Millimeter-wave Absorption Research Articles

Trends in alkali metal hydrosulfides: A combined Fourier transform microwave/millimeter-wave spectroscopic study of KSH ($\tilde X^1 A^\prime $X̃1A′)

The pure rotational spectrum of KSH (X(1)A') has been measured using millimeter-wave direct absorption and Fourier transform microwave (FTMW) techniques. This work is the first gas-phase experimental study of this molecule and includes spectroscopy of KSD as well. In the millimeter-wave system, KSH was synthesized in a DC discharge from a mixture of potassium vapor, H2S, and argon; a discharge-assisted laser ablation source, coupled with a supersonic jet expansion, was used to create the species in the FTMW instrument. Five and three rotational transitions in the range 3-57 GHz were recorded with the FTMW experiment for KSH and KSD, respectively, in the K(a) = 0 component; in these data, potassium quadrupole hyperfine structure was observed. Five to six transitions with K(a) = 0-5 were measured in the mm-wave region (260-300 GHz) for the two species. The presence of multiple asymmetry components in the mm-wave spectra indicates that KSH has a bent geometry, in analogy to other alkali hydrosulfides. The data were analyzed with an S-reduced asymmetric top Hamiltonian, and rotational, centrifugal distortion, and potassium electric quadrupole coupling constants were determined for both isotopolgues. The r0 geometry for KSH was calculated to be r(S-H) = 1.357(1) Å, r(K-S) = 2.806(1) Å, and θ(M-S-H) (°) = 95.0 (1). FTMW measurements were also carried out on LiSH and NaSH; metal electric quadrupole coupling constants were determined for comparison with KSH. In addition, ab initio computations of the structures and vibrational frequencies at the CCSD(T)/6-311++G(3df,2pd) and CCSD(T)/aug-cc-pVTZ levels of theory were performed for LiSH, NaSH, and KSH. Overall, experimental and computational data suggest that the metal-ligand bonding in KSH is a combination of electrostatic and covalent forces.

Millimeter-wave spectroscopy of carbonyl diazide, OC(N3)2

Millimeter-wave absorption spectra for carbonyl diazide (OC(N3)2) are reported in the frequency range of 243–360GHz, at both 293K and 213K. Transitions for two of the three possible conformations, one with both of the azide groups syn to the carbonyl group, or with one syn and the other anti, were observed in the spectra. Theoretical calculations at the CCSD(T)/ANO1 level do an excellent job of predicting the ground state rotational constants and 4th order centrifugal distortion terms for both conformers. Relative line intensities, along with theoretically predicted dipole moments, were used to estimate the energy difference of the two observed forms, yielding a result in good agreement with the ab initio potential energy surface. The spectra of the ν12, ν7, ν9 and 2ν12 excited vibrational states for the more abundant syn–syn conformer have been assigned, and a great many transitions for each of them have been fit using partial 6th and 8th order centrifugal distortion Hamiltonians. Anharmonic vibration–rotation interaction constants from the CCSD(T)/ANO1 calculations are in excellent agreement with the experimentally determined constants in the case of ν7 and ν9, but not for ν12.

Keto–Enol Tautomerism and Conformational Landscape of 1,3-Cyclohexanedione from Its Free Jet Millimeter-Wave Absorption Spectrum

The free jet millimeter-wave absorption spectrum of 1,3-cyclohexanedione has been investigated in the 59.6-74.4 GHz frequency range, and the rotational spectra of two conformational species, the chair-diketo and boat-diketo, and probably one excited vibrational state belonging to the chair-diketo form have been assigned. Quantum chemical calculations, performed at the B3LYP/6-311++G** and MP2/6-311++G** levels, were used to characterize the potential energy surface minima. The potential energy surface related to the interconversion of the observed diketonic species was modeled at the DFT level.

The pure rotational spectrum of VS (X4Σ−): A combined Fourier transform microwave and millimeter-wave study

The pure rotational spectrum of the vanadium sulfide radical, VS (X4Σ−), has been measured in the frequency range 5–310GHz using a combination of millimeter-wave direct absorption and Fourier transform microwave (FTMW) techniques. In the millimeter-wave region, the radical was produced in an AC discharge from the reaction of VCl4, the vanadium donor, and CS2. In the FTMW instrument, the molecule was created in a supersonic jet, coupled with a laser ablation/DC discharge source (DALAS), from a mixture of metal vapor and H2S, heavily diluted in argon. A total of 8 rotational transitions were measured for VS, in which both the quartet fine structure and vanadium hyperfine splittings were resolved. The spectra were analyzed with a Hund’s case (b) Hamiltonian, and rotational, spin–rotation, spin–spin, and hyperfine parameters were determined. The precision of the constants from previous optical studies was refined and, for the first time, the vanadium quadrupole constant, eQq=−7.6 (4.0)MHz, and the third order Fermi contact correction, bS=−0.293 (94)MHz, were established. From the fine structure parameters, the nearby 4Π and 2Σ+ states were estimated to lie ∼6560cm−1 and ∼7170cm−1 above the ground state. The hyperfine constants suggest that the bonding in VS is partly ionic, with a significant degree of covalent character.

A Broadband Waveguide Calorimeter in the Frequency Range From 50 to 110 GHz

A new broadband rectangular waveguide calorimeter for radio-frequency (RF) power meter calibration in the millimeter-wave region is reported. The calorimeter has a quasi-twin structure exploiting the WR-10 and WR-15 rectangular waveguides, and its operational frequency ranges from 50 to 110 GHz. This calorimeter is characterized by its high measuring speed and sensitivity. A thermal feedback control system reduces the typical reading time per frequency to less than 10 min, and the minimum measurable incident RF power is 0.2 mW. A new technique for reducing the equivalence error by utilizing a flat millimeter-wave absorber is also discussed.

Contactless Liquid-Level Measurement With Frequency-Modulated Millimeter Wave Through Opaque Container

A novel contactless method for measuring liquid level through an opaque container is proposed. A millimeter-wave Doppler sensor is developed to “see” (i.e., sense) through a target container and measure the liquid level on the basis of the absorption of millimeter waves in liquid. One of the challenges is to accurately measure liquid level (within sub-millimeter error) despite the inherently large beam diameter of the millimeter wave due to diffraction. A piezoelectric vibrator enables accurate measurement by reflecting a limited portion of the spread beam and modulating it in frequency to distinguish it from the other portion of the beam. A prototype measurement system is fabricated and evaluated. The feasibility of our proposed method for clearly detecting an air-liquid interface concealed in an opaque container is confirmed experimentally. The nonlinearity error of the measured liquid level is within ±0.5 mm .

Absorbing coatings for high power millimeter-wave devices and matched loads

In the electron cyclotron frequency range the handling of high power is critical. In some cases an unpredictable amount of stray radiation can reach some components or accumulate in localized regions, with risk of damages caused by thermal overloads, and any uncontrolled reflection represents a danger for the sources. A possibility to mitigate the problem consists in covering some regions exposed to radiation with absorbers. Enhanced absorption of stray radiation lowers requirements on active protection systems in microwave diagnostics. The released heat can be extracted by dedicated cooling systems. The chromium oxide (Cr2O3), largely tested at IFP-CNR, has been routinely used as internal coating for matched loads. The performances of a variable thickness coating has been tested at high power at Karlsruhe Institute of Technology (KIT), with a 140GHz gyrotron of the W7-X ECRH system and an averaged power density absorbed at the coating surface higher than 1MW/m2 for 3min. Also boron carbide (B4C) has been tested at low power and patented as a millimeter-wave absorber. In the paper, the results of some tests performed on these coatings are given, together with some simulations of the absorption capability based on low power measurements on samples. Finally, some calculations are presented for a coating obtained combining together Cr2O3 and B4C.

Structural studies of aromatic carboxylic acids via computational chemistry and microwave spectroscopy

The structures of three simple aromatic carboxylic acids: benzoic, isophthalic, and terephthalic have been investigated using a combination of theoretical high-level quantum chemical calculations and experimental millimeter-wave Stark-modulated free-jet absorption spectroscopy. Rotational and centrifugal distortion constants have been measured for one conformer of each of the species and for its -COOD isotopologue, leading to the experimental determination of the coordinates of the carboxyl hydrogen atom. Consideration of the observed inertial defect is consistent with a planar equilibrium structure for each species. Calculated structures, relative energies, and electric dipole moments, using ab initio methods at the MP2/cc-pVTZ level, are reported for all the lower-energy conformers of each species. The theoretical calculations lead to the unambiguous identification of the conformers involved in the observed microwave spectra. The match between theoretical and spectroscopic measurements was used to gauge the reliability of the quantum chemical structure optimization calculations.

Millimeter-Wave Absorption as a Quality Control Tool for M-Type Hexaferrite Nanopowders

Millimeter wave (MMW) absorption measurements have been conducted on commercial samples of large (micrometer-sized) and small (nanometer-sized) particles of BaFe12O19 and SrFe12O19 using a quasi-optical MMW spectrometer and a series of backwards wave oscillators encompassing the 30-120 GHz range. Effective anisotropy of the particles calculated from the resonant absorption frequency indicates lower overall anisotropy in the nano-particles. Due to their high magnetocrystalline anisotropy, both BaFe12O19 and SrFe12O19 are expected to have spin resonances in the 45-55 GHz range. Several of the sampled BaFe12O19 powders did not have MMW absorptions, so they were further investigated by DC magnetization and x-ray diffraction to assess magnetic behavior and structure. The samples with absent MMW absorption contained primarily iron oxides, suggesting that MMW absorption could be used for quality control in hexaferrite powder manufacture.

Hard magnetic ferrite with a gigantic coercivity and high frequency millimetre wave rotation

Magnetic ferrites such as Fe3O4 and Fe2O3 are extensively used in a range of applications because they are inexpensive and chemically stable. Here we show that rhodium-substituted ε-Fe2O3, ε-RhxFe2−xO3 nanomagnets prepared by a nanoscale chemical synthesis using mesoporous silica as a template, exhibit a huge coercive field (Hc) of 27 kOe at room temperature. Furthermore, a crystallographically oriented sample recorded an Hc value of 31 kOe, which is the largest value among metal-oxide-based magnets and is comparable to those of rare-earth magnets. In addition, ε-RhxFe2−xO3 shows high frequency millimetre wave absorption up to 209 GHz. ε-Rh0.14Fe1.86O3 exhibits a rotation of the polarization plane of the propagated millimetre wave at 220 GHz, which is one of the promising carrier frequencies (the window of air) for millimetre wave wireless communications.

Single and dual band 77/95/110 GHz metamaterial absorbers on flexible polyimide substrate

Ultra thin millimeter-wave absorbers on flexible polyimide substrate utilizing metamaterials are implemented for single and dual frequency bands in an emerging frequency spectrum of 77, 95, and 110 GHz. The dual band absorber is designed using a novel approach of imbedding high frequency resonator inside low frequency resonator capable of absorbing electromagnetic energy at both 77 and 110 GHz bands simultaneously. The total thickness of the absorber is just 126 μm (almost 1/20th of the wavelength). Measured peak absorptions for single frequency absorbers are 92, 94, and 99% at 77.2, 94.8, and 109.5 GHz, respectively, and for dual band absorber 92% at 77 GHz and 94% at 109.8 GHz.

Anomalous microwave absorption in multi-walled carbon nanotubes filled with iron

The interaction of millimeter-wave radiation (MWR) with vertically oriented carbon nanotubes filled with iron (Fe-MWCNT) in solid monolith has been investigated. Fe-MWCNT demonstrated high-efficiency towards electromagnetic shielding due to a high conductivity of CNT and iron-containing filling compound. The shielding efficiency and the reflection loss depending on the CNT orientation have been measured in a rectangular waveguide. A resonant absorption of millimeter waves has been revealed with the orthogonal Fe-MWCNT orientation with respect to the MWR electric field in the waveguide.

Modelling millimetre wave propagation and absorption in a high resolution skin model: the effect of sweat glands

The aim of this work was to investigate the potential effect of sweat gland ducts (SGD) on specific absorption rate (SAR) and temperature distributions during mm-wave irradiation. High resolution electromagnetic and bio-heat transfer models of human skin with SGD were developed using a commercially available simulation software package (SEMCAD X™). The skin model consisted of a 30 µm stratum corneum, 350 µm epidermis and papillary dermis (EPD) and 1000 µm dermis. Five SGD of 60 µm radius and 300 µm height were embedded linearly with 370 µm separation. A WR-10 waveguide positioned 20 µm from the skin surface and delivering 94 GHz electromagnetic radiation was included in the model. Saline conductivity was assigned inside SGD. SAR and temperatures were computed with and without SGD. Despite their small scale, SAR was significantly higher within SGD than in the EPD without SGD. Without SGD, SAR and temperature maxima were in the dermis near EPD. With SGD, SAR maximum was inside SGD while temperature maximum moved to the EPD/stratum-corneum junction. Since the EPD participates actively in perception, the effect of SGD should be taken into account in nociceptive studies involving mm-waves. This research represents a significant step towards higher spatial resolution numerical modelling of the skin and shows that microstructures can play a significant role in mm-wave absorption and induced temperature distributions.

A Millimeter-Wave Tunable Electromagnetic Absorber Based on $\varepsilon$-Al$_{\rm x}$Fe$_{2-{\rm x}}$O$_{3}$ Nanomagnets

High-performance millimeter-wave absorber composed of ε-Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2-x</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> nanomagnets is prepared by sol-gel techniques. Free space quasi-optical spectrometer equipped with a set of tunable backward-wave oscillators as power sources is used in this work to study the absorption property of this series of materials. Special aluminum-disc sample container with thickness of 2 mm is fabricated. Transmittance spectra of ε-Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2-x</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> with different x values (x = 0.66 and 0.49) are recorded using our free space spectrometer. Strong magnetic absorption at 95 GHz (corresponding to x = 0.66) and 105 GHz (x = 0.49) are found. Combining the data obtained in this and previous work, second order polynomial curve is simulated to study the x parameter dependence of absorption frequencies. The simulation results are in good agreement with the experimental ones.

Millimeter-Wave Absorption by Cutaneous Blood Vessels: A Computational Study

The aims of the present study were to calculate the specific absorption rate (SAR) and E-field distributions inside cutaneous blood vessels and in surrounding tissues (dermis and fat) depending on the frequency of millimeter wave exposure. Most calculations were performed using the finite-difference time-domain (FDTD) technique. A rectangular block of homogeneous or multilayer tissue with blood vessels located in the center of the block was used as the basic geometry. We found that the SAR reached its maximal value in a long blood vessel oriented parallel to the E-field. It exceeded the SAR in the surrounding dermis by 40%-42% at 42.25 GHz. However, in the same blood vessel oriented perpendicularly to the E-field, the SAR was lower than that of the surrounding dermis. Absorption of millimeter waves in a cutaneous blood vessel was higher at 61.22 GHz than at 42.25 GHz. The SAR distribution in a blood vessel was nearly uniform. Because of the small sizes of cutaneous blood vessels relative to the wavelength, the SAR distributions in these blood vessels can be calculated by using quasi-static theory.

A Selection of Channels for a Proposed Atmospheric Temperature Sounder of ISRO

This paper presents a new assortment of temperature-sounding channels for a proposed low-Earth-orbit polar Sun-synchronous satelliteborne millimeter-wave atmospheric sounder of the Indian Space Research Organization (ISRO). The newness owes its origin to the exploration of the millimeter-wave O2 absorption spectrum in quest of optimal off-resonance frequencies that impose fewer restrictions on channel bandwidth and temperature sensitivity and yet can sound up to 40 km in the atmosphere with a 4-km vertical resolution. This is ISRO's first leap toward millimeter-wave technology. The overall receiver-noise figure for the channels in the 5-mm band (50-60 GHz) has been pessimistically estimated at 5 dB which will severely degrade the system temperature sensitivity. Therefore, channel bandwidth is at premium. The purpose of this design is to limit the number of passbands and simplify the design of frequency-selective filters by choosing center frequencies that have sufficient interleaving and also provide a scope of allotting a reasonable bandwidth. The set of temperature-sounding channels in the current design have 15 channels in 17 passbands in contrast with 15 channels in 29 passbands of the operational Advanced Microwave Sounding Unit (AMSU)-A.

Absorbing Frequency-Selective-Surface for the mm-Wave Range

We report on a millimeter-wave (mm-wave) absorber based on the frequency selective surface. It consists of a periodic array of resistive patches on a grounded dielectric layer. By varying the shape of the patches and the distance between them, the device can be tuned to absorb in a given frequency band. We designed and fabricated several devices consisting of square arrays of Nichrome circles or rings. The design was targeted to the mm-wave range, 75-110 GHz. Our measurements of the mm-wave reflection from these devices show good agreement with computer simulations. We discuss the use of our device as a microbolometer array.

Evolution of Chemical Bonding during HCN⇄HNC Isomerization as Revealed through Nuclear Quadrupole Hyperfine Structure

Hyperfine reporter: Millimeter-wave absorption measurements and ab initio calculations show that nuclear quadrupole hyperfine structure (hfs) is highly sensitive to the extent of bending excitation in HCN and HNC. These results indicate that hfs serves as a reporter of progress along the isomerization reaction coordinate and can reveal mechanistic information about this process.

Outflowing atomic and molecular gas at z∼ 0.67 towards 1504 + 377

Abstract We report the detection of OH 1667-MHz and wide H i 21-cm absorption at z∼ 0.67 towards the red quasar 1504 + 377, with the Green Bank Telescope and the Giant Metrewave Radio Telescope. The H i 21-cm absorption extends over a velocity range of ∼600 km s−1 blueward of the quasar redshift (z= 0.674), with the new OH 1667-MHz absorption component at ∼ −430 km s−1, nearly coincident with earlier detections of millimetre-wave absorption at z∼ 0.6715. The atomic and molecular absorption appear to arise from a fast gas outflow from the quasar, with a mass outflow rate and a molecular hydrogen fraction . The radio structure of 1504 + 377 is consistent with the outflow arising as a result of a jet–cloud interaction, followed by rapid cooling of the cloud material. The observed ratio of HCO+ and OH column densities is ∼20 times higher than typical values in Galactic and high-z absorbers. This could arise because of small-scale structure in the outflowing gas on sub-parsec scales, which would also explain the observed variability in the H i 21-cm line.

The Rotational Spectrum of Anti‐Ethylamine (CH3CH2NH2) from 10 to 270 GHz: A Laboratory Study and Astronomical Search in Sgr B2(N)

The pure rotational spectrum of the lowest energy (anti-) conformer of ethylamine (CH3CH2NH2) has been measured in the frequency range of 10-270 GHz. The spectrum was recorded using both millimeter-wave absorption spectroscopy and Fourier transform microwave (FTMW) techniques. Ten rotational transitions of this molecule were recorded in the frequency range of 10-40 GHz using FTMW methods, resulting in the assignment of 53 quadrupole-resolved hyperfine lines; in the millimeter-wave region (48-270 GHz), nearly 600 transitions were assigned to the ground (anti-) state. The amine group in CH3CH2NH2 undergoes inversion, resulting in a doubling that is frequently small and most apparent in the low-frequency K-doubling transitions. In addition, seemingly random rotational levels of this molecule were found to be significantly perturbed. The cause of these perturbations is presently uncertain, but torsion-rotation interactions with the higher lying gauche conformers seem to be a likely explanation. An astronomical search was conducted for ethylamine toward Sgr B2(N) using the Kitt Peak 12 m antenna and the Sub-Millimeter Telescope (SMT) of the Arizona Radio Observatory. Frequencies of 70 favorable rotational transitions were observed in this search, which covered the range 68-263 GHz. Ethylamine was not conclusively detected in Sgr B2(N), with an upper limit to the column density of (1–8) × 1013 cm−2 with f(CH3CH2NH2/H2) ∼ (0.3–3) × 10−11, assuming a rotational temperature of 50-220 K. These observations indicate a gas-phase CH3CH2NH2/CH3NH2 ratio of <0.001-0.01, in contrast to the nearly equal ratio suggested by the acid hydrolysis of cometary solids from the Stardust mission.