A Stark-effect microwave spectrograph of high sensitivity.
01 Nov 1949-Review of Scientific Instruments (American Institute of Physics)-Vol. 20, Iss: 11, pp 821-826
TL;DR: A microwave spectrograph is described in which the absorption of the gas being studied is modulated by the application of a periodic Stark‐effect field so that the output of the crystal detector contains a radiofrequency component which can be amplified with a narrow‐band amplifier.
Abstract: A microwave spectrograph is described in which the absorption of the gas being studied is modulated by the application of a periodic Stark‐effect field so that the output of the crystal detector contains a radiofrequency component which can be amplified with a narrow‐band amplifier. By using a very narrow band‐width, achieved by employing phase‐sensitive detection, high sensitivity is obtained. The klystron is swept mechanically over the frequency range. Details of the components are given.
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TL;DR: The CP-FTMW spectrometer produces an equal sensitivity spectrum with a factor of 40 reduction in measurement time and a reduction in sample consumption by a factors of 20, and displays good intensity accuracy for both sample number density and rotational transition moment.
Abstract: Designs for a broadband chirped pulse Fourier transform microwave (CP-FTMW) spectrometer are presented. The spectrometer is capable of measuring the 7-18 GHz region of a rotational spectrum in a single data acquisition. One design uses a 4.2 Gsampless arbitrary waveform generator (AWG) to produce a 1 mus duration chirped pulse with a linear frequency sweep of 1.375 GHz. This pulse is sent through a microwave circuit to multiply the bandwidth of the pulse by a factor of 8 and upconvert it to the 7.5-18.5 GHz range. The chirped pulse is amplified by a traveling wave tube amplifier and broadcast inside the spectrometer by using a double ridge standard gain horn antenna. The broadband molecular free induction decay (FID) is received by a second horn antenna, downconverted, and digitized by a 40 Gsampless (12 GHz hardware bandwidth) digital oscilloscope. The second design uses a simplified pulse generation and FID detection scheme, employing current state-of-the-art high-speed digital electronics. In this spectrometer, a chirped pulse with 12 GHz of bandwidth is directly generated by using a 20 Gsampless AWG and upconverted in a single step with an ultrabroadband mixer. The amplified molecular emission is directly detected by using a 50 Gsampless digital oscilloscope with 18 GHz bandwidth. In both designs, fast Fourier transform of the FID produces the frequency domain rotational spectrum in the 7-18 GHz range. The performance of the CP-FTMW spectrometer is compared to a Balle-Flygare-type cavity-FTMW spectrometer. The CP-FTMW spectrometer produces an equal sensitivity spectrum with a factor of 40 reduction in measurement time and a reduction in sample consumption by a factor of 20. The CP-FTMW spectrometer also displays good intensity accuracy for both sample number density and rotational transition moment. Strategies to reduce the CP-FTMW measurement time by another factor of 90 while simultaneously reducing the sample consumption by a factor of 30 are demonstrated.
493 citations
TL;DR: In this article, the microwave spectra of formamide, and the isotopic species 1, 2 N di-deutero formamide (cis) and (trans), have been investigated.
Abstract: The microwave spectra of formamide, and the isotopic species 1, 2 N di‐deutero formamide, 1 N mono‐deutero formamide (cis) and (trans), have been investigated. The rotational constants obtained from the frequencies of assigned rotational transitions lead to the following effective structural parameters: for the bond distances, there results C–O = 1.243±0.007 A, C–N = 1.343±0.007 A, N–H = 0.995±0.007 A, and for the bond angles ∠HNH = 118.98±0.50°, ∠ NCO = 123.58±0.35°, ∠NCH = 103.9±1.2°. The molecule is planar. Stark effect measurements yield values for the dipole moment components which, when combined with assumed approximate bond moments and the structural parameters, give the magnitude and the direction of the dipole moment. The dipole moment is thus equal to 3.714±0.06 Debyes and makes an angle of 39.6° with the C–N bond. The quadrupole coupling constants, Xa = 1.9 mc, Xb = 1.7 mc, Xc = ‐3.6 mc, were calculated from the frequency shifts of the hyperfine splittings observed for several rotational transi...
257 citations
TL;DR: The microwave spectra of five further isotopic species of nitric acid, H18ONO2, cis-HON18OO, trans−HONO18O, cis−HO15N18OO and D15NO3, have been investigated as mentioned in this paper.
Abstract: The microwave spectra of five further isotopic species of nitric acid, H18ONO2, cis‐HON18OO, trans‐HONO18O, cis‐HO15N18OO, and D15NO3, have been investigated. The data have been used to refine the previous structural determination of Millen and Morton. In the calculation of structure, particular consideration has been given to the use of ground‐state moments of inertia in Kraitchman's equations for near‐oblate planar molecules. The structure was calculated using several different methods, including the double‐substitution method due to Pierce, in order to substantiate the following structural parameters: O–H=0.964, (H)O–N=1.406, cis–N–O=1.211, trans‐N–O=1.199 A, ∠HON=102°9′, ∠cis‐ONO=115°53′, ∠trans‐ONO=113°51′, and O···O (NO2 group)=2.184 A.This determination clearly demonstrates a 2° tilt of the NO2 group away from the hydrogen atom. The NO2 parameters are close to those found in a number of related molecules, but there is some evidence that the cis‐NO bond in HNO3 is slightly longer than the trans‐NO b...
156 citations
TL;DR: In this paper, the normal propyl fluoride molecule has been confirmed by microwave spectroscopy to exist in two rotational isomers, trans and gauche, and the barrier height hindering methyl rotation has been determined, using lines in the first excited state of methyl rotation, to be 2.69 kcal/mole.
Abstract: The normal propyl fluoride molecule has been confirmed by microwave spectroscopy to exist in two rotational isomers, trans and gauche. Rotational constants in the ground vibrational state are A = 26 986.73, B = 3748.32, C = 3509.88 Mc/sec for the trans form and A = 14 503.69, B = 5085.71, C = 4295.28 Mc/sec for the gauche form. The dihedral angle of the gauche form is about 63° from the cis position.The barrier height hindering methyl rotation has been determined, using lines in the first excited state of methyl rotation, to be 2.69 kcal/mole and 2.87 kcal/mole for the trans and the gauche forms, respectively. The dipole moment and its components along the principal axes have been determined by the Stark effect: μa = 1.970±0.026, μb = 0.566±0.064, μtotal = 2.050±0.043D for the trans form and μa = 1.137±0.008, μb = 1.450±0.033, μc = 0.472±0.137, μtotal = 1.902±0.064 D for the gauche form. The total dipole moment is nearly parallel to the C–F bond for both forms; the angle between them is 10.6° in the trans...
129 citations
TL;DR: The 3-fluoropropene molecule has been confirmed by microwave spectroscopy to exist in two rotational isomers, cis and gauche as mentioned in this paper, and the molecular structure is determined separately for the two isomers.
Abstract: The 3‐fluoropropene molecule has been confirmed by microwave spectroscopy to exist in two rotational isomers, cis and gauche. Rotational constants in the ground vibrational state are A = 17236.638, B = 6002.915, C = 4579.827 Mc/sec for the cis and A = 27720.349, B = 4263.628, C = 4131.984 Mc/sec for the gauche form. Rotational transitions are also assigned for isotopic species, eight of the cis and nine of the gauche form. By using Costain's method the molecular structure is determined separately for the two isomers. The dipole moment and its components along the principal axes of inertia are μa=0.742±0.008, μb=1.601±0.011, μtotal=1.765±0.014 D for the cis and μa=1.595±0.006, μb=0.908±0.012, μc=0.623±0.014, μtotal=1.939±0.015 D for the gauche form. Through the temperature dependence of the intensity ratio the cis form is found more stable than the gauche form by 166±67 cal/mole.The energies of the first and the second excited states of the C–C torsion are determined by the relative intensity measurement t...
127 citations
References
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TL;DR: In this paper, the rotational spectra of a number of isotopes and vibrational states of OCS, C1CN, BrCN, and ICN near one centimeter wave-length have been made.
Abstract: Measurements of the pure rotational spectra of a number of isotopes and vibrational states of OCS, C1CN, BrCN, and ICN near one centimeter wave-length have been made. Experimental techniques used and interpretation of the spectra of linear $\mathrm{XYZ}$ molecules are discussed. Tables include frequencies and intensities of lines and comparison with theoretical values, rotational constants ${B}_{0}$, rotation-vibration constants $\ensuremath{\alpha}$, $l$-type doubling constants, internuclear distances, half-width parameters of lines, quadrupole coupling constants, and nuclear quadrupole moments. Agreement between experimental results and available theory is good in all cases except for the values of $l$-type doubling constants.
86 citations
TL;DR: In this article, the authors investigated whether other gases show similar absorption for microwaves and as a result fourteen additional gases have been found whose absorption is comparable to that of ammonia, including dimethyl ether, a variety of amines and alkyl halides.
Abstract: The absorption by ammonia of electromagnetic waves having a length in the one‐centimeter range has been known for some years. An investigation has been made to determine whether other gases show similar absorption for microwaves and as a result fourteen additional gases have been found whose absorption is comparable to that of ammonia. Among these gases are dimethyl ether, a variety of amines and alkyl halides, and several others. Measurements on the absorption coefficient and dielectric constant of these gases taken at 1.25 cm at room temperature and a pressure of one atmosphere are given. The frequency at which the absorption coefficient attains its maximum value may be inferred from the curve: absorption coefficient vs. pressure. Data on the absorption of several gas mixtures are given. Possible molecular mechanisms adequate to account for the large absorptions observed are discussed together with the conclusions reached.
31 citations
TL;DR: In this article, the minimum detectable absorption coefficient for an isolated spectral line as measured in a matched wave guide of optimum length is [4kTNΔf/(P0/e)]αc, where N is the noise factor and Δf the band width of the receiver, αc is the attenuation constant for the wave guide, and P 0/e is the average microwave power available at the receiver.
Abstract: It is shown that the minimum detectable absorption coefficient for an isolated spectral line as measured in a matched wave guide of optimum length is [4kTNΔf/(P0/e)]αc, where N is the noise factor and Δf the band width of the receiver, αc is the attenuation constant for the wave guide, and P0/e is the average microwave power available at the receiver. The equivalence between cavity and wave guide methods is developed. The Stark modulation method of Hughes and Wilson is analyzed, assuming a quadratic Stark effect, and expressions are developed for the modulation factor of a microwave at twice and four times the frequency of the Stark field. For a matched guide the modulation factor is αgl/4. Also the shift in line position is calculated, and finally, the directions in which improvement in sensitivity may be expected are indicated.
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