Ely E. Bell

Measurement of the far infrared optical properties of solids with a michelson interferometer used in the asymmetric mode: Part I, mathematical formulation∗☆

Abstract A tutorial formulation of the relationships between the interferograms obtained with and without a sample in one beam of a Michelson interferometer is given. The relationship between the interferograms and the impulse response function of the sample is developed and shown to be useful in measuring the optical constants of the sample. The results obtained in the 20–200 cm−1 region with Mylar and polyethylene films in transmission and a KBr crystal in reflection are given to demonstrate the method. An elementary analysis to show the alteration of the ideal results produced by sampling the interferogram, by apodizing the interferogram, and by the noise errors on the interferogram is included. The extra phase information obtained from the asymmetric interferograms that cannot be obtained by conventional spectroscopy together with the signal-to-noise improvement afforded by the multichannel technique are seen to be the important advantages of this method for measuring optical properties of solids.

Measurement of the far infrared optical properties of solids with a Michelson interferometer used in the asymmetric mode: Part II, the vacuum interferometer∗

Abstract A vacuum far infrared Michelson interferometer for use in the measurement of optical constants of samples placed in one arm of the instrument has been constructed. The features of this instrument are the following : stability of the system ; uncollimated optics to permit the use of small sized samples; provision for the measurement of the samples in transmission or in reflection ; easy exchange of beam-splitters ; easy introduction of filters and polarizers ; simple programing of the optical path sampling positions. The optical and mechanical plan of the interferometer is described and examples, Mylar film transmission and metal mesh transmission, are given to illustrate its performance.

Spectral Radiance of Sky and Terrain at Wavelengths between 1 and 20 Microns. II. Sky Measurements

Quantitative data are presented to illustrate the infrared spectral radiance of the sky under a variety of situations. A qualitative discussion of the dominant spectral features is given to show the importance of the emission and scattering processes, the effect of angle of elevation, angle of azimuth, sun angle, and cloud cover.

An Infrared Spectrograph for Use in the 40–150-Micron Spectral Region*

Problems relating to the design of a far infrared spectrograph are discussed. The design of a spectrograph which was constructed is described. A platinum strip coated with thorium-oxide is used as a source. Discrimination against short-wave radiation is provided by quartz, paraffin, turpentine soot, a compensated potassium bromide chopper, reststrahlen plates, and a grating (used as a reflection filter) with groove separation somewhat less than the wavelength being investigated. As a dispersing device, an echelette grating ruled with 180 lines per inch is used. The detector is a Golay pneumatic cell. A low noise vacuum-tube amplifier renders the signal from the detector suitable for operation of a strip-chart recorder. The spectrograph is evacuable.Spectral operating characteristics of the instrument are shown in ammonia and atmospheric water vapor spectra in the region between 45 and 150 microns. Absorption lines separated by less than 1 cm−1 are well resolved throughout this spectral region.

Spectral Radiance of Sky and Terrain at Wavelengths between 1 and 20 Microns. I. Instrumentation

A description is given of a mobile laboratory equipped with two spectrographs for radiance measurements of the sky and terrain between 1 and 20 μ. The laboratory has its own ac power supply and means for controlling its temperature. Procedures are described for calibrating the instrumentation and transforming the data into spectral radiance (in microwatts cm−2 steradian−1 micron−1) as a function of wavelength.

The Infra‐Red Spectrum of Acetylene

New measurements of the infra‐red absorption spectrum of acetylene between 16 and 2.5μ have established many new bands. A cooling experiment has demonstrated that the central line of the parallel band ν41+ν51 (1328.18 cm−1) belongs to a difference band, 2ν40+ν51—ν41 (1328.46 cm−1). Resolution sufficient to distinguish the center of the fundamental ν3 (3282.5 cm−1) from the combination band ν2+ν41+ν51 (3295.56 cm−1) has been obtained. An interesting Πu—Πg‐band, identified as ν1+ν51—ν41, is shown. Frequencies of the infra‐red inactive fundamentals are derived and many anharmonic coefficients are evaluated. Diagrams of the absorption regions and tables of the line frequencies are included.

The Forbidden Transition ν 2 in the Infrared Spectrum of Methane

A preliminary discussion is given concerning the measurements on the forbidden transition ν2 in the infrared spectrum of methane, first observed by Coblentz. This band becomes active in the infrared through Coriolis interaction with the frequency ν4. Although no detailed discussion of the structure of this band can be given before the theory has been studied in detail, many of its characteristics can be accounted for. Finally, with the aid of the value of ν2*, the corresponding frequency in the infrared spectrum of CD4, an estimate of the normal frequency ω2 can be made.

INFRARED TARGET AND BACKGROUND RADIOMETRIC MEASUREMENTS-CONCEPTS, UNITS, AND TECHNIQUES

Abstract This report discusses concepts units and techniques for making and describing measurements of radiation from targets and backgrounds.

The Rotational Spectra of Regular and Deuterated Phosphine and Arsine in the Region between 50 and 200 K

The far infrared spectrograph at The Ohio State University was used to obtain the spectra of phosphine and arsine molecules. Tabular and graphical data show series of lines for phosphine, deuterated phosphine, arsine, and deuterated arsine in the ground vibrational state. Two previously unpredicted series of lines are clearly shown in the spectrum of phosphine and indicated in that of arsine. These additional series are ascribed to rotational transitions occurring while the molecules are in excited vibrational states and their rotational constants as well as the equilibrium rotational constants have been computed.

Pure Rotational Spectra of the Partially Deuterated Ammonias in the Far Infrared Spectral Region

Rotational spectra of NH2D and NHD2 have been measured in the spectral range from 30 to 200 cm—1. The inversion splitting of the NH2D lines has been observed. The spectra have been analyzed to obtain term values for many of the low J rotational levels of the ground states of those molecules. The effects of centrifugal distortion on the energy levels is discussed. Molecular constants and dimensions are obtained.