transmittance of YBa2Cu3O7 films on LaAlO3 substrates are measured using a Fourier-transform infrared spectrometer at wavelengths from 1 to 100 μm at room temperature. The reflectance of these superconducting films at 10 K in the wavelength region from 2.5 to 25 μm is measured using a cryogenic reflectance accessory. The film thickness varies from 10 to 200 nm. By modeling the frequency-dependent complex conductivity in the normal and superconducting states and applying electromagnetic-wave theory, the complex refractive index of YBa2Cu3O7 films is obtained with a fitting technique. It is found that a thickness-independent refractive index can be applied even to a 25 nm film, and average values of the spectral refractive index for film thicknesses between 25 and 200 nm are recommended for engineering applications.

Development of Neutral-Density Infrared Filters Using Metallic Thin Films

1994 ◽  
Vol 374 ◽  
Author(s):  
Z. M. Zhang ◽  
R. U. Dada ◽  
L. M. Hanssen
Keyword(s):  
Optical Constants ◽  
Wavelength Region ◽  
Single Layer ◽  
Alloy Film ◽  
Neutral Density ◽  
Metallic Thin Films ◽  
Future Design ◽  
Infrared Spectrometer ◽  
Spectrometer Calibration ◽  
Reflectance Measurements

AbstractBroadband infrared filters with uniform spectral transmittance are used for spectrometer calibration and other applications. However, commercially available neutral-density filters with optical density (GD) greater than 2 exhibit significant variations in OD over the wavelength region from 2 μm to 25 μ m. In this work, we found a single-layer alloy film that, for appropriate thicknesses, yields a flat transmittance for OD near 3 and 4. The transmittance and reflectance of the filters were measured using a Fourier-transform infrared spectrometer. The optical constants of the alloy films were obtained from transmittance and reflectance measurements, which can be used for future design optimization.

CALCULATION OF OPTICAL CONSTANTS IN POROUS SILICON THIN FILMS USING DIFFUSED AND SPECULAR REFLECTANCE MEASUREMENT

2002 ◽  
Vol 09 (05n06) ◽  
pp. 1821-1825 ◽  
Author(s):  
C. PEÑA ◽  
J. TORRES
Keyword(s):  
Thin Films ◽  
Porous Silicon ◽  
Optical Constants ◽  
Refraction Index ◽  
Summation Method ◽  
Electrochemical Anodization ◽  
Reflectance Measurement ◽  
Theoretical Simulation ◽  
Specular Reflectance ◽  
Two Samples

In this work a method is proposed to calculate the optical constants of porous silicon (PS) thin films. The method is based on the theoretical simulation of the experimental reflectance spectra. In the optical system of this method the PS is considered a homogeneous, absorbing thin film, deposited on a silicon substrate of semi-infinite dimension. The theoretical form of the systems reflectance is calculated using the summation method proposed by Airy. Light scattering is included in the model by introducing the Davies–Bennett relation. The refraction index of the material is fit with the simple harmonic oscillator, proposed by Wemple–DiDomenico. The model was tested on two samples fabricated with anodization times of 25 and 35 min; the values for the refraction indexes, absorption coefficients, thickness and roughness were calculated for both samples. The PS samples were fabricated by electrochemical anodization of single crystal p-type silicon substrates in HF (25%) + isopropyl alcohol solution.

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