John G. Baker

Torsion–Vibration–Rotation Interactions in Methanol. I. Millimeter Wave Spectrum

In a study of internal rotation in methanol, the millimeter wave spectra of CH3OH, CD3OH, and CH3OD have been investigated between 90 and 200 Gc/sec. In the analysis of the spectra, torsion–vibration–rotation interactions were treated as adjustable parameters in semiempirical formulas. Kivelsons formula for a‐type ΔK = 0 transitions was tested over a wide range of quantum numbers. It reproduced the CH3OH and CD3OH spectra quite well, but the approximations used in the calculations appear to start breaking down for the larger asymmetry of CH3OD. For assignment of b‐type ΔK = ± 1 transitions, a method was developed based on the wide spectral range of the millimeter wave spectrometer. Sufficient b‐type data were obtained for CH3OH to permit a test of Kirtmans formula for origins of Q branches. Convergence difficulties in the Q‐branch least‐squares fit prompted a re‐examination of the theory, which revealed an interesting linear relation coupling six of the parameters. This relation shows that for any molec...

Millimeter‐Wavelength Microwave Spectrum of Nitrogen Dioxide

Two low‐N millimeter‐wavelength transitions of 14N16O2 were observed. The 19 additional hyperfine components were combined with previously reported data and hyperfine coupling constants of much greater reliability were obtained. Some improvement in the accuracy of rotational constants was made.

The millimeter-wave spectrum of acetaldehyde in its two lowest torsional states

Abstract A large number of new millimeter-wave spectral lines of gaseous acetaldehyde have been measured at frequencies up to 250 GHz. These lines arise from rotational transitions of acetaldehyde in its two lowest ( v t = 0,1) torsional states and involve angular momentum quantum numbers J ≤ 12 and K ≤ 10. A global data set consisting of 562 lines has been obtained by combining the millimeter-wave lines with previously measured lower frequency data involving the two lowest torsional states. This data set has been analyzed via an internal axis method previously used to study the spectra of CH 3 OH and CH 3 SH. The root-mean-square deviation of the fit is only 685 kHz. An analogous least-squares fit to 335 v t = 0 lines yields a root-mean-square deviation of 269 kHz.

Active millimeter wave detection of concealed layers of dielectric material

Extensive work has been published on millimetre wave active and passive detection and imaging of metallic objects concealed under clothing. We propose and demonstrate a technique for revealing the depth as well as the outline of partially transparent objects, which is especially suited to imaging layer materials such as explosives and drugs. The technique uses a focussed and scanned FMCW source, swept through many GHz to reveal this structure. The principle involved is that a parallel sided dielectric slab produces reflections at both its upper and lower surfaces, acting as a Fabry-Perot interferometer. This produces a pattern of alternating reflected peaks and troughs in frequency space. Fourier or Burg transforming this pattern into z-space generates a peak at the thickness of the irradiated sample. It could be argued that though such a technique may work for single uniform slabs of dielectric material, it will give results of little or no significance when the sample both scatters the incident radiation and gives erratic reflectivities due to its non-uniform thickness and permittivity . We show results for a variety of materials such as explosive simulants, powder and drugs, both alone and concealed under clothing or in a rucksack, which display strongly directional reflectivities at millimeter wavelengths, and whose location is well displayed by a varying thickness parameter as the millimetre beam is scanned across the target. With this system we find that samples can easily be detected at standoff distances of at least 4.6m.

A scanning temperature control system for laser diodes

A computer-adjustable laser diode temperature control system yielding +or-2 mK temperature stability is described. An integrated circuit temperature sensor and a Peltier heater/cooler are used as the basis of this arrangement. Rather than a conventional bridge, a 16-bit A/D converter is incorporated to determine the temperature to high accuracy. The system is designed primarily to generate rapid temperature scans but is also capable of maintaining a set point temperature for long periods.

Quantitative millimetre wave spectrometry. Part I: Design and implementation of a tracked millimetre wave confocal Fabry-Perot cavity spectrometer for gas analysis

This paper describes the design criteria and construction of a compact Fabry-Perot cavity spectrometer with frequency modulation (FM) signal recovery, for use in the 70–220 GHz region. A solid state Gunn oscillator was frequency locked to a radiofrequency synthesiser and used as the primary source. The confocal cavity comprised two brass spherical mirrors, one made part of the sample chamber and the other attached to a precision translation stage with piezoelectric displacement. The sample chamber used a PTFE cup with a central thin window 30 mm diam. supported by the rest of the cup structure which also acted as a spatial filter to suppress non-axial modes. The high quality factor cavity (Q = 1 × 104) was maintained resonant with the source frequency by locking the detected FM signal in phase with the incident FM impressed upon the source, in a control loop which adjusted the cavity dimensions using the piezoelectric transducer. The first harmonic (2F) of the FM (F) was measured to recover the second derivative of the spectral line profile, as the source was scanned across it. Examples of spectra arising from 9 ppm HDO, 2.7 ppm OCS (in an excited vibrational state), HCH18O, (2000 ppm natural abundance), HCH17O (373 ppm natural abundance) and acrylonitrile at approximately 300 ppm, demonstrate the high sensitivity and versatility of the technique.

Quantitative millimetre wave spectroscopy. part II: Determination of working conditions in an open Fabry-Perot cavity

Abstract A confocal Fabry-Perot frequency modulated cavity spectrometer of quality factor 1.25 × 10 5 operating inside a chamber maintained at ambient temperature and pressure of 1 Pa to 1 KPa was employed for spectrometric measurements in the region of 72 GHz and 140 to 160 GHz. The spectrometer used a spatial filter to suppress unwanted, non-axial modes. The solid state microwave source frequency was derived from a phase-locked frequency synthesizer and detection was by a liquid helium cooled bolometer. Transitions in acrylonitrile, formaldehyde, and sulphur dioxide were studied demonstrating parts per million sensitivity for these species in atmospheric samples, whilst carbonyl sulphide samples were detected at sub-parts per million concentration. The effect of pressure on line intensities was studied in order to determine the optimum operating regime. It was found that the technique was not restricted to the 5–50 Pa region characteristic of centimetric wave spectroscopy, but was able also to function in the 0.1 to 1 KPa regime. Furthermore the intensities in this latter region were found to be not critically dependent on sample pressure. A treatment of the effect of pressure and depth of frequency modulation on absorption signals was carried out and the resulting theory applied to the observed intensity-pressure relationships. There was good quantitative agreement between the frequency modulation depth and cavity response characteristics and qualitative agreement between the pressure, frequency modulation and spectral line intensity characteristics. It became clear that the possibility of power saturation, coupled with the non-uniform power distribution within the cavity, was affecting the fits of theoretical curves to the observed data, and that taking this into account produced marked improvement in the fits. Nonetheless the treatment permitted some practically useful conclusions: at low modulation depths and pressures, the sharp spectral absorption peak makes identification of the target species easy, but extraction of quantitative information more difficult, as the intensity will depend critically on the power level in the cavity. At pressures in the 100 Pa region however, the signal obtained is maximal, power broadening minimal and comparative intensity measurements possible over a range of sample species and concentrations.

Quantitative millimetre wave spectroscopy. part III: Theory of spectral detection and quantitative analysis in a millimetre wave confocal Fabry-Perot cavity spectrometer

The quantitative response of a confocal millimetre wave Fabry-Perot cavity spectrometer is described theoretically. The treatment is based on consideration of the cavity as a lossy resonator, with the gas sample acting as an additional loss with a frequency dependence due to its spectral profile. Frequency modulation of the source frequency causes a variation of the transmitted signal due to the changing power level in the cavity as well as that caused by power absorption by the sample as the source is swept across the resonant frequency of each. Fourier analysis of the resulting cavity signals leads to a rather straightforward relationship between the modulated power output from the cavity integrated over the spectral range scanned and the product of the fractional abundance (concentration) of the absorbing species in the cavity with the peak absorption coefficient of a pure sample. The integrated spectral line power absorption was tested as an indicator of gas concentration within the cavity, using as trial samples vibrationally excited states of 14N14N16O and naturally abundant (0.365%) 14N15N16O, each of which display absorptions in the neighbourhood of 176 GHz. The results obtained were at first surprisingly poor, the integration algorithm showing anomalous behaviour at low sample pressures. This was demonstrated to be caused by the integration interval (in this case a frequency increment) being too large to support a 256 point trapezium or Simpsons rule numerical integration procedure. Only when these were replaced by a 8096 point Romberg interactive integration process based on the theoretically computed line profile did the area algorithm become stable at all linewidths tested. Experiments on the modulation depth dependence of the integrated spectral line absorption displayed further anomalies in the line area to cavity background ratio possibly caused by the pulling of a cavity resonance containing a spectral line during our frequency sweep. These effects were intriguing rather than serious, and the experiments did indicate a region for which the ratio was almost independent of the modulation depth at a constant pressure of 7 Pa, varying for a range of modulation from 0.132 MHz to 1.32 MHz by only a factor of 4 in the worst case (14N15N16O) and 1.4 in the best (14N14N16O in the 01−10 state). Measurements were conducted by diluting N2O at 7 Pa by addition of air up to 92 Pa. The integrated spectral line absorption for the mixtures rose gradually from a value of 0.99 for the pure sample to 1.92 for a sample containing 7 Pa N2O + 32 Pa air, and then dropped to 1.64 again in the high pressure limit (7 Pa N2O + 92 Pa air). This drop can be explained by the frequency scan not encompassing the broadened wings of the spectral line, but the initial increase in the lower pressure range is not readily explained. Various procedures and algorithms were essayed to improve the signal to noise and background ratios. Savitzky-Golay, Gaussian and derivative Gaussian convolutions all serve to highlight the spectral features, but suffer from the inherent problem that those that remove the background also set the integral over these features to zero. Further studies are addressing this dilemma. In spite of its imperfections, the theoretical model used has enabled the definition of an operating regime in which absorption signals are almost independent of the instrument parameters. It also accounts correctly for the observed amplitudes and shapes of these spectra over a wide range of modulation depths and sample pressures.

The near infrared overtone spectrum of propyne taken by diode laser in the 12700 cm−1region

Detailed regions of the near infrared spectrum of propyne between 12737 cm-1 and 12778cm-1 have been measured using a simple and inexpensive home-made laser diode spectrometer. This part of the spectrum covers the overlapping 3v1 + v3 + v5 and 3v1 + v3 + v5 + v10—v10 bands. Combining a global fit and an individual profile analysis made possible the determination of the vibrational and rotational constants in each band and in some cases the resolution of the individual K structure in each transition cluster.

Frequency stabilization of a 0.78 mu m GaAlAs laser diode to acetylene absorption lines

A GaAlAs single mode laser diode operated in the 0.78 mu m region has been stabilized to the spectral peak of an overtone absorption in acetylene with a long-term frequency stability of 0.8 MHz. This represents a hundred fold improvement in the performance provided by the current and temperature control of the laser diode.