Previous Microwave Research Articles

Rotational spectroscopy of methyl tert-butyl ether with a new Ka band chirped-pulse Fourier transform microwave spectrometer.

Chirped-pulse Fourier transform microwave (CP-FTMW) spectroscopy is a powerful tool for performing broadband gas-phase rotational spectroscopy, and its applications include discovery of new molecules, complex mixture analysis, and exploration of fundamental molecular physics. Here we report the development of a new Ka band (26.5-40 GHz) CP-FTMW spectrometer that is equipped with a pulsed supersonic expansion source and a heated reservoir for low-volatility samples. The spectrometer is built around a 150 W traveling wave tube amplifier and has an instantaneous bandwidth that covers the entire Ka band spectral range. To test the performance of the spectrometer, the rotational spectrum of methyl tert-butyl ether (MTBE), a former gasoline additive and environmental pollutant, has been measured for the first time in this spectral range. Over 1000 spectroscopic transitions have been measured and assigned to the vibrational ground state and a newly-identified torsionally excited state; all transitions were fit using the XIAM program to a root-mean-square deviation of 22 kHz. The spectrum displays internal rotation splitting, nominally forbidden transitions, and an intriguing axis-switching effect between the ground and torsionally excited state that is a consequence of MTBE's extreme near-prolate nature. Finally, the sensitivity of the spectrometer enabled detection of all singly-substituted 13C and 18O isotopologues in natural abundance. This set of isotopic spectra allowed for a partial r0 structure involving the heavy atoms to be derived, resolving a structural discrepancy in the literature between previous microwave and electron diffraction measurements.

Reinvestigation of the ν3-ν6 Coriolis interaction in trifluoroiodomethane.

The lowest-frequency fundamental ν6 of trifluoroiodomethane (CF3I) has never been directly observed and analyzed at high resolution in the gas phase. The ν6(e) level interacts with ν3(a1) at 286.297 cm-1via a b-axis Coriolis interaction, which perturbs the rotational structure of both levels. In this work, we report low-J microwave transitions (for J ranging from 0-2) within the ν6 vibrational level. The l-type doubling observed in our spectrum agreed poorly with the predictions of previously published models of the interacting ν3-ν6 levels, prompting us to refine the model. We performed ab initio anharmonic force-field calculations, which were used to constrain some of the parameters, and which served as a check on some of the floated parameters. We fit a dataset consisting of 3593 transitions, which combined our measurements with previous microwave, millimeter wave, and high-resolution infrared observations. A reasonable set of fit parameters is obtained with ν6 = 267.28 cm-1, but we cannot rule out a lower value of ν6 = 261.5 cm-1 consistent with analyses of the vibrational level structure.

Weak hydrogen bonding to halogens and chirality communication in propanols: Raman and microwave spectroscopy benchmark theory.

Constitutional and conformational isomers of bromopropanol are vibrationally and rotationally characterised with parallels drawn to the structural chlorine analogues. A previous microwave spectroscopic study of the chloropropanols is re-examined and all systems are explored by Raman jet spectroscopy. For bromine, the entire nuclear quadrupole coupling tensors are accurately determined and compared to their chlorine counterparts. Tensor asymmetry parameters are determined and linked with the hydrogen bond strength as indicated by the downshift of the OH-stretching frequency. The spectroscopic constants derived from the observed transitions are used as benchmarks for a large variety of electronic structure methods followed by harmonic and anharmonic rovibrational treatments. The CCSD(T) electronic structure calculations provide the best performance, in particular once anharmonic and relativistic corrections are applied or implied. Standard DFT approaches vary substantially with respect to their systematic error cancellation across the investigated species, and cost-effective compromises for the different observables are proposed.

Design of a composite metamaterial toward perfect microwave absorption and excellent load-bearing performance

In this paper, an ultrathin layer of arrayed electromagnetic resonators is introduced on the CFRP laminate to form a meta-CFRP composite. It is quite an exciting and promising design, as the composite metamaterial not only inherits the advantages of the CFRP composite in mechanical performance but can also obtain on-demand microwave absorption properties rather than purely reflective as neat CFRP laminates. Here, several meta-CFRP samples have been prepared by a co-curing process. Near-perfect microwave absorption (i.e., S11 ≤ -10 dB) has been achieved in a series of situations such as under normal incidence and a constant polarization angle, under oblique incidence and an arbitrary polarization angle, and for a relative broadband microwave absorption (11.28 GHz ∼ 12.52 GHz). Moreover, the mechanical properties of the fabricated meta-CFRP samples, including tension, compression, bending, and shear, have also been investigated, which showed an increase of more than 680% compared with previous microwave absorbers. The present work provides a new idea for designing electromagnetic and mechanical dual functional materials, based on which many new materials, for example, strong materials with tunable microwave absorption, can be further developed in the future.

A Long-term Consistent Artificial Intelligence and Remote Sensing-based Soil Moisture Dataset

The Consistent Artificial Intelligence (AI)-based Soil Moisture (CASM) dataset is a global, consistent, and long-term, remote sensing soil moisture (SM) dataset created using machine learning. It is based on the NASA Soil Moisture Active Passive (SMAP) satellite mission SM data and is aimed at extrapolating SMAP-like quality SM back in time using previous satellite microwave platforms. CASM represents SM in the top soil layer, and it is defined on a global 25 km EASE-2 grid and for 2002–2020 with a 3-day temporal resolution. The seasonal cycle is removed for the neural network training to ensure its skill is targeted at predicting SM extremes. CASM comparison to 367 global in-situ SM monitoring sites shows a SMAP-like median correlation of 0.66. Additionally, the SM product uncertainty was assessed, and both aleatoric and epistemic uncertainties were estimated and included in the dataset. CASM dataset can be used to study a wide range of hydrological, carbon cycle, and energy processes since only a consistent long-term dataset allows assessing changes in water availability and water stress.

Metasurface-Based Time-Reversal Focusing for Brain Tumor Microwave Hyperthermia

We introduce a metasurface-based system for the microwave hyperthermia treatment of brain tumors, where the metasurface is designed by the time-reversal technique for optimal focusing at the location of the tumor. Compared to previous time-reversal microwave hyperthermia systems, which are based on antenna arrays, the proposed system offers two advantages: 1) it provides superior focal resolution, due to the finer sampling capability of its deeply-subwavelength metasurface particles compared to that of typical array antenna elements, 2) it features much lower complexity and cost, due to its direct illumination principle compared to the heavy feeding network of the array system. The system leverages reflecting metallic walls and a rich scattering structure in combination with the metasurface to achieve high hyperthermia performance with maximal simplicity. The metasurface system is demonstrated by full-wave simulation results, which show that it represents a viable novel noninvasive hyperthermia technology.

Constraints on primordial magnetic fields from their impact on the ionization history with Planck 2018

ABSTRACT We update and extend our previous cosmic microwave background anisotropy constraints on primordial magnetic fields through their dissipation by ambipolar diffusion and magnetohydrodynamic decaying turbulence effects on the post-recombination ionization history. We derive the constraints using the latest Planck 2018 data release which improves on the E-mode polarization leading to overall tighter constraints with respect to Planck 2015. We also use the low-multipole E-mode polarization likelihood obtained by the SROLL2 map making algorithm and we note how it is compatible with larger magnetic field amplitudes than the Planck 2018 baseline, especially for positive spectral indices. The 95 per cent confidence level constraints on the amplitude of the magnetic fields from the combination of the effects is $\sqrt{\langle B^2 \rangle } \lt 0.69 (\lt 0.72)$ nG for Planck 2018 (SROLL2) by marginalizing on the magnetic spectral index. We also investigate the impact of a damping scale allowed to vary and the interplay between the magnetic field effects and the lensing amplitude parameter.

On the planarity of benzyl cyanide

The microwave spectra of benzyl cyanide, C6H5CH2CN, were recorded using a molecular jet Fourier transform microwave spectrometer operating in the frequency range 2–26.5 GHz. Analyses of the 14N quadrupole hyperfine structures were performed simultaneously with the rotational frequencies. Rotational constants, centrifugal distortion constants, and nuclear quadrupole coupling constants χaa and χbb − χcc were all determined with very high accuracy in a fit containing 465 lines with standard deviation within the measurement accuracy. Results from quantum chemical calculations at many levels of theory employing the MP2 method indicate a non-planar heavy atom skeleton structure, where the CH2CN group is tilted up to 50° with respect to the phenyl ring. This conflicts with results from many calculations employing density functional theory methods and also with the conclusion of a previous microwave spectroscopic work (X.-Z. Liu, R.K. Bohn, S.A. Sorenson, N.S. True, J. Mol. Struct.243 (1991) 325–339), stating that the CH2CN group and the phenyl ring are in-plane. By comparing the planar defect with those of several planar molecules reported in the literature, and with extensive support from quantum chemistry, we present for the first time evidence suggesting that the heavy atom structure of benzyl cyanide is probably not planar.

Global analysis of pure-rotational spectra in both the vt = 0 and vt = 1 torsional states of CD3SH

The CD3SH molecule, a multiply deuterated isotopologue of methyl mercaptan, can be detected in the interstellar medium with the aid of new high-sensitivity telescopes. Following our previous microwave studies on the ground and first torsional states (J. Mol. Spectrosc. 337(2017) 32–36 and J. Mol. Struc. 1217(2020) 128339), we performed a global analysis for both vt = 0 and vt = 1 torsional states. In this study, 354 transitions (119 (A species) and 76 (E species) in the vt = 0 state and 67 (A species) and 92 (E species) in the vt = 1 state) were newly assigned in addition to the previous assignments. A total of 2130 transition frequencies were analyzed by the least-squares-method using the pseudo-principal axis method (pseudo-PAM) Hamiltonian. Forty-eight parameters consisting of rotational, centrifugal distortion, and internal rotational constants, and centrifugal distortion parameters related to pnJzm terms with n + m = 4 were determined. The last terms will be discussed from a new perspective.

Rovibrational spectra of Ar–SO2 and (SO2)2 van der Waals complexes in the v1 region of SO2

Rovibrational absorption spectra of Ar–SO2 and (SO2)2 van der Waals complexes generated in a slit supersonic jet expansion have been recorded in the v1 symmetric stretching region of SO2 monomer using an external-cavity quantum cascade laser. Two vibrational bands for (SO2)2 and one for Ar–SO2 are assigned and analyzed. All these vibrational bands are of a/c hybrid type. The observed infrared transitions are fitted together with previous radio frequency and microwave spectra, yielding precise molecular constants in both the ground and excited vibrational states for these two complexes. The band origins of (SO2)2 are determined to be 1150.30319(13) cm−1 for vibration on the electron-donor side and 1153.40911(10) cm−1 for vibration on the electron-acceptor side, which is shifted from that of the SO2v1 band by about −1.41 cm-1 and + 1.70 cm−1, respectively. The vibrational energies of the two tunneling components of Ar–SO2 are determined to be 1151.61315(11) cm−1 and 1151.64840(11) cm−1. The tunneling splitting of Ar–SO2 increases from 982.881(32) MHz in the ground state to 1056.7(34) MHz in the excited vibrational state, indicating a decrease of the tunneling barrier upon excitation of the SO2 symmetric stretching vibration.

New infrared spectra of CO2–Xe: modelling Xe isotope effects, intermolecular bend and stretch, and symmetry breaking of the CO2 bend

The infrared spectrum of the weakly bound CO2–Xe complex is studied in the region of the carbon dioxide ν3 fundamental vibration (≈2350 cm−1), using a tunable OPO laser source to probe a pulsed supersonic slit jet expansion. The Xe isotope dependence of the spectrum is modelled by scaling the vibrational and rotational parameters, with the help of previous microwave data. The scaling model provides a good simulation of the observed broadening and (partial) splitting of transitions in the fundamental band, and it is essential for understanding the intermolecular bending combination band where some transitions are completely split by isotope effects. The combination band is influenced by a significant bend–stretch Coriolis interaction and by the relatively large Xe isotope dependence of the intermolecular stretch frequency. The weak CO2–Xe spectrum corresponding to the (0111) ← (0110) hot band of CO2 is also detected and analysed, providing a measurement of the symmetry breaking of the CO2 bending mode induced by the nearby Xe atom. This in-plane/out-of-plane splitting is determined to be 2.14 cm−1.

Long spin coherence times in the ground state and in an optically excited state of Er3+167:Y2SiO5 at zero magnetic field

Spins in solids are an ideal candidate to act as a memory and interface with superconducting qubits due to their long coherence times. We spectroscopically investigate erbium-167-doped yttrium orthosilicate as a possible microwave-addressed memory employing its microwave frequency transitions that occur without applying an external magnetic field. We obtain coherence times of 380 $\mu$s in a ground state spin transition and 1.48 ms in an excited state spin transition. This is 28 times longer compared to previous zero field measurements, as well as 200 times longer than a previous microwave memory demonstration in the same material. These long coherence times show that erbium-167-doped yttrium orthosilicate has potential as a microwave-addressed quantum memory.

Microwave spectroscopy of CD3SH in the first torsionally excited state

The CD3SH molecule, an isotopologue of methyl mercaptan, may be detected in interstellar space with the aid of the new high-sensitivity telescopes. Following our previous microwave study for its ground vibrational state (J. Mol. Spectrosc. 337(2017) 32–36), that for the first torsionally excited state (the vt = 1 torsional state) is reported. In the present study, 371 A-species transitions up to J = 46 and Ka = 7 and 317 E-species transitions up to J = 33 and Ka = 7 in the vt = 1 torsional state were newly assigned. The transition frequency data were least-squares-analyzed by using the pseudo-principal axis method (PAM) Hamiltonian to determine 31 parameters consisting of rotational, centrifugal distortion, and internal-rotational constants.

Microwave Non-destructive Evaluation of Glass Reinforced Epoxy and High Density Polyethylene Pipes

Owing to their inherent advantages, non-metallic structures such as glass reinforced epoxy (GRE) pipes and high-density polyethylene (HDPE) pipes are rapidly replacing their steel counterparts in pipeline infrastructures. However, non-metallic pipes are vulnerable to a variety of defects which compromise their structural integrity. Hence, periodic inspection of these structures is incumbent to ensure their structural integrity and avoid catastrophic in-service failures. Microwave and millimeter wave non-destructive evaluation techniques have shown great potential for inspecting non-metallic structures due to their high sensitivity, resolution, and cost-effectiveness. However, most of the previous investigations performed on microwave non-destructive testing (NDT) of these particular structures are deemed limited since these investigations considered one probe type and often conducted over narrow frequency band. Furthermore, many of the previous microwave NDT investigations considered unrealistically large defects which renders the reported conclusions about the performance envelope of microwave NDT of non-metallic pipes rather inaccurate. Consequently, an uncertainty still exists in the domain with regards to detection capability of the method when the defects in the pipes are actually small in size. Unlike all previous investigations, a comprehensive and realistic microwave imaging of GRE and HDPE pipes with embedded defects of practical importance is demonstrated in this paper. In order to image these defects, various near-field probes operating at wide range of frequencies are employed and compared herein. The relative performance and the effectiveness of microwave NDT for non-metallic pipe inspection using these probes are quantitatively reported. Furthermore, the images of the HDPE pipe produced by microwave probes are benchmarked to phased array ultrasonic testing.

Material Identification Using a Microwave Sensor Array and Machine Learning

In this paper, a novel methodology is proposed for material identification. It is based on the use of a microwave sensor array with the elements of the array resonating at various frequencies within a wide range and applying machine learning algorithms on the collected data. Unlike the previous microwave sensing systems which are mainly based on a single resonating sensor, the proposed methodology allows for material characterization over a wide frequency range which, in turn, improves the accuracy of the material identification procedure. The performance of the proposed methodology is tested via the use of easily available materials such as woods, cardboards, and plastics. However, the proposed methodology can be extended to other applications such as industrial liquid identification and composite material identification, among others.

Photonics Generation of Pulsed Arbitrary-Phase-Coded Microwave Signals Based on the Conversion Between Intensity Modulation and Phase Modulation

This article proposes a pulsed arbitrary-phase-coded microwave signals (APCMSs) photonics generation system. The key to this scheme is to achieve the single-sideband modulated (SSBM) optical signal with the conversion capability between intensity modulation and phase modulation by utilizing dual-parallel Mach–Zehnder modulator (DPMZM) and optical bandpass filter (OBPF). Through heterodyne detection of the SSBM optical signal, pulsed APCMSs can be generated. Unlike previous continuous-wave (CW) APCMSs and pulsed binary phase-coded microwave signals (PCMSs) photonics generation schemes, our scheme provides a pulsed APCMSs generation scheme. The scheme realizes the generation of APCMSs without CW background signals, and the scheme is not only suitable for binary phase codes but also suitable for arbitrary phase codes. The useless CW background signals are effectively suppressed without cascading an intensity modulator for gating operation and the coded phases are linear with the input pre-encoding signals amplitudes, leading to great improvement of practical application values. A theory analysis, a system simulation, and a proof-of-concept experiment are provided. Simulation demonstrates the generations of 2-Gbps 14-GHz and 1-Gbps 16-GHz pulsed APCMSs. The generations of 2-Gbps 14-GHz and 1-Gbps 16-GHz pulsed PCMSs with a 13-bit binary Barker-code and a 16-bit quaternary Frank-code are verified in experiment.

Magnon Bose–Einstein Condensate and Supercurrents Over a Wide Temperature Range

Magnon Bose–Einstein Condensates (BECs) and supercurrents are coherent quantum phenomena, which appear on a macroscopic scale in parametrically populated solid state spin systems. One of the most fascinating and attractive features of these processes is the possibility of magnon condensation and supercurrent excitation even at room temperature. At the same time, valuable information about a magnon BEC state, such as its lifetime, its formation threshold, and coherence, is provided by experiments at various temperatures. Here, we use Brillouin Light Scattering (BLS) spectroscopy for the investigation of the magnon BEC dynamics in a single-crystal film of yttrium iron garnet in a wide temperature range from 30 K to 380K. By comparing the BLS results with previous microwave measurements, we revealed the direct relation between the damping of the condensed and the parametrically injected magnons. The enhanced supercurrent dynamics was detected at 180 K near the minimum of BEC damping.

Solving the mystery of the 1960 Hawaiian lava flow: implications for estimating Earth's magnetic field

Studying historic lava flows provides a rare comparison between direct measurements of the Earth's magnetic field and the field information recorded in the rock record. Connecting direct and indirect measurements provides a way to bridge the gap between historic data in the last 50–100 yr to geologic data over kyr to Gyr. The field strength in Hawaii in 1960 was directly measured at the Honolulu observatory to be 36.47 µT, so our palaeointensity analysis of the 1960 flow is expected to give the same value. Two vertical sections of the 1960 flow (section 1 and section 2) were the focus of a previous microwave palaeointensity study. The microwave experiments were run using the non-standard, perpendicular modified Thellier-type protocol and produced either apparently good quality data that were biased to low values (section 1) or more scattered results averaging close to the expected value (section 2). The cause of the non-ideal behaviour observed in the data from the 1960 flow is a long-standing mystery that it is important to resolve to confirm the reliability of palaeointensity measurements in general, and the microwave demagnetization mechanism in particular. Here, we test the hypothesis that higher quality, unbiased (only random noise) measurements are possible using an improved Thellier-type protocol coupled to an updated microwave system. New palaeointensity experiments were performed primarily using the IZZI protocol (which allows alteration checks during the experiment) adapted for the microwave system. The specimens from section 1 produced more linear Arai plots and gave an estimate of 36.8±3.4 µT, whereas those from section 2 gave an estimate of 39.1±4.6 µT. Our new experiments demonstrate the microwave system's ability to produce accurate results and efficiently run any Thellier-style experiment. We investigate correcting perpendicular data for undetected alteration and find that using too strict selection criteria can be counterproductive to obtaining accurate and precise microwave palaeointensity results.

Submillimeter spectroscopy and astronomical searches of vinyl mercaptan, C2H3SH

Context. New laboratory investigations of the rotational spectrum of postulated astronomical species are essential to support the assignment and analysis of current astronomical surveys. In particular, considerable interest surrounds sulfur analogs of oxygen-containing interstellar molecules and their isomers. Aims. To enable reliable interstellar searches of vinyl mercaptan, the sulfur-containing analog to the astronomical species vinyl alcohol, we investigated its pure rotational spectrum at millimeter wavelengths. Methods. We extended the pure rotational investigation of the two isomers syn and anti vinyl mercaptan to the millimeter domain using a frequency-multiplication spectrometer. The species were produced by a radiofrequency discharge in 1,2-ethanedithiol. Additional transitions were remeasured in the centimeter band using Fourier-transform microwave spectroscopy to better determine rest frequencies of transitions with low-J and low-Ka values. Experimental investigations were supported by quantum chemical calculations on the energetics of both the [C2,H4,S] and [C2,H4,O] isomeric families. Interstellar searches for both syn and anti vinyl mercaptan as well as vinyl alcohol were performed in the EMoCA spectral line survey carried out toward Sgr B2(N2) with ALMA. Results. Highly accurate experimental frequencies (to better than 100 kHz accuracy) for both syn and anti isomers of vinyl mercaptan are measured up to 250 GHz; these deviate considerably from predictions based on extrapolation of previous microwave measurements. Reliable frequency predictions of the astronomically most interesting millimeter-wave lines for these two species can now be derived from the best-fit spectroscopic constants. From the energetic investigations, the four lowest singlet isomers of the [C2,H4,S] family are calculated to be nearly isoenergetic, which makes this family a fairly unique test bed for assessing possible reaction pathways. Upper limits for the column density of syn and anti vinyl mercaptan are derived toward the extremely molecule-rich star-forming region Sgr B2(N2) enabling comparison with selected complex organic molecules.

Infrared spectra of Rg1,2-C6H6 complexes, Rg = He, Ne, Ar

Infrared spectra of Rg1,2 - C6H6 complexes (Rg = He, Ne, Ar) are observed in the region of the ν12 fundamental of C6H6 using a pulsed supersonic jet expansion and a tunable optical parametric oscillator laser source. The mixed trimer He - Ne - C6H6 is also detected. Four bands are analyzed for each complex, namely ν12 itself (≈3048 cm−1) and three linked combination bands (≈3079, 3100, and 3102 cm−1). The results are consistent with previous ultraviolet and microwave results, with Ne2 - C6H6 and He - Ne - C6H6 being analyzed spectroscopically here for the first time.