Millimeter-wave Spectrometer Research Articles

Remote determination of oxygen and water at millimetre wave frequencies using a fibre optic communications network

This paper describes the development of a remote millimetre wave (MMW) spectrometer capable of operation in the 57–66, 114–128 and 171–189 GHz bands. A 9.5–10.5 GHz signal from a yttrium iron garnet (YIG) source is carried via an infrared (IR) laser down a 1 km fibre-optic cable using a high-speed communications modulator. The 6th harmonic of the transmitted microwave signal is generated directly with an active sextrupler, which permits working in the 57–66 GHz band. For operation at 114–128 and 171–189 GHz, the 57–66 GHz output from the sextrupler is doubled or tripled by a further harmonic generator. Absorption line strength measurements and hence sample concentration determinations are undertaken using a Fabry–Perot cavity absorption cell. The spectroscopic data are recovered from the remote spectrometer by transmitting the rectified signal back over a further fibre-optic cable. Also described are the methods of cavity stabilisation and control across this fibre optic network. Oxygen determinations in the 57–66 and 114–128 GHz bands are performed to evaluate the performance of the spectrometer. A determination of water vapour in air at atmospheric pressure, at 183 GHz, is also presented, over a range of ∼5×10 −5 to ∼0.025 volume fraction in air.

Quantitative Millimetre Wavelength Spectrometry

Quantitative Millimetre Wavelength Spectrometry

Fourier transform millimeter-wave spectroscopy of the deuterated vinyl radical, C2D3

The 101–000 rotational transition of the C2D3 radical in the ground electronic state has been detected for the first time with the Fourier transform millimeter-wave spectrometer. The C2D3 radical is produced by discharging the C2D3Br gas diluted in Ar. Thirty-two fine and hyperfine components of the 101–000 transition are measured in the frequency region around 44.4 GHz. The rotational constant (B+C)/2, the spin-rotation interaction constant, and the hyperfine interaction constants for the s and a states split by the tunneling motion associated with the CDα rocking mode are determined by a least-squares analysis. The energy splitting due to the tunneling motion is discussed on the basis of the hyperfine interactions. The angle between the CDα bond and the a-axis is estimated to be 148.5° from the nuclear quadrupole interaction constant χaa.

Submillimeter-wave spectra of HCS and DCS

The a-type R-branch K−1=0 rotational transitions of the HCS and DCS radicals have been measured in the frequency range of 161 to 644 GHz using source modulation spectrometers. For DCS, the seven fine and hyperfine components of the 101–000 rotational transition are also measured at 35 GHz using a Fourier transform millimeter-wave spectrometer. The spectra are found to be perturbed by the K−1=1 state through the off-diagonal spin–rotation interaction (εab+εba)(NaSb+SbNa+NbSa+SaNb). In particular for DCS, strong perturbations are observed. The rotational constants, A, B+C, and B−C, of DCS are determined through an analysis of the perturbation. The r0 structure of HCS has been determined as follows: r0(CH)=1.079(3) Å, r0(CS)=1.562 28(3) Å, and α0(HCS)=132.8(3)°. The quasilinearity parameter, γ0, is evaluated to be 0.80 for DCS, indicating that HCS is not a simple bent molecule.

Hyperfine Interaction Constants of the HCCS and DCCS Radicals Studied by Fourier Transform Millimeter-Wave Spectroscopy

The rotational spectral lines of HCCS and DCCS have been observed with a Fourier transform millimeter-wave spectrometer in combination with a pulsed discharge nozzle. The HCCS radical is produced by discharging a mixture of C2H2 and CS2 diluted in Ar. The DCCS radical is produced by using C2D2 instead of C2H2. The spectral lines of HCCS and DCCS in both the 2Π3/2 and 2Π1/2 states are measured in the frequency range from 16 to 48 GHz, and the molecular constants are determined accurately from a joint least-squares analysis with the reported millimeter- and submillimeter-wave data. The hyperfine interaction constants of the hydrogen and deuterium nuclei are determined for the first time, and are discussed in relation to the Renner–Teller effect on this molecule.

Fourier transform millimeter-wave spectroscopy of chlorocarbene (HCCl)

The 101–000 and 202–101 rotational transitions of HC35Cl and HC37Cl in the X̃ 1A′ ground vibronic state have been observed with a Fourier transform millimeter-wave spectrometer. The HCCl molecule is produced by discharging a gaseous sample of CH2Cl2 diluted in Ar with a pulsed discharge nozzle. The effective rotational constant (B+C)/2, the centrifugal distortion constant ΔJ, the nuclear quadrupole interaction constants, and the nuclear-spin rotation interaction constant are determined for each isotopic species. The nuclear-spin rotation interaction is found to make a significant contribution to the hyperfine structure of this molecule, which originates from the relatively low-lying electronic excited state. The nuclear quadrupole interaction tensor is highly asymmetric, indicating a significant π character of the C–Cl bond. This can be interpreted in terms of the backdonation of π electrons from the chlorine atom to the carbon atom.

Microwave spectrum of the CH3CCS radical in the E3/22 electronic ground state

The rotational spectrum of a symmetrical-top radical CH3CCS in the E3/22 electronic ground state has been detected by microwave spectroscopy. The CH3CCS radical is produced by discharging a gaseous mixture of CH3CCH and CS2, and 50 spectral lines with the K range from −4 to 6 are observed in the 132–261 GHz region with a source modulation millimeter-wave spectrometer. In addition, 19 low-J lines with K=0 and 1 are measured in the 11–21 GHz region with a Fourier transform microwave spectrometer. Ten molecular constants including hyperfine interaction constants are determined accurately by a least-squares method. The obtained hyperfine constants are found to be about a half of those of a related radical, CH3S, indicating that the structure of CH3CCS would be expressed in a classical resonant form; H3C–C≡C–Ṡ↔H3C–Ċ=C=S.

Millimeter-wave intracavity-jet OROTRON-spectrometer for investigation of van der Waals complexes

A highly sensitive intracavity millimeter-wave spectrometer was developed for the investigation of the absorption spectra of van der Waals complexes in a supersonic jet. The key element of the spectrometer is a tunable oscillator, called OROTRON, which generates the millimeter-wave radiation through the interaction of an electron beam with the electromagnetic field of a high quality (Q≈104) Fabry–Perot resonant cavity. This cavity consists of a movable spherical mirror and a fixed planar mirror with the periodic structure imprinted on its surface. The electron beam moves along the periodic structure of the plane mirror. This part separated from the rest of the resonator by a mica foil is kept under ultrahigh vacuum conditions. The molecular jet is injected by a pulsed valve into the other part of the resonator. The absorption in the jet is sensitively detected by measuring the electric current in a special collector circuit of the OROTRON. The spectral purity of the OROTRON radiation is 10–15 kHz providing the capability of sub-Doppler spectral resolution without phase locking. An increase in sensitivity of a factor of about 100 in comparison with the usual single pass arrangement was evaluated from the measurements of the absorption lines of the CO rare isotopomers, the Ar–CO and Ne–CO van der Waals complexes. The high sensitivity, wide spectral range, and simple tunability of the spectrometer make it a very efficient tool for the searching of weakly absorbing species in a jet.

Microwave Spectrum of the FCO Radical in the 2A′ Electronic Ground State

The 101–000, 202–101, and 303–202 rotational transitions of the FCO radical are observed at 22.3, 44.5, and 66.8 GHz, respectively, using a Fourier transform millimeter-wave spectrometer with a pulsed discharge nozzle. The FCO radical is produced by discharging F2CO diluted in the Ar buffer gas. Twelve fine and hyperfine components for the three transitions are observed, and the effective rotational constant, the centrifugal distortion constant, the spin–rotation constant with its centrifugal correction term, and three hyperfine constants are determined. Furthermore, the vibrational satellites for the v1=1, v2=1, v3=1, and v3=2 states are also observed in the 22.3 GHz region. From the dipolar interaction constants, the principal axis of the dipolar interaction tensor is estimated and is discussed in relation to the distribution of the unpaired electron.

Millimeter-Wave Spectroscopy of Kr-CO and Xe-CO Using a Coaxial Jet Spectrometer

A new pulsed supersonic jet millimeter-wave spectrometer with coaxial propagation of the molecular jet and millimeter-waves has been constructed. The coaxial configuration provides greater sensitivity which has been demonstrated by observing b-type transitions of five isotopomers of Kr-12C16O and seven isotopomers of Xe-12C16O. These results were analyzed together with a-type transitions obtained by FTMW spectroscopy to determine improved rotational parameters for each isotopomer. The ground vibrational state K = 1 ← 0 intervals have thus been precisely determined for Kr-CO and Xe-CO.

Scientific case for a millimeter-wave spectrometer for high redshift galaxy surveys

We propose a low resolution millimeter-wave grating spectrometer with wide spectral coverage for conducting redshift surveys of the recently discovered class of far-infrared galaxies. A summary of ...

The Rotational Spectrum of OCCCS Revisited

The rotational spectrum of the linear chain molecule tricarbon oxide sulfide, OCCCS, was recorded at room temperature between 118 and 179 GHz using a new commercial millimeter-wave spectrometer. The observed transitions were analyzed together with hitherto unassigned transitions from measurements between 78 and 118 GHz. About 1900 rotational transitions of OCCCS in the vibrational ground state and in 13 excited bending states could be assigned, the latter including the eighth, ninth, and tenth excited state of the low-lying bending mode ν7 as well as the second excited state of the bending mode ν6. Besides the determination of effective constants for these newly identified vibrational states, our data also served to considerably improve the effective constants for some of the bending states already characterized in previous publications.

Microwave spectrum and molecular structure of the HSC radical

The HSC radical, that is a geometrical isomer of the HCS radical, has been identified and characterized by microwave spectroscopy. The HSC radical has been produced in a discharge plasma of a gaseous mixture of H2S and CO, and its rotational spectral lines are observed with a source-modulation spectrometer and a Fourier-transform millimeter-wave spectrometer. The spectrum of the deuterated species, DSC, has also been measured with a source-modulation spectrometer. Rotational constants, centrifugal distortion constants, and spin–rotation interaction constants with their centrifugal distortion corrections for HSC and DSC are determined from the observed transition frequencies by a least-squares method. Furthermore hyperfine interaction constants of the hydrogen nucleus are also determined for HSC. The Fermi contact term of the hydrogen nucleus takes a large positive value, 288.845±0.185 MHz, which indicates that the HSC radical is a σ radical (2A′) in the ground electronic state. The harmonic force field is evaluated from the obtained centrifugal distortion constants and inertial defects. The zero-point vibrational average structure is determined as follows; rz(H–S)=1.390±0.003 Å, rz(S–C)=1.6403±0.0005 Å and αz(HSC)=104.2±0.2°.

The rotational spectrum of rhomboidal SiC3

Rhomboidal SiC3, a planar ring with C2v symmetry and a transannular C–C bond, was detected at centimeter wavelengths in a pulsed supersonic molecular beam with a Fourier transform microwave (FTM) spectrometer, and was subsequently observed in a low-pressure dc glow discharge with a free-space millimeter-wave absorption spectrometer. The rotational spectrum of SiC3 is characterized by large harmonic defects and large splitting of the K-type doublets. Lines in the centimeter-wave band were very strong, allowing the singly substituted isotopic species to be observed in natural abundance. Measurements of the normal and five isotopically substituted species with the FTM spectrometer provided conclusive evidence for the identification and yielded an experimental zero-point (r0) structure. Forty-six transitions between 11 and 286 GHz with Ka⩽6 were measured in the main isotopic species. Three rotational and nine centrifugal distortion constants in Watson’s A-reduced Hamiltonian reproduce the observed spectrum to within a few parts in 107 and allow the most intense transitions up to 300 GHz to be calculated with high accuracy. The spectroscopic constants confirm that SiC3 is a fairly rigid molecule: the inertial defect is comparable to those of well-known planar rings and the centrifugal distortion constants are comparable to molecules of similar size. The number of SiC3 molecules in our supersonic molecular beam in each gas pulse is at least 3×1011, so large that electronic transitions may be readily detectable by laser spectroscopy.

Millimeter-wave magnetooptics: New method for characterization of ferrites in the millimeter-wave range

This paper presents a new free-space millimeter-wave method based on the transverse magnetooptical effect, which provides the complete and accurate characterization of the ferrites for the first time in the millimeter-wave range. The described data analysis and processing methods lead to the separation of dielectric and magnetic effects and to the simultaneous determination of both millimeter-wave permittivity and permeability. It is demonstrated that this new method can be used for the characterization of soft ferrites as well as hard anisotropic ferrites. The magnetooptical measurements were realized using a millimeter-wave spectrometer combined with the electromagnet providing a transverse magnetic field up to 1 T. The backward-wave oscillators were used as a source of tunable coherent millimeter-wave radiation. It was shown that the freespace method applying the polarized Gaussian beams generates precise transmission data and enables the accurate determination of the optical constants of ferrites in the entire millimeter-wave range.

Dimethyl Ether: Laboratory Assignments and Predictions through 600 GHz

Dimethyl ether (CH3OCH3) is a well-known interstellar molecule with a large abundance in hot core regions. We have measured many rotational-torsional transition lines of this molecule in the 100-550 GHz frequency range using two experimental techniques: a klystron-based traditional millimeter-wave spectrometer and a new fast scan spectrometer (designated "FASSST") with a voltage-tunable backward wave oscillator. The lines arising from the ground vibrational state have been combined with previous data, mainly at lower frequencies, to form a global data set consisting of over 1600 lines that have been assigned and fitted by an effective Hamiltonian method. The spectral constants obtained from the fit allow us to predict the frequencies of almost 6000 additional lines of dimethyl ether through 600 GHz.

Fourier transform millimeter-wave spectroscopy of the HCS radical in the A′2 ground electronic state

The 101–000 rotational transition of the HCS radical in the X̃ A′2 ground electronic state has been observed with a Fourier transform millimeter-wave spectrometer in combination with a pulsed discharge nozzle. The radical is produced by discharging a mixture of CH4 and H2S diluted in Ar. Six fine and hyperfine components are detected, and the effective rotational constant, spin–rotation interaction constant, and hyperfine interaction constants are determined accurately. The Fermi contact term of the hydrogen nucleus is found to be much smaller than that for the isovalent radical, HCO, indicating that the HCS radical is more close to a linear structure than the HCO radical.

New Measurements and Assignments in the Millimeter-Wave Spectrum of CD3OH

The ground state rotational spectrum of CD3OH has been revisited in the millimeter-wave range. A total of 216 transition frequencies have been measured and assigned in the 117–179 GHz spectral range, including about 40 transitions previously reported. The spectrum was recorded at the Justus–Liebig University in Gießen, Germany using a frequency modulated millimeter-wave spectrometer. The assignments for the CD3OH transitions were predicted based on energy levels calculated using preliminary results of the global fit of microwave, millimeter-wave, and far-infrared data of Walshet al.(Paper FC04 presented at the 52nd International Symposium of Molecular Spectroscopy, Columbus, OH, 1997). The new measurements have substantially enlarged the accurate millimeter-wave component of the data set available for the global fit and have allowed Walshet al.to obtain significant improvement in the CD3OH molecular parameters (J. Mol. Spectrosc.188,85–93, 1998). The low residuals between observed and calculated frequencies highlight the quality of the global fit results.

Low-noise cooled planar Schottky diode receivers for ground-based spectral ozone measurements at 142 GHz

Two new low-noise cooled receivers based on planar Schottky diode mixers were designed and built for the Lebedev Physics Institute (LPI) spectrometer for ground-based measurements of the atmospheric ozone spectral line of 142.175 GHz central frequency. The receivers differ in the intermediate frequency (IF) bands, of around 3.7 and 1.5 GHz. Review description of the spectrometer equipped with the 3.7–GHz IF receiver is given, and its performance is compared with other millimeter-wave ground-based ozone spectrometers. Special attention was paid to design of the input Gaussian optics and the mixers. Techniques of laboratory tests of the receivers and results of the tests through all 2–mm wavelength range are considered and discussed. SSB mixer noise temperature of 460±60 K was obtained at 151 GHz for room temperature mixer, and the value of 180±30 K was measured at 134 GHz under cooling to 85 K in liquid nitrogen cryostat. SSB mixer conversion losses were less than 5.5 dB in both the cases. SSB noise temperature of the spectrometer is less than 1500 K without cooling and less than 700 K at cryogenic operation at 142 GHz. This provides sensitivity of about 0.2 and 0.1 K for the narrowest spectral channel width of 0.1 MHz and signal integration time of 1 hour. Using the optimized spectrum analyzers delivers data on high-accuracy retrieval of the ozone vertical profile in the atmosphere at altitudes about from 15 to 75 km. Examples of the ozone observation data are given.

The millimeter wave spectrum of CH3 18OH in the 119–165 GHz spectral range

The rotational millimeter-wave spectrum of CH318OH has been revisited, and 162 transitions of both a- and b-types have been measured in the 119-165 GHz spectral range. The spectrum was recorded using the frequency-modulated millimeter-wave spectrometer at the/ustus-LiebigUniversit~it in Giegen. The CH318OH transition assignments were based on energy levels obtained from the far-infrared analysis of S. Zhao (Ph.D. thesis, University of New Brunswick, 1993) using the "Ritz" program of G. Moruzzi et al. (J. Mol. Spectrosc. 167, 156 (1994)) for direct energy level fitting. The relatively low residuals between calculated and observed frequencies highlight the good quality of the results from this program. The newly measured transitions combined with those existent in the literature have been included in least-squares fits to improve the set of rotational, torsional and centrifugal distortion constants for O-18 methanol.