Michael E. Thomas

High-temperature infrared properties of sapphire, AlON, fused silica, yttria, and spinel

Abstract A vacuum emissometer, utilizing a carbon dioxide laser for high-temperature sample heating, has been designed and built for use with a Fourier Transform spectrometer. A two-color pyrometer technique is used to measure sample surface temperature. Oxides such as sapphire, spinel, yttria, aluminum oxynitride, and fused silica are experimentally characterized in temperature from 600 to 2000 K and in frequency from 500 to 5000 cm−1. A glowing yttria sample has also been characterized over the spectral range of 8500 to 13u2008500 cm−1. Good agreement with temperature dependent classical oscillator and quantum mechanical multiphonon models for the complex index of refraction is obtained.

Frequency and temperature dependence of the refractive index of sapphire

Because the index of refraction is temperature dependent, a temperature gradient across a window causes image blur and bore sight error. Prior to this paper, there have been no direct temperature-dependent measurements reported on the mid-infrared refractive index for sapphire, a popular infrared window material of high durability. Measurements of dn/dT are reported on the ordinary ray of sapphire in the 4 μm region for the first time. Accurate temperature and frequency dependent refractive index models can now be constructed from visible measurements of the refractive index, far-infrared reflectance measurements, thermo-optic coefficient measurements, and infrared measurements of the absorption coefficient. Visible measurements determine the contribution to the refractive index from electronic transitions. Far-infrared measurements determine the contributions from fundamental lattice vibrations (phonons). Infrared absorption data are used to determine parameters in a multiphonon sum band model. By applying the Hilbert transform to this multiphonon absorption model, a model for the multiphonon refractivity is obtained. Two- and three-phonon contributions to the refractive index are important for an accurate model that includes temperature dependence. Results for the ordinary- and extraordinary-rays are obtained.

Line shape model for describing infrared absorption by water vapor

A complete representation of water vapor IR absorption as a function of frequency, partial pressure, and temperature is accomplished by a semiempirical total line shape model. Water vapor dimer models accounting for continuum absorption are found to be unnecessary.

Infrared- and millimeter-wavelength continuum absorption in the atmospheric windows: measurements and models

Abstract Attenuation of electromagnetic waves by atmospheric gases in an important consideration in a variety of radar and electro-optical applications. Molecular absorption by strong bands of H 2 O and CO 2 defines the atmospheric window regions. The window regions are not totally transparent and feature weak line absorption and continuum absorption. The experimental character of continuum absorption by water vapor in atmospheric window regions at millimeter wavelengths and 10, 4 and 2.2 μm and nitrogen at 4 μm is surveyed. This includes recent measurements at the The Johns Hopkins University Applied Physics Laboratory on H 2 O at 2.2 μm and the nitrogen collision-induced band at 4.3 μm. Also, some of the concepts and models used to characterize these phenomena will be reviewed. The search for a unified theory on the water vapor continuum has been elusive, yet the frequency and pressure dependence is consistent with current far-wing theories. The temperature dependence is not totally understood. A model describing the temperature dependence of the integrated intensity of the collision-induced nitrogen vibrational band is emphasized.

The N2-broadened water vapor absorption line shape and infrared continuum absorption—I. Theoretical development☆

Abstract Attenuation of infrared radiation in the troposphere is dominated by water vapor absorption. Most past work on the spectroscopy of water vapor has been on the analysis of the rotational and vibrational bands. As a result, a thorough listing of line positions, line strengths, and half-widths exists today. Because of the weak continuum absorption in the water vapor windows centered at 10 and 4 μm, efforts to measure and model the continuum have not been as succesful as efforts in the analysis of the bands. Yet, a precise understanding of continuum absorption is important for long path energy transmission. For this reason, the study of the water vapor windows has had a long and speculative history. The purpose of this study is to demonstrate the importance of far wing phenomena in characterizing H2O continuum absorption. A total line shape for water vapor-nitrogen interactions valid under tropospheric conditions is derived. The complexity of such a model is substantial and many approximations are necessary to obtain a tractable theory. This effort represents one of the initial attempts at a far wing line shape solution of continuum absorption. Application of the model to existing experimental data is presented in the following article.

The N2-broadened water vapor absorption line shape and infrared continuum absorption—II. Implementation of the line shape☆

Abstract The total line shape model of the previous paper is tested using a set of experimental room temperature H2O continuum measurements of high quality. Parameters of the far wing component of the total line shape are determined from near band experimental data. Grating spectrometer measurements from 300 to 650 cm−1 are used to determine unknown far wing parameters of the pure rotational band of H2O. CO and HF laser measurements taken in the 5 and 3 μm regions are used to determine the far wing parameters of the ν2 and ν1, ν3 fundamental bands, respectively. The total line shape model is applied to the 10 and 4 μm transmission windows with encouraging success. A significant increase in the self-broadening ability of H2O over N2 is predicted in the far wing. This procedure allows the proper modeling of the absorption coefficient vs H2O partial pressure dependence in all window regions. A negative temperature is predicted by the model in the continuum. The observed rate of the temperature decrease is not predicted by the model; however, this limitation is related to the approximations made on the interaction potentials and the perturbation expansion of the Hamiltonians. Although the total line shape has limitations, it does demonstrate the importance of considering far wings of absorption lines in continuum absorption.

Infrared properties of CVD β-SiC

Abstract The temperature and frequency dependent infrared properties of polycrystalline CVD β-SiC have been measured. This was accomplished using both broadband and narrowband (laser) measurements as a function of temperature from room temperature up to 900 K. Calculated multiphonon absorption shows good agreement with experiments. Furthermore, the thermo-optic coefficient was measured in the 2.5–5 μm region and the BSDF for CVD β-SiC was measured at 0.6328 μm for the first time.

Modeling of the frequency- and temperature-dependent absorption coefficient of long-wave-infrared (2–25 µm) transmitting materials

A semiempirical multiphonon model based on quantum-mechanical oscillators under a Morse potential is applied to the absorption coefficient of far-infrared transmitting materials. Known material properties are combined with absorption coefficient data to fit the empirical parameters of the model. This provides an accurate means of predicting the intrinsic absorption of the materials in their multiphonon regions. Extinction data are obtained by measuring material transmittances with a Fourier-transform spectrometer and comparing them with the lossless transmittances predicted by Sellmeier models. Where appropriate, scatter models are used to separate the extinction into loss due to scatter and absorption. Data and model parameters are presented for GaAs, GaP, ZnS, and ZnSe.

Infrared properties of the extraordinary ray multiphonon processes in sapphire

Transmission measurements using a linearly polarized diode laser, operating between 4.9 and 5.0 microm, and a sapphire sample with the c axis in the plane of the sample surface are used to determine the first multiphonon absorption coefficient measurements of the extraordinary ray.

THE ENVIRONMENT CREATED BY AN OPEN-AIR SOLID ROCKET PROPELLANT FIRE

Abstract A 91 kg (200 lbm) block of aluminized solid rocket propellant was burned in open air to simulate an accidental propellant fire. A suite of remote optical instruments measured the temperature and radiative properties of the plume. Solid molybdenum calorimeters provided data for heat flux estimates. Various refractory oxide and metallic witness samples placed in the fire provided temperature benchmarks and insight into how such samples may be dispersed by the fire. A thermochemical analysis assessed the overall energy balance. The results indicate that temperatures reached 3000±100 K and heat fluxes reached 200±80 W/cm2 under the propellant, which burned for 120 s, creating a severe environment.