Wavenumber Resolution Research Articles

Apparatus for the study of the IR multiple-photon resonances of polyatomic molecules via the optothermal technique. I. Description of the apparatus

An apparatus for observing the IR multiple-photon resonances of jet-cooled polyatomic molecules using the optothermal technique is described. For experiments in the spectral region 9/11 μm, a commercially available, high-pressure CO2 laser with an improved wave number resolution (∼0.05 cm−1) is used as the pulsed radiation source. Although a moderate pumping capacity is available, high intensity, supersonic beam pulses are obtained by an expressly developed nozzle source. A superconducting bolometer with a responsivity of ∼2×104 V/W and an effective time constant of ≂60 ns (for radiation) has been developed to detect the energy flux carried by the irradiated molecular beam. The time-resolved detector signal is then passed through an analogic processor which separates the fast, small amplitude component generated by the irradiated molecules from the larger component due to the unexcited beam. After further amplification, the useful signal is digitized and stored in a computer to be averaged over several repetitions and then time integrated. A detector input NEP of ∼1.5×10−12 W/(Hz)1/2 is obtained in a single laser pulse. This figure can be further reduced by averaging. Complementary information such as the laser fluence, the intensity of the molecular beam pulse, the optoacoustic signal from a reference gas, as well as the time-of-flight of the molecules can be simultaneously monitored. Part I of this article gives a general description of the apparatus, while Part II gives a detailed description of the bolometer and the related electronics.

Statistical reliability of wavenumber spectrum estimates with crossed-line multiplicative arrays

In a previous paper [J. Acoust. Soc. Am. Suppl. 164, S124, (1978)], it was shown that in a spatially homogeneous noise field the mean response of a multiplicatively processed pair of crossed-line arrays with apertures 2L1 and 2L2 and appropriate shading is identical to the mean-squared response of a rectangular, separably shaded planar array with aperture L1 × L2. Thus the number of elements required for wavenumber spectrum measurement may be considerably reduced (from N2 to 4N for a square array) or the wavenumber resolution may be significantly increased for the same number of elements by the use of multiplicative crossed-line arrays. However, there is an additional trade-off of statistical reliability of the measurement for finite averaging time. This paper compares the variance of the wavenumber spectrum estimate obtained with rectangular planar arrays and with crossed-line multiplicative arrays.

Linked gas chromatography infrared mass spectrometry.

For almost 20 years the idea of linking a gas chromatograph with both an infrared and a mass spectrometer to form an integrated analysis system (GC/IR/MS) has intrigued analytical chemists. For example, in one of the earliest GC/IR papers, which reported GC/IR spectra of 10-18 wavenumber resolution with microgram-level sensitivity, Low and Freeman suggested the potential value of a GC/IR/MS system. Of course, it was obvious then (in 1968) that a number of significant improvements in both IR spectroscopy and computer technology would be required for the practical realization of this visionary suggestion. Although powerful and inexpensive computers easily capable of the experiment control and data reduction needs are readily available today, that was most emphatically not the case in 1968. In fact, at that time computer systems with 8K words (16K bytes) of 16-bit memory equipped with paper tape input-output, a teletype, and modest data acquisition and control capabilities were available for $35,000-$40,000exclamation Programming support was equally limited (e.g., multipass paper tape-oriented FORTRAN compilers, memory resident BASIC compilers that occupied most of memory, or assembly language). Consequently, development of requisite real-time control, data acquisition, data storage, and interpretation software would have been a formidable and extremely expensive undertaking. Mostmore » of these early difficulties have been overcome, and substantial progress has been made. It is therefore timely to review the current status and historical development of GC/IR/MS. The present article should complement the excellent related reviews by Griffiths and co-workers on chromatography-Fourier transform IR (FT-IR) spectrometry and Henion and co-workers on liquid chromatography-mass spectrometry (LC/MS). All these techniques are possible because of major technical advances made during the past 10 years or so.« less

Cryogenic instrumentation and detector limits in FTS

The desire to measure atmospheric emission spectra of the upper atmosphere has led to the development of cryogenically cooled infrared Fourier transform spectrometers. These spectrometers combine the sensitivity advantage of a Fourier transform spectrometer with the sensitivity gains obtainable with modern infrared detectors operated at low photon backgrounds. The temperature required to obtain maximum sensitivity is primarily determined by the wavelength region of interest, longer wavelengths requiring lower temperatures. In order to take full advantage of reduced background operation, the preamplifier must be cooled in order to reduce the noise. Low noise preamplifiers capable of operation down to liquid helium temperatures have been developed. Three cryogenically cooled spectrometers of significantly different design are discussed. The first instrument is a standard Michelson interferometer cooled to 10 K with supercritical helium and capable of 1 wave number resolution. This interferometer has been successfully flown on a rocket and obtained measurements of atmospheric limb emission at tangent altitudes from 70 km to 150 km. The second is a field widened prism interferometer which operates at liquid nitrogen temperature and has been flown on a rocket and obtained measurements of atmospheric emission in the 2 to 8 micrometer region during an aurora. The third is a balloon borne liquid nitrogen cooled interferometer which incorporates cat's eye retroreflectors for end mirrors and is capable of 0.1 wave number resolution. This spectrometer has successfully flown on a balloon several times and obtained measurements of atmospheric limb emission at tangent altitudes from 0 km to 30 km.

外向長波放射と発散風循環との関連について

The wide band high resolution radiometer on the NIMBUS 7 satellite provides a detailed view of the net outgoing long wave radiation around the 11.5μm band. The availability of this data during 1979 made it possible to explore a statistical relationship among the radiance data and the divergent circulations based on the 200mb wind from the analysis of the Global Experiment data sets. This study reveals a very strong relationship between these fields at a horizontal resolution of wave number 8 (two dimensional). Examples of the monthly averaged and daily maps of the divergent wind, observational and statistically derived at this resolution, are compared. The use of this relationship, especially over the data sparse tropical latitudes, for numerical weather prediction and climate studies is suggested.

Improved Wavenumber Resolution for Small Arrays

The objective of the method described is to compute the Fourier transform of a function of space and time with a significant improvement in the resolution of spatial frequencies, particularly in the case of small arrays for which the size of the array is on the order of one wavelength (or less). This improvement is initially developed by considering the conversion of spatial frequency into temporal frequency through the oscillatory motion of a single receiver. A more practical scheme is next presented in which the oscillatory motion is created synthetically by the appropriate sequential sampling of individual elements in a receiving array. Both forms of this method (realand synthetic motion) besides providing some degree of ?super-resolution? are linear and indicative of amplitude as well as phase.

Extended-Range Forecasts for the Southern Hemisphere with the ANMRC Spectral Model

A series of twelve 4-day forecasts for the Southern Hemisphere was made, including both summer and winter cases using a hemispheric spectral model. The model had a resolution of wavenumber 31 in the horizontal and nine levels in the vertical. The model forecasts were verified in terms of various objective measures and a subjective evaluation of the synoptic charts. From this limited study it appears that the winter forecasts are significantly better than the summer forecasts. Given the present data network, analysis procedures and the parameterization of physical processes in the model used, the useful forecast period is about two days for summer and three days for winter.

Cw far-infrared laser scattering from a laboratory plasma

The first far-infrared laser (λ0=1.22 and 0.447 mm) scattering experiment in a laboratory plasma is described. cw laser radiation is scattered at large angles (0 °–20 °) from ion acoustic waves launched in a low-density (ne ≃1010 cm−3) plasma thereby allowing detailed comparison with Langmuir probe data. Measurement of the wave-number resolution and studies of both linear and nonlinear ion waves are presented. Future application areas are briefly described.

Frequency‐wavenumber spectral analysis of borehole acoustic waves

In a laboratory experiment an acoustic pulse excited waves in a fluid‐filled borehole that had been drilled in a solid block of epoxy. Typical wavelengths were the same order of magnitude as the hole radius. Waveforms were received at many different ranges and conventional and high resolution, maximum likelihood methods (MLM) of frequency‐wavenumber (ω, k) spectral analysis are applied to this data. The spectra obtained are compared to theoretical dispersion calculations. Good agreement is obtained. Encouraging improvements in wavenumber resolution over conventional spectral estimation are obtained by the use of the MLM techniques.

Angularly resolved coherent Raman spectroscopy (ARCS)

This new crossed beam configuration produces an angularly separated Coherent Anti-Stokes Raman spectrum, eliminating the need for a spectrometer. Since one of the driving laser beams is always linearly opposed to the dye laser, it may also be used as a dye pump and the experiment performed within the dye cavity for efficient laser energy utilization. With this arrangement, a broad bandwidth dye laser produces a several wavenumber resolution spectrum from a small volume with relatively simple alignment. The spectrum is scanned by varying the incidence angle of the probe beam or taken is a single pulse using a fan shaped probe beam.

Assignments of the Two-Phonon Infrared Absorption Spectrum of LiF

Measurements of the far-infrared lattice absorption in LiF at several temperatures are reported. An approximate calculation of the relative two-phonon absorption intensity has been made considering the anharmonic mechanism only and using the lattice-dynamical data, resulting from a modified deformation-dipole model, of Karo and Hardy. The wave-vector density used was 64000 points per zone (1685 independent wave-vector positions). Such a density with a simple bin-sampling smoothing procedure gave a fairly good wave-number resolution in the computed two-phonon spectra. The agreement with the two-phonon difference absorption presented here and the summation results reported elsewhere is poor, but the discrepancies may be seen to be in the lattice-dynamical data. By identifying the phonon branches responsible for the features in the computed spectrum and considering various criteria necessary for strongly combining branches, it was possible to take the more accurate frequencies recently measured by inelastic neutron scattering and satisfactorily explain the experimental features. Phonon branches of less symmetry than the three major branches were found to contribute appreciably. No evidence was found for sizeable three-phonon absorption in the far infrared.

On the Utilization of a Flexible Bar as a Continuous Spatial Filter

In order to determine empirically the wavenumber-frequency spectra of the wall pressure fluctuations beneath a turbulent boundary layer at nonconvective wavenumbers, a spatial filter with high wavenumber resolution is necessary. Such a filter can be realized by using a continuous system such as a flexible bar or by using a discrete array of finite-sized transducers. The bar offers the advantage that high wavenumber resolution can be achieved without the use of the elaborate equipment necessary for the transducer arrays. In order to use the bar as a spatial filter all that is required is a measurement of the modal displacement response at some spatial position. In this paper, the theoretical formulation of the bar as a continuous spatial filter is developed. Calibration procedures and measurement problems such as those associated with vibrational noise are discussed. Finally, measurements of the low-wavenumber (nonconvective) spectra of the wall pressure fluctuations obtained by using the bar as a spatial filter are reported. One important result of this study is that it demonstrates that the primary wavenumbers associated with the pressure fluctuations contributing to the bar response are not the convective wavenumbers. This implies that Corcos's model, which has been empirically verified only for wavenumbers near the convective wavenumbers [D. M. Chase, J. Acoust. Soc. Amer. 46, 1350–1365 (1969)], may not be adequate for predicting panel response. The measured low-wavenumber (nonconvective) spectra indicate the limitations of Corcos' model in this region. [Work supported by grant from the National Aeronautics and Space Administration.]

High-resolution frequency-wavenumber spectrum analysis

The output of an array of sansors is considered to be a homogeneous random field. In this case there is a spectral representation for this field, similar to that for stationary random processes, which consists of a superposition of traveling waves. The frequency-wavenumber power spectral density provides the mean-square value for the amplitudes of these waves and is of considerable importance in the analysis of propagating waves by means of an array of sensors. The conventional method of frequency-wavenumber power spectral density estimation uses a fixed-wavenumber window and its resolution is determined essentially by the beam pattern of the array of sensors. A high-resolution method of estimation is introduced which employs a wavenumber window whose shape changes and is a function of the wavenumber at which an estimate is obtained. It is shown that the wavenumber resolution of this method is considerably better than that of the conventional method. Application of these results is given to seismic data obtained from the large aperture seismic array located in eastern Montana. In addition, the application of the high-resolution method to other areas, such as radar, sonar, and radio astronomy, is indicated.

Resolution of Wide-range Grating Spectrometers

WHILE it is common experience that the wavenumber resolution of grating spectrometers tends to be constant over a wide wave-length range1, the theoretical basis for this observation does not appear to have been clearly formulated. The reason for this omission is doubtless due to concentration of interest on a particular grating, or set of gratings, and to the diversity of sources of radiation and detectors used for different spectral regions. However, if an unlimited range of gratings be assumed, so that maximum diffracted energy can always be assured, and attention is confined to a black-body source, some simple relationships may readily be deduced.

A double-beam automatic prism-grating infrared spectrophotometer

Abstract The design and performance of a prism-grating, double-beam infrared spectrophotometer primarily intended for chemical analysis is described. Ease of operation, rapidity of scanning, and the presentation of spectra as unbroken recordings on charts of a size to fit standard filing cabinets have retained for this instrument the convenience of the conventional prism spectrophotometer. This has been achieved by completely automatic programming of all the operations for interchanging the three gratings and synchronizing their scan with the foreprism to cover the range from 3800 cm−1 to 450 cm−1 (from 2·6 μ to 22 μ). Two of the gratings are used in the first order only and one in the first and second orders. The spectra are presented linear in transmittance on a scale registered as absorbance, vs. a linear wave-number scale. Scanning time averages about 15 min with automatic scan speed suppression. With the 127 mm x 102 mm gratings a resolution of one-half wave-number can be used throughout the spectrum under routine conditions of signal-to-noise ratio and recording time. For all but the few light gas samples, a standard slit program is used which provides ~1 cm−1 resolution with a signal-to-noise ratio of 600 to 1 at a 1 sec time constant. Under these conditions complete spectral detail is shown and over 90 per cent of the bands are recorded at 95 per cent or greater of their true absorbance value.