Wave Spectrometer Research Articles

Submillimeter wave spectrum of the SH +3 ion. J = 2-1, K = 0 and 1 transitions

The observation of the rotational transitions of SH + 3 has been extended to the 600 GHz region. The SH + 3 ion was generated by an electric dc discharge in the gas mixture of H 2S and H 2 in a hollow-cathode discharge cell. The J = 2-1, K=0 and 1 transitions have been observed using a source-modulated submillimeter wave spectrometer. The rotational constant B 0 and the centrifugal distortion constants D j and D JK are determined from the three observed rotational transitions. The molecular constants determined agree well with those reported by previous infrared studies.

Airborne radar measurements of ocean wave spectra and wind speed during the grand banks ERS‐1 SAR wave experiment

Abstract Measurements of ocean directional wave spectra, significant wave height, and wind speed over the Grand Banks of Newfoundland were made using the combined capabilities of the radar ocean wave spectrometer (ROWS) and scanning radar altimeter (SRA). The instruments were flown aboard the NASA P‐3A aircraft in support of the Grand Banks ERS‐1 Synthetic Aperture Radar (SAR) Wave Experiment. The NASA sensors use proven techniques, which differ greatly from SAR, for estimating the directional long‐wave spectrum; thus they provide a unique set of measurements for use in evaluating SAR performance. ROWS and SRA data are combined with spectra from the SAR aboard the Canadian Centre for Remote Sensing (CCRS) CV‐580 aircraft, the first‐generation Canadian Spectral Ocean Wave Model (CSOWM) hindcast, and other available in situ measurements to assess the ERS‐1 SAR's ability to correctly resolve wave field components along a 200‐ to 300‐km flight line for four separate satellite passes. Given the complex seas pr...

Airborne measurements of the ocean's Ku‐band radar cross‐section at low incidence angles

Abstract Ocean backscatter data obtained with a Ku‐band airborne radar are presented along with coincident altimeter and directional wave spectral estimates. These data were collected using one sensor, NASA's radar ocean wave spectrometer (ROWS). The measurements are compared with an electromagnetic scattering model for perfectly conducting Gaussian random surfaces. The normalized radar cross‐section (NRCS) data cover those incidence angles (0–20°) where both quasi‐specular and Bragg scattering mechanisms are expected. Under certain conditions, identification and separation of these two mechanisms is possible. The scanning radar allows observations of the azimuthal variations in NRCS that are at times indicative of short‐scale wave generation in the wind direction.

Study of the conformational equilibrium of 1-chlorobutane by free-jets and conventional microwave spectroscopy

The microwave spectrum of 1-chlorobutane has been investigated with a free-jet millimetre wave spectrometer in the frequency range 60–78 GHz. Additional information has been obtained using a conventional spectrometer in the frequency range 28–40 GHz. The rotational spectra have been assigned to three conformers.

Optical control of microwaves with semiconductor n-i-p-i structures

We control the microwave transmission of a GaAs n-i-p-i structure by illuminating it with a cw argon ion laser. Tests in a broadband microwave modulator wave spectrometer show that an optical intensity of 800 mW/cm2 produces a 50% change in transmission for microwaves between 10 and 50 GHz.

Radar measurement of directional ocean wave spectra at low incidence angles†

Abstract There exist several techniques for the measurement of directional ocean wave spectra. The most conventional technique is the employment of pitch-roll buoys but a disadvantage of this technique is that it measures at a fixed point. Another promising technique is the use of airborne or spaceborne radars. We present here results of a comparison between the data from an airborne radar, which is measuring near vertical incidence, and measurements of directional wave spectra obtained by means of a pitch-roll buoy and processed by using the Long-Hasselmann iterative algorithm. Although preliminary, these results constitute a step towards the employment of the airborne radar based on the Radar Ocean Wave Spectrometer (ROWS) principle as defined by Jackson (1981), as a validation tool for spaceborne synthetic aperture radars.

An untuned cavity as a high effective temperature millimetre wave calibration source

An untuned cavity has been developed and characterized as a microwave source of high effective radiometric temperature. This source is intended for use in the calibration of the Fabry-Perot millimetre wave spectrometers in use on the Joint European Torus (JET). Radiation input to the cavity is provided by backward wave oscillators operating between 100 and 170 GHz at a typical bandwidth of 0.5 GHz. The cavity is shown to give an isotropic output which simulates that from a black body source observed with a bandwidth smaller than the resolution of the spectrometer.

Rotational spectra of p-isopropylbenzaldehyde: assignment of two conformers and observation of torsionally excited states

Rotational spectra of p-isopropylbenzaldehyde were obtained at a very low temperature in a pulsed-beam Fourier transform-microwave spectrometer and over a range of much higher temperatures in a continuous wave spectrometer. Two species with ground-state rotational constants A=2975.15(14) MHz, B=550.1631(1) MHz, C=516.3000(1) MHz and A=3183.80(30) MHz, B=538.8124(1) MHz, C=512.0049(1) MHz were observed in the beam and are consistent with structures having the methine hydrogen coplanar with the benzene ring and respectively anti and syn to the carbonyl oxygen of the para formyl group. Relative intensities of the rotational transitions indicate that the two conformers have almost equal energies. Near room temperature, in addition to R-branch a-type band series consistent with the anti and syn conformers, a very intense band series with an intermediate B+ C value is observed in the low-resolution microwave spectrum of p-isopropylbenzaldehyde, as previously reported [N.S. True, J. Phys. Chem., 87 (1983) 4622]. Temperature-dependent relative intensity measurements do not support the proposal of these authors that the intensity of this series reflects the population in torsional states above the torsional barrier and can be used to estimate the internal rotation barrier height. Additional factors not directly related to the free rotor population must contribute to the observed intensity.

Rotational spectra of p-anisaldehyde. Assignment of the planar conformers and observation of torsionally excited states

Rotational spectra of p-anisaldehyde were obtained at a very low temperature in a pulsed beam Fourier transform-microwave spectrometer and over a range of much higher temperatures in a continuous wave spectrometer. Two species with ground state rotational constants A = 4661.95(4) MHz, B=698.3951(1) MHz and C=610.0577(1) MHz, and A=4160.64(2) MHz, B=717.8407(1) MHz and C=614.8932(1) MHz, were observed in the beam and are consistent with planar anti and syn conformers, respectively. Relative intensities of the rotational transitions indicate that the two planar conformers have nearly equal energies. The anti and syn conformers produce low-resolution microwave (LRMW) band series with B+ C values of 1311.2(5) MHz and 1334.9(3) MHz, respectively. A third band series with an intermediate B+ C value, 1323.2(2) MHz, is observed in the LRMW spectrum of p-anisaldehyde. At 298 K the third series is the most intense spectral feature and its relative intensity doubles with a 50° increase in temperature. This temperature dependence precludes assigning the third series to a third conformational species. This series may originate from many weakly bound and/or freely rotating torsional satellites with similar rotational constants.

A high-resolution Raman gain spectrometer for spectral lineshape studies

The details and performance of a shot-noise-limited high-resolution (≤ 2 MHz) Raman gain spectrometer are discussed and compared to a "standard" continuous wave (cw) spectrometer. For a single pass cell and 1 s integration time, a signal-to-noise ratio greater than 1000 is obtained for the Q(2) line of D2 at pressures above a few atmospheres. The quality of the spectrometer for the determination of spectral profiles is demonstrated by examining the Dicke narrowing of the line.

Rotational spectra of benzyl cyanide assignment of the planar conformer and evidence of a low barrier to internal rotation

Rotational spectra of benzyl cyanide were obtained at a very low temperature in a pulsed beam Fourier transform microwave spectrometer, and over a range of much higher temperatures in a continuous wave spectrometer. The ground state rotational constants, A = 4346.815 (5) MHz, B = 1057.373(3) MHz and C=856.642(3) MHz, and five centrifugal distortion constants were derived from transitions measured between 7.5 and 17 GHz and are only consistent with a heavy-atom planar structure. The 14N quadrupole coupling constant, X aa = −1.313(2) MHz and X bb = −0.657 (3) MHz, is also only consistent with a planar heavy-atom structure if the electric field gradient in the CN group is assumed cylindrically symmetrical with X zz = −3.9 MHz. Temperature dependent low resolution microwave spectra between 18 and 26.5 GHz display R-branch a-type bands from two types of species. The planar ground state produces a band series with B + C = 1927(2) MHz. At 300 K three series of bands of steadily diminishing intensity are displaced to higher frequency from the ground state bands; they are assigned to three torsionally excited species. Relative intensities of these satellites indicate a low internal rotation barrier. Superposed on this satellite pattern are broader bands of another species whose characteristics are incompatible with another conformational isomer. Its intensity relative to the series assigned to the ground state increases from 0.5 to 4 when the temperature is increased from 280 to 360 K. This series may originate from many weakly bound and/or freely rotating torsional satellites with similar rotational constants.

Estimating aircraft SAR response characteristics and ocean wave spectra in the Labrador Sea Extreme waves Experiment

The Labrador Extreme Waves Experiment (LEWEX) was an international effort to assess methods of measuring and modeling the directional aspects of wind-generated ocean waves, especially their evolution in the presence of rapidly turning winds. The author describes data-processing methods that have been developed to derive estimates of two-dimensional wave height-variance spectra from the ocean imagery obtained in LEWEX by a C-band synthetic aperture radar (SAR) system aboard a Canadian CV-580 aircraft. SAR spectra at altitudes of 20000 and 12000 ft are compared for both large ( approximately 50 degrees ) and small ( approximately 25 degrees ) radar look angles. Independence spectral estimates from both a surface contour radar and a radar ocean wave spectrometer aboard a NASA aircraft are used to verify the surface wave spectrum.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

A time-resolved study of rotational energy transfer into A and E symmetry species of 13CH3F

Rotational energy transfer processes into the A and E symmetry species of the symmetric top molecule 13CH3F have been studied. In this time-resolved double resonance experiment a tunable millimeter/submillimeter wave spectrometer was used to monitor the change in strength of rotational transitions in the ν3 vibrational state after a Q-switched CO2 laser pumped the K=3, J=5 level in ν3. A simple numerical simulation of rotational energy transfer allowed the 13CH3F system to be modeled and collisional energy transfer rates to be obtained from the data. Two important processes were studied. The first, a process that obeys the spin statistic selection rule ΔK=3n has a rate of 29±6 ms−1 mTorr−1. The second, a vibrational quantum number swapping collision that effectively transfers population between the A and E symmetry species and thereby transcends the spin statistic selection rule, has a rate of 6.6±0.7 ms−1 mTorr−1, about 1.4 gas kinetic collisions. The numerical simulations and these rates, along with previous measurements of the ΔJ=±1 rate and vibrational decay rates, provide an accurate characterization for a large body of varied experimental data.

Comparison of Shuttle Imaging Radar‐B ocean wave image spectra with linear model predictions based on aircraft measurements

During October 1984, coincident shuttle imaging radar‐B synthetic aperture radar (SAR) imagery and wave measurements from airborne instrumentation were acquired. The two‐dimensional wave spectrum was measured by both a radar ocean wave spectrometer and a surface contour radar aboard the aircraft. In this paper we compare two‐dimensional SAR image intensity variance spectra with these independent measures of ocean wave spectra to verify previously proposed models of the relationship between such SAR image spectra and ocean wave spectra. The results illustrate both the functional relationship between SAR image spectra and ocean wave spectra and the limitations imposed on the imaging of short‐wavelength, azimuth‐traveling waves.

Numerical Simulation of Synthetic Aperture Radar Image Spectra for Ocean Waves

A numerical model for predicting the synthetic aperture radar (SAR) image of a moving ocean surface is described, and results are presented for two SIR-B data sets collected off the coast of Chile. Wave height spectra measured by the NASA radar ocean wave spectrometer (ROWS) were used as inputs to this model, and results are compared with actual SIR-B image spectra from orbits 91 and 106. Additional parametric variations are presented to illustrate the effects of nonlinearities in the imaging process.

Frequency modulated (FM) time delay photoacoustic and photothermal wave spectroscopies. Technique, instrumentation, and detection. Part III: Mirage effect spectrometer, dynamic range, and comparison to pseudo-random-binary-sequence (PRBS) method

A detailed comparison is presented between the time delay and the pseudo-random-binary-sequence (PRBS) methods of excitation and mirage effect response. Both time delay domain dynamic system response and frequency domain spectral functions were calculated via FFT methods. The results show that the FM time delay spectrometer exhibits superior performance to the PRBS device and is optimally suitable for nondestructive and depth-profiling studies. The detailed examination of, and comparison between, the time delay and spectral dynamic functions of our FM time delay photothermal wave spectrometer and those of a PRBS-driven device has proven that the former apparatus is capable of producing superior quality time delay and spectral function information when tested on a fast, flat frequency response mirage effect system. The FM time delay photoacoustic/photothermal wave technique and instrumentation of this work holds excellent promise for nondestructive evaluation and depth-profiling applications in scientific research as well as industrial development laboratories.

Measurements of ocean wave spectra and modulation transfer function with the airborne two‐frequency scatterometer

The results of this research show that the directional spectrum and the microwave modulation transfer function (MTF) of ocean waves can be measured with the airborne two‐frequency scatterometer‐microwave resonance technique. By combining two‐frequency backscatter resonance intensity with estimates of the directional wave spectrum (obtained by the airborne surface contour radar) and the known radar beam geometry, it was possible to solve for the unknown MTF using a system equation that has been verified in previous studies. Similar to tower‐based operations, the aircraft measurements of the MTF show that it is strongly affected by both wind speed and sea state. Also detected were small differences in the magnitudes of the MTF between downwind and upwind radar look directions, and variations with ocean wave number. Unexpected results were obtained, which indicate the MTF inferred from the two‐frequency radar is larger than that measured using single‐frequency/wave orbital velocity/Doppler techniques such as tower‐based radars or ROWS (remote ocean wave spectrometer) from low‐altitude aircraft. Possible reasons for this are discussed. When independent MTF data are combined with two‐frequency resonance intensities over a suitable variety of azimuthal look directions, then estimates of the surface wave directional spectrum can be made, to within the accuracy of the uncertainties in the individual data sets.

Delayed Double Resonance Between Quadrupolar Levels: Observation of a Nuclear Spin Emission Signal in s-Triazine

A delayed double resonance experiment was carried out using a Robinson-type continuous wave spectrometer. The sample chosen was s-triazine at liquid helium temperature, where the relaxation times are of the order of ten hours. Line pairings between the two Nitrogen-14 NQR sites were confirmed. Emission signals from ν- transitions were observed after successively saturating first the ν- line and then the ν+ line. The results are understood in terms of a simple model of spin population dynamics.

Self- and air-broadened linewidth of the 183 GHz absorption in water vapor

The linewidth of the 3 1.3 ← 2 2.0 rotational transition of water vapor at 183 GHz has been investigated using a video millimeter wave spectrometer equipped with a signal digitizer. Self- and air-broadening parameters have been obtained for four temperatures over the 299−251 K range; the results are compared with previous experimental and theoretical data.

A comparison of in situ and airborne radar observations of ocean wave directionality

The directional spectrum of a fully arisen ∼3 m sea as measured by an experimental airborne radar, the NASA Ku‐band radar ocean wave spectrometer (ROWS), is compared to reference pitch‐roll buoy data and to the classical SWOP (stereo wave observation project) spectrum for fully developed conditions. The ROWS spectrum, inferred indirectly from backscattered power measurements at 5‐km altitude, is shown to be in excellent agreement with the buoy spectrum. Specifically, excellent agreement is found between the two nondirectional height spectra, and mean wave directions and directional spreads as functions of frequency. This agreement is found despite certain discrepancies between the radar and buoy angular harmonics which are believed to be due to buoy instrumental effects. A comparison of the ROWS and SWOP spectra shows the two spectra to be very similar, in detailed shape as well as in terms of the gross spreading characteristics. Both spectra are seen to exhibit bimodal structures which accord with the Phillips' resonance mechanism. This observation is thus seen to support Phillips' contention that the SWOP modes were indeed resonance modes, not statistical artifacts.