Modeling liquid organic thin films on substrates

Bernacki, Bruce E.; Johnson, Timothy J.; Myers, Tanya L.; Blake, Thomas A.

doi:10.1117/12.2299873

Cited by 8 publications

(8 citation statements)

References 10 publications

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“…9,51 Conclusion Laboratory, industrial, and standoff IR spectroscopies continue their exponential growth. 52,53 It has already been demonstrated 54 that laboratory reference data can readily be used for field detection of both solids and liquids, as well as gases 55 but increasingly the n/k vectors are used for modeling 5,56 the target signal both for liquids 4 and solids. 54 Obtaining n/k for solids is more challenging, and one goal of this study was thus to (determine how to) prepare pellets with high surface smoothness, since the single-angle method measures only specular reflectance.…”

Section: Discussionmentioning

confidence: 99%

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

et al. 2020

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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.

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Section: Discussionmentioning

confidence: 99%

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

et al. 2020

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“…This uncertainty for k increases to about 0.1 for regions with spectral modulation or about 5%. These data along with the methods reported in this paper are needed for improving standoff and remote sensing IR detection by providing the optical constants that can be applied to model spectral reference data 9,39 for the target material under various morphological conditions (e.g., layer thickness, substrate, etc.). This capability is important since a single bulk spectrum for a target material is not sufficient and could lead to missed detections as many materials in the environment often exist as residues or deposits on surfaces, for example, resulting in a different reflectance spectrum from the bulk.…”

Section: Resultsmentioning

confidence: 99%

“…Instead, modeling the chemical spectrum to account for the different morphological conditions may be a preferable route, but requires both the real ( n ) and imaginary ( k ) optical constants of the complex refractive index. 5,6,9 These optical constants are not available for many of the materials of interest; spectral libraries that contain the optical constants for both solids and liquids are lacking and are clearly needed.…”

Section: Introductionmentioning

confidence: 99%

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

et al. 2020

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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.

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“…4,5 Many materials have been studied with HSI and some even studied in multiple phases; much of the early HSI work focused on gas-phase detection 6,7 and a few studies also reported liquid-phase detection. [8][9][10] More recent HSI studies, however, have focused on detection of solid materials, typically using reflected visible or near-infrared wavelengths. 11,12 The solid-phase studies have grown at an exponential pace, particularly for vegetation [13][14][15] as well as for rocks, minerals, and other geological specimens.…”

Section: Introductionmentioning

confidence: 99%

Hyperspectral imaging of minerals in the longwave infrared: the use of laboratory directional-hemispherical reference measurements for field exploration data

Myers

Johnson

Gallagher

et al. 2019

J. Appl. Rem. Sens.

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Hyperspectral imaging (HSI) continues to grow as a method for remote detection of vegetation, materials, minerals, and pure chemicals. We have used a longwave infrared (7.7 to 11.8 μm) imaging spectrometer in a static outdoor experiment to collect HSI data from 24 minerals and background materials to determine the efficacy with which HSI can remotely detect and distinguish both pure minerals and mineral mixtures at a 45-deg tilt angle relative to ground using two different backgrounds. Measurements were obtained separately for the minerals and materials mounted directly on both a bare plywood board and a board coated with aluminum foil: 19 powders (3 mixtures and 16 pure mineral powders) held in polyethylene bottle lids as well as five samples in rock form were taped directly to the boards. The primary goal of the experiment was to demonstrate that a longwave infrared library of solids and minerals collected as directional-hemispherical reflectance spectra in the laboratory could be used directly for HSI field identification along with simple algorithms for a rapid survey of the target materials. Prior to the experiment, all 24 mineral/inorganic samples were measured in the laboratory using a Fourier transform infrared spectrometer equipped with a gold-coated integrating sphere; the spectra were assimilated as part of a larger reference library of 21 pure minerals, 3 mixtures, and the polyethylene lid. Principal component analysis with mean-centering was used in an exploratory analysis of the HSI images and showed that, for the aluminum-coated board, the first principal component captured the difference between the signal that resembled a blackbody and the highly reflective aluminum background. In contrast, the second, third, and fourth principal components were able to discriminate the materials including phosphates, silicates, carbonates, and the mixtures. Results from generalized least squares target detection clearly showed that laboratory reference spectra of minerals could be utilized as targets with high fidelity for field detection.

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Modeling liquid organic thin films on substrates

Cited by 8 publications

References 10 publications

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

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

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

Hyperspectral imaging of minerals in the longwave infrared: the use of laboratory directional-hemispherical reference measurements for field exploration data

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Modeling liquid organic thin films on substrates

Cited by 8 publications

References 10 publications

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

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

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

Hyperspectral imaging of minerals in the longwave infrared: the use of laboratory directional-hemispherical reference measurements for field exploration data

Contact Info

Product

Resources

About

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

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