Frequency-domain Spectrum Research Articles

Multiple Core and Vibronic Coupling Effects in Attosecond Stimulated X-Ray Raman Spectroscopy (SXRS).

Attosecond Stimulated X-ray Raman Spectroscopy (SXRS) is a promising technique for investigating molecular electronic structure and photochemical processes with high spatial and temporal resolution. We present a theoretical study of SXRS from multiple core excitation sites of the same element. Two issues are addressed: interference between pathways contributing the signals from different sites; and how nuclear vibrations influence the signals. Taking furan as a model system, which contains two types of carbons Cα and Cβ, we performed time-dependent density functional theory calculations and computed the SXRS signals with two pulses tuned at the carbon K-edge. Our simulations demonstrate that the SXRS signal from the Cα and Cβ sites are non-additive, owing to the significant mixed contributions (Cα 1s excitations by the pump pulse followed by Cβ 1s excitations by the probe, or vice verse). Harmonic vibrations linearly coupled to the electronic transitions are incorporated using the cumulant expansion. The nuclei act as a bath for electronic transitions which accelerate the decay of time-domain signal. The frequency-domain spectrum is modified by a small red shift and high-resolution fine-structure features are introduced.

Application of Simulated Annealing Particle Swarm Optimization in Response Spectrum Fitting of Simulated Earthquake Wave

In order to get a simulated earthquake wave whose response spectrum fitted well to the smooth design response spectrum, a model was established by making the standard error between the response spectrum of simulated earthquake wave and the design response spectrum as the minimal optimization objective. Simulated annealing particle swarm optimization algorithm, which was an improvement algorithm of particle swarm optimization, was used to solve the model. This spectrum fitting method was compared with the conventional spectrum fitting method, which adjusted Fourier amplitude spectrum in frequency domain. The results show that the method of response spectrum fitting by applying simulated annealing particle swarm optimization algorithm has a good convergence. And the response spectrum of simulated earthquake wave generated by simulated annealing particle swarm optimization algorithm agrees better with the design response spectrum than that by conventional spectrum fitting method.

Application of Simulated Annealing Particle Swarm Optimization in Response Spectrum Fitting of Simulated Earthquake Wave

In order to get a simulated earthquake wave whose response spectrum fitted well to the smooth design response spectrum, a model was established by making the standard error between the response spectrum of simulated earthquake wave and the design response spectrum as the minimal optimization objective. Simulated annealing particle swarm optimization algorithm, which was an improvement algorithm of particle swarm optimization, was used to solve the model. This spectrum fitting method was compared with the conventional spectrum fitting method, which adjusted Fourier amplitude spectrum in frequency domain. The results show that the method of response spectrum fitting by applying simulated annealing particle swarm optimization algorithm has a good convergence. And the response spectrum of simulated earthquake wave generated by simulated annealing particle swarm optimization algorithm agrees better with the design response spectrum than that by conventional spectrum fitting method.

An Application of the Frequency-Domain Experience Mode Decomposition to Enhance Deconvolution Results

To improve the resolution of the seismic section after deconvolution, a method based on frequency-domain experience mode decomposition was proposed. Empirical mode decomposition (EMD) method is usually used to analyze the time domain non-stationary signal, in order to better recover original reflection coefficient sequence, empirical mode decomposition was implemented for frequency-domain amplitude spectrum. Through the different characteristics between the equivalent filter amplitude after deconvolution and reflection coefficient sequence amplitude in frequency-domain, the real reflection coefficient sequence was recovered. Simulation results indicate that the method is effective and feasible.

Terahertz optical parameters and lattice vibration-induced resonance of Er3+-doped Y3Al5O12 crystal

In this paper, the terahertz time-domain spectra of undoped and Er3+-doped YAG crystals with the components of Y3Al5O12, (Er0.2Y0.8)3Al5O12, and (Er0.5Y0.5)3Al5O12 were detected and the corresponding frequency-domain spectra were also obtained after Fourier transform. It is shown that the three kinds of crystals are weakly dispersive and nearly transparent below 2.10 THz, whereas in the frequency region between 2.10 and 2.60 THz, dielectric resonance and obvious absorption are investigated which can be attributed to the low-frequency lattice phonon. It is also found that with the increase in Er3+ concentration, the resonant frequency exhibits a red shift and the values of refractive index show a small increase, which could be explained by the larger ionic mass and larger polarizability of Er3+ compared to Y3+, respectively. This work contributes to the research on YAG laser materials as well as novel materials applicable to THz devices.

Study on hydration process of early-age concrete using embedded active acoustic and non-contact complex resistivity methods

The microstructure development of early-age concrete during hydration process was investigated by means of embedded active acoustic and non-contact complex resistivity monitoring methods. Embedded active acoustic method utilized cement-based piezoelectric sensors and FEbLIN acoustic system to continuously emit and receive compressional acoustic wave. Signal-based parameters such as velocity, attenuation and frequency domain spectrum variation were evaluated to analyze the microstructure development of early-age concrete. Meanwhile, non-contact complex resistivity method was utilized to measure real-time electrical impedance of early-age concrete. These two innovative non-destructive testing methods have distinguished merit. Compressional acoustic wave is sensitive to the connectivity of the solid phase network in the fresh concrete. It continuously hunts for the shortest propagation path during monitoring. While the inducted current generated in impedance measurement is more sensitive to the liquid phase network conditions of the fresh concrete. It strictly adopts the transition path with lowest electrical impedance. In this study, the behaviors and properties of fresh concrete are interpreted based on the measurement results of these two methods. It is found that the hydration stages can be distinguished according to the characteristics of acquired parameters, and so does the maturity of concrete.

Using terahertz time-domain spectroscopical technique to monitor cocrystal formation between piracetam and 2,5-dihydroxybenzoic acid

Far-infrared vibrational absorption of cocrystal formation between 2,5-dihydroxybenzoic acid (2,5-DHBA) and piracetam compounds under solvent evaporation and grinding methods have been investigated using terahertz time-domain spectroscopy (THz-TDS) at room temperature. The experimental results show large difference among absorption spectra of the formed cocrystals and the involved individual parent molecules in 0.20–1.50 THz region, which probably originated from the intra-molecular and inter-molecular hydrogen bonds due to the presence of two hydroxyl groups in 2,5-DHBA and amide moieties in piracetam compound. The THz absorption spectra of two formed cocrystals with different methods are almost identical. With grinding method, the reaction process can be monitored directly from both time-domain and frequency-domain spectra using THz-TDS technique. The results indicate that THz-TDS technology can absolutely offer us a high potential method to identify and characterize the formed cocrystals, and also provide the rich information about their reaction dynamic process involving two or more molecular crystals in situ to better know the corresponding reaction mechanism in pharmaceutical fields.

Impact of Pellet Thickness on Quantitative Terahertz Spectroscopy of Solid Samples in a Polyethylene Matrix

Pellets composed of different weight-percent (wt-%) of lactose within a polyethylene (PE) matrix are used to examine how the physical thickness of solid samples impact analytical measurements performed over terahertz (THz) frequencies when using time-domain THz spectroscopy. Results indicate that the thickness of each pellet depends on the mass and physical properties of the individual components that comprise the pellet. Thickness of mixture pellets depends on the porosity of the individual pellet components. Porosity measurements presented here for PE and lactose give values of 25.6 ± 0.3 and 14.5 ± 0.1, respectively, which indicate that more air is trapped within the compressed PE matrix compared to that for lactose. This difference in porosity creates different pellet thicknesses for pellets of the same nominal mass but with different relative amounts of PE and lactose. For this binary matrix, the thickness of each pellet is found to be a linear combination of the compressed densities of the individual components. Analysis of the time-domain THz spectra reveals that thinner samples are confounded by a fringe pattern observed in the frequency-domain spectra. This fringe pattern is created by an etalon corresponding to the air/pellet interfaces for the sample in the optical path. Spectra collected from thicker pellets are confounded by a sloping baseline caused by scattering effects within the pellet matrix. The quantitative impact of pellet thickness is determined by comparing the mean standard error of calibration (MSEC) and mean standard error of prediction (MSEP) for a set of leave-three-out cross validation multivariate calibration models based on the partial least-squares (PLS) algorithm. Results indicate that PLS models are capable of analytical measurements with MSEC and MSEP values between 0.04 and 0.20 wt-%. Analysis of spectral variance captured within the corresponding spectral loadings for each model indicates that spectral variance is lowest for the 300 mg samples where the impact of scattering is minimal under conditions when the sample etalon is nonexistent.

Refined Distances Between Paramagnetic Centers of a Multi‐Copper Nitrite Reductase Determined by Pulsed EPR (iDEER) Spectroscopy

Refined Distances Between Paramagnetic Centers of a Multi‐Copper Nitrite Reductase Determined by Pulsed EPR (<i>i</i>DEER) Spectroscopy

Mn2+ induced luminescence regulation and enhancement of Lu-based nanocrystals

Transformation from Lu-based nanocrystals in hexagonal and cubic mixed phases to pure cubic phase was observed through adjusting the doping concentration of Mn2+. The mechanism for the phase transformation was discussed in detail. Studies on the time and frequency domain spectra indicated that the semi-pure red emissions in cubic Na5Lu9F32: 40% Mn2+, 20% Yb3+, 2% Ln3+ (Ln=Er3+, Ho3+) nanocrystals were caused by a two-step energy transfer between Mn2+ and Ln3+ ions. After incorporating of Mn2+ ions into the host lattices, the local symmetry around the luminescent ion was reduced, which induced the increase of radiative rates for transitions that were mainly contributed by electric dipole radiations. Considerable enhancements in upconversion and downconversion luminescence were accompanied. The result of the current study has great application potential in bioimaging and solar cells.

高分辨宽测绘带星载合成孔径雷达频域重构算法

高分辨宽测绘带星载合成孔径雷达频域重构算法

Real and imaginary modulation spectral subtraction for speech enhancement

In this paper, we propose a novel spectral subtraction method for noisy speech enhancement. Instead of taking the conventional approach of carrying out subtraction on the magnitude spectrum in the acoustic frequency domain, we propose to perform subtraction on the real and imaginary spectra separately in the modulation frequency domain, where the method is referred to as MRISS. By doing so, we are able to enhance magnitude as well as phase through spectral subtraction. We conducted objective and subjective evaluation experiments to compare the performance of the proposed MRISS method with three existing methods, including modulation frequency domain magnitude spectral subtraction (MSS), nonlinear spectral subtraction (NSS), and minimum mean square error estimation (MMSE). The objective evaluation used the criteria of segmental signal-to-noise ratio (Segmental SNR), PESQ, and average Itakura–Saito spectral distance (ISD). The subjective evaluation used a mean preference score with 14 participants. Both objective and subjective evaluation results have demonstrated that the proposed method outperformed the three existing speech enhancement methods. A further analysis has shown that the winning performance of the proposed MRISS method comes from improvements in the recovery of both acoustic magnitude and phase spectrum.

Improved Stepped-Frequency SAR Imaging Algorithm With the Range Spectral-Length Extension Strategy

In order to achieve a high range resolution and low instantaneous bandwidth, stepped frequency synthetic aperture radar (SAR) is widely used with stepped-frequency chirps and frequency-domain spectrum reconstruction method. To combine the spectrum reconstruction with SAR image algorithms, “Image for each sub-band and then reconstruct the wideband spectrum” is a more adaptable flow since its validity for almost all imaging algorithms. However, due to the two-dimensional (2-D) spectrum twisting, a special problem called range spectrum aliasing may occur and lead the flow to invalidity, especially in high-resolution SAR systems. In this paper, we discuss the extended range spectrum by the analysis of the 2-D twisting spectrum, judge its influence through an azimuth threshold resolution, and propose an improved stepped-frequency SAR imaging algorithm with the range spectral-length extension (RSLE) strategy, which can eliminate the range spectrum aliasing. After theoretic analyses, simulations and experimental results are provided to demonstrate the proposed algorithm.

Learning gait of quadruped robot without prior knowledge of the environment

Walking is the basic skill of a legged robot, and one of the promising ways to improve the walking performance and its adaptation to environment changes is to let the robot learn its walking by itself. Currently, most of the walking learning methods are based on robot vision system or some external sensing equipment to estimate the walking performance of certain walking parameters, and therefore are usually only applicable under laboratory condition, where environment can be pre-defined. Inspired by the rhythmic swing movement during walking of legged animals and the behavior of their adjusting their walking gait on different walking surfaces, a concept of walking rhythmic pattern(WRP) is proposed to evaluate the walking specialty of legged robot, which is just based on the walking dynamics of the robot. Based on the onboard acceleration sensor data, a method to calculate WRP using power spectrum in frequency domain and diverse smooth filters is also presented. Since the evaluation of WRP is only based on the walking dynamics data of the robot's body, the proposed method doesn't require prior knowledge of environment and thus can be applied in unknown environment. A gait learning approach of legged robots based on WRP and evolution algorithm(EA) is introduced. By using the proposed approach, a quadruped robot can learn its locomotion by its onboard sensing in an unknown environment, where the robot has no prior knowledge about this place. The experimental result proves proportional relationship exits between WRP match score and walking performance of legged robot, which can be used to evaluate the walking performance in walking optimization under unknown environment.

An arbitrary waveform generator based chirped pulse Fourier transform spectrometer operating from 260 to 295 GHz

The performance of a chirped-pulse Fourier transform millimeter-wave spectrometer operating from 260 to 295GHz is described. The spectrometer uses a high-speed arbitrary waveform generator (AWG) to create both a chirped excitation pulse and the single-frequency local oscillator (LO) used for the final down conversion detection stage. The mm-wave excitation source is an active multiplier chain (factor of 24 frequency multiplication) with power output of greater than 10mW across the 260–295GHz frequency range. The LO, produced by a separate active multiplier chain (factor of 12 frequency multiplication), drives a subharmonic mixer which downconverts the molecular emission to the microwave region for digitization on a 100GS/s digital oscilloscope. All frequency sources in the experiment are locked to a 10MHz Rb-disciplined oscillator providing direct frequency calibration for molecular transitions in the Fourier transform frequency-domain spectrum. Benchmark measurements are presented on ethyl cyanide and 1-butyne and are used to illustrate advantages and tradeoffs compared with direct absorption millimeter-wave spectroscopy.

Mechanisms of bridge-mediated electron transfer: A TDDFT electronic dynamics study

We present a time-dependent density functional theory approach for probing the dynamics of electron transfer on a donor-bridge-acceptor polyene dye scaffold. Two kinds of mechanisms, namely, the superexchange mechanism and the sequential mechanism, may be involved in the electron transfer process. In this work, we have focused on the crossover between these two charge transfer mechanisms on a series of donor-bridge-acceptor polyene dye systems with varying lengths of conjugated bridges. A number of methods and quantities are used to assist in the analysis, including the phase relationship of charge evolution and frequency domain spectra of the time-dependent dipole. Our simulations show that the superexchange mechanism plays a dominant role in the electron transfer from donor to acceptor when the bridge length is small, and the sequential mechanism becomes more important as the polyene bridge is lengthened. Full Ehrenfest dynamics with nuclear motion show that molecular vibrations play a very small role in such ultrafast charge transfer processes.

Characteristic and Modeling of Human Body Motions for Body Area Network Applications

Many current and future medical devices are wearable and human body is used as a carrier for wireless communication, which implies human body to be a crucial part of the transmission medium in body area networks (BANs). In order to understand the propagation characteristics around human body, the statistical model is derived for communication links in the medical implant communication service band, industrial scientific medical band and ultra-wideband based on the narrowband measurement. The channel model of diffracting components around human body were different from one scenario to another. Moreover, second order statistics, including level crossing rate and fade duration, are presented for each scenario to evaluate the link quality and outage performance for on-body to on-body scenario. Using a network analyzer, Doppler spread spectrum in frequency domain and coherence time in time domain from temporal variations of human body movements are also analyzed from diverse perspectives. Additionally, the shape of Doppler spread spectrum is fitted to describe the relationship of power and frequency. The proposed on-body to on-body channel model for human body motions can be used to better design wireless network protocols for BANs.

A rule-based intelligent method for fault diagnosis of rotating machinery

To better equip with a non-expert to carry out the diagnosis operations, a new method for intelligent fault identification of rotating machinery based on the empirical mode decomposition (EMD), dimensionless parameters, fault decision table (FDT), MLEM2 rule induction algorithm and improved rule matching strategy (IRMS) is proposed in this paper. EMD is used to preprocess the vibration signals for mining the fault characteristic information more accurately. Then, dimensionless parameters are extracted from both the decomposed signals in time domain and envelop spectrum in frequency domain respectively to form the conditional attributes of a FDT. Moreover, MLEM2 algorithm is run directly on the FDT to generate decision rules imbedded in the data. To make the following classification process more robust, the IRMS is adopted to resolve the conflicting and non-matching problems. Finally, data of rolling element bearings with four typical working conditions is used to evaluate the performance of the proposed method. The testing result demonstrates that the method has high accuracy and systematically good performance. It is proved to be a convenient, concise, interpretable and reliable way to diagnose bearings’ faults. The advantages are also confirmed by the comparisons with the other two approaches, i.e. the principal component analysis (PCA) and probabilistic neural network (PNN) based method as well as the wavelet transform (WT) and genetic algorithm (GA) based one. Furthermore, thank to the FDT working as a data interface, the method is more transplantable, therefore it may be applied to diagnose other types of rotating machines effectively.

Vibration Fault Diagnosis of Rotating Machine Based on the Principle of Entropy Increase

Information fusion is the main method to improve diagnosis accuracy. In this paper, fusion information entropy is defined by fusing speed with three kinds of information entropy that are singular spectrum entropy in time domain, power spectrum entropy in frequency domain and wavelet energy spectrum entropy in time-frequency domain from multiple perspectives to describes vibration fault characteristics. According to the principle of entropy increase, condition information entropy increase of vibration fault are defined and used as new criterion for fault diagnosis, and the grey correlation between sample fault and typical fault is defined to realize the vibration fault diagnosis. Finally, the method’s effectiveness is verified by vibration simulation data from vibration fault simulator. The result shows that this method can identify 6 typical vibration faults correctly.

Frequency-domain terahertz transmission spectra of Mn3 and Mn12 single-molecule magnets

Frequency-domain terahertz transmission spectra of Mn3 and Mn12 single molecule magnets (SMMs) have been measured at different temperatures, and hence the anisotropic parameters D2 and D4 of the spin Hamiltonian \(\hat H = D_2 \hat S_z^2 + D_4 \hat S_z^4\) have been calculated. For Mn12 SMM, D2=−10.9 GHz and D4=−2.59×10−2 GHz, while for Mn3 SMM, D2=−22.0 GHz and D4 can be considered negligible. This suggests Mn3 SMM can be considered as a simpler and more suitable candidate for magnetic quantum tunneling research.