Frequency-domain Experiments Research Articles

High-Resolution Frequency-Domain Spectroscopic and Modeling Studies of Photosystem I (PSI), PSI Mutants and PSI Supercomplexes.

Photosystem I (PSI) is one of the two main pigment-protein complexes where the primary steps of oxygenic photosynthesis take place. This review describes low-temperature frequency-domain experiments (absorption, emission, circular dichroism, resonant and non-resonant hole-burned spectra) and modeling efforts reported for PSI in recent years. In particular, we focus on the spectral hole-burning studies, which are not as common in photosynthesis research as the time-domain spectroscopies. Experimental and modeling data obtained for trimeric cyanobacterial Photosystem I (PSI3), PSI3 mutants, and PSI3-IsiA18 supercomplexes are analyzed to provide a more comprehensive understanding of their excitonic structure and excitation energy transfer (EET) processes. Detailed information on the excitonic structure of photosynthetic complexes is essential to determine the structure-function relationship. We will focus on the so-called "red antenna states" of cyanobacterial PSI, as these states play an important role in photochemical processes and EET pathways. The high-resolution data and modeling studies presented here provide additional information on the energetics of the lowest energy states and their chlorophyll (Chl) compositions, as well as the EET pathways and how they are altered by mutations. We present evidence that the low-energy traps observed in PSI are excitonically coupled states with significant charge-transfer (CT) character. The analysis presented for various optical spectra of PSI3 and PSI3-IsiA18 supercomplexes allowed us to make inferences about EET from the IsiA18 ring to the PSI3 core and demonstrate that the number of entry points varies between sample preparations studied by different groups. In our most recent samples, there most likely are three entry points for EET from the IsiA18 ring per the PSI core monomer, with two of these entry points likely being located next to each other. Therefore, there are nine entry points from the IsiA18 ring to the PSI3 trimer. We anticipate that the data discussed below will stimulate further research in this area, providing even more insight into the structure-based models of these important cyanobacterial photosystems.

Solving viscoelastic problems with a Laplace transform approach supplanted by ARX models, suggesting a way to upgrade Finite Element or spectral codes

Finite Element codes used for solving the mechanical equilibrium equations in transient problems associated to (time-dependent) viscoelastic media generally relies on time-discretized versions of the selected constitutive law. Recent concerns about the use of non-integer differential equations to describe viscoelasticity or well-founded ideas based upon the use of a behavior's law directly derived from Dynamic Mechanical Analysis (DMA) experiments in frequency domain, could make the Laplace domain approach particularly attractive if embedded in a time discretized scheme. Based upon the inversion of Laplace transforms, this paper shows that this aim is not only possible but also gives rise to a simple algorithm having good performances in terms of computation times and precision. Such an approach, which fully relies on the Laplace-defined Behavioral Transfer Function (LTBF) can be promoted if it uses AutoRegressive with eXogeneous input parametric models perfectly substitutable to the real LTBF. They avoid the hitherto prohibitive pitfall of having to store all past data in the computer's memory while maintaining an equal computation precision.

Spatially separated heterodyne grating interferometer for eliminating periodic nonlinear errors.

Periodic nonlinear errors caused by frequency mixing are serious obstacles for increasing the resolution of heterodyne grating interferometers. To eliminate the periodic nonlinear errors, a spatially separated heterodyne grating interferometer is proposed in this study. Two modulated beams with different frequencies are transferred respectively by two fibers, which form a spatially separated construction. A couple of comparison experiments in both time domain and frequency domain are designed and conducted. The results of the frequency-spectrum analysis experiment showed that the periodic nonlinear errors were no larger than 0.086 nm, which proved that the proposed system was effectual in eliminating periodic nonlinear errors.

Control of Nonadiabatic Passage through a Conical Intersection by a Dynamic Resonance.

Nonadiabatic processes, dominated by dynamic passage of reactive fluxes through conical intersections (CIs), are considered to be appealing means for manipulating reaction paths, particularly via initial vibrational preparation. Nevertheless, obtaining direct experimental evidence of whether specific-mode excitation affects the passage at the CI is challenging, requiring well-resolved time- or frequency-domain experiments. Here promotion of methylamine-d2 (CH3ND2) molecules to spectral-resolved rovibronic states on the excited S1 potential energy surface, coupled to sensitive D photofragment probing, allowed us to follow the N-D bond fission dynamics. The branching ratios between slow and fast D photofragments and the internal energies of the CH3ND(X̃) photofragments confirm correlated anomalies for predissociation initiated from specific rovibronic states. These anomalies reflect the existence of a dynamic resonance that strongly depends on the energy of the initially excited rovibronic states, the evolving vibrational mode on the repulsive S1 part during N-D bond elongation, and the manipulated passage through the CI that leads to CH3ND radicals excited with C-N-D bending. This resonance plays an important role in the bifurcation dynamics at the CI and can be foreseen to exist in other photoinitiated processes and to control their outcome.

Novel Method for Processing the Dynamic Calibration Signal of Pressure Sensor.

Dynamic calibration is one of the important ways to acquire the dynamic performance parameters of a pressure sensor. This research focuses on the processing method for the output of calibrated pressure sensor, and mainly attempts to solve the problem of extracting the true information of step response under strong interference noise. A dynamic calibration system based on a shock tube is established to excite the time-domain response signal of a calibrated pressure sensor. A key processing on difference modeling is applied for the obtained signal, and several generating sequences are established. A fusion process for the generating sequences is then undertaken, and the true information of the step response of the calibrated pressure sensor can be obtained. Finally, by implementing the common QR decomposition method to deal with the true information, a dynamic model characterizing the dynamic performance of the calibrated pressure sensor is established. A typical pressure sensor was used to perform calibration tests and a frequency-domain experiment for the sensor was also conducted. Results show that the proposed method could effectively filter strong interference noise in the output of the sensor and the corresponding dynamic model could effectively characterize the dynamic performance of the pressure sensor.

A finite element formulation for smart piezoelectric composite shells: Mathematical formulation, computational analysis and experimental evaluation

A formulation of a smart shell finite element for laminated curved structures with piezoelectric layers working under either the d31 or d33 effects has been developed in order to simulate dynamic tests. The electrical domain was modeled using a first order theory for the electrical field and a layer-wise approach. The mechanical domain is modeled using a degenerated shell theory with implicit curvature, considering a first order shear deformation theory and an equivalent single layer approach. The final formulation was implemented within Abaqus™ commercial finite element analysis package using its User Element (UEL) subroutine. First, some results provided by the proposed formulation were compared to numerical analyses shown by the literature. Second, the implemented finite element was evaluated using modal and frequency domain experiments for a cantilever aluminum beam with two piezoelectric transducers working under the d31 effects, and a free-free curved composite plate with four piezoelectric transducers working under the d33 effects. Comparisons between the natural frequencies and the frequency response functions amplitudes obtained from the experiments and the computational analyses were performed in order to discuss the limitations and potentialities of the proposed formulation.

Self-Injection Locking and Phase-Locked States in Microresonator-Based Optical Frequency Combs

Microresonator-based optical frequency combs have been a topic of extensive research during the last few years. Several theoretical models for the comb generation have been proposed; however, they do not comprehensively address experimental results that show a variety of independent comb generation mechanisms. Here, we present frequency-domain experiments that illuminate the transition of microcombs into phase-locked states, which show characteristics of injection locking between ensembles of comb modes. In addition, we demonstrate the existence of equidistant optical frequency combs that are phase stable but have nondeterministic phase relationships between individual comb modes.

Fully absorptive 3D IR spectroscopy using a dual mid-infrared pulse shaper.

This paper presents the implementation of 3D IR spectroscopy by adding a second pump beam to a two-beam 2D IR spectrometer. An independent mid-IR pulse shaper is used for each pump beam, which can be programmed to collect its corresponding dimension in either the frequency or time-domains. Due to the phase matching geometry employed here, absorptive 3D IR spectra are automatically obtained, since all four of the rephasing and non-rephasing signals necessary to generate absorptive spectra are collected simultaneously. Phase cycling is used to isolate the fifth-order from the third-order signals. The method is demonstrated on tungsten hexacarbonyl (W(CO)6) and dicarbonylacetylacetonato rhodium (I), for which the eigenstates are extracted up to the third excited state. Pulse shaping affords a high degree of control over 3D IR experiments by making possible mixed time- and frequency-domain experiments, fast data acquisition and straightforward implementation.

Time and frequency domain analysis of hydrogen permeation across PdCu metallic membranes for hydrogen purification

Hydrogen permeation is a process used in the industry for purification purposes. Palladium alloys (PdAg and PdCu) are commonly used as membrane material. In this communication, we report on the kinetics of hydrogen permeation across Pd 0.47Cu 0.53 metallic membranes which can be used in catalytic crackers of biofuels. The permeation mechanism is a multi-step process including surface chemisorption of molecular hydrogen (upstream side of the membrane), hydrogen diffusion across bulk regions, hydrogen recombination (downstream side of the membrane) and evolution. The role of different operating parameters (temperature, surface state, sample microstructure) is analyzed and discussed using both time and frequency domain experiments. Experimental pneumato-chemical impedance diagrams show that there is no significant rate-limitation at surfaces, except at low temperatures close to room temperature. Diffusion-controlled transport of hydrogen across the membrane is rate-determining. However, the value of the hydrogen diffusion coefficient does not rise exponentially with operating temperature in the 40–400 °C temperature range under investigation, as expected for a thermally activated diffusion process. At temperatures as low as 300 °C, new rate-limitations appear. They can be attributed to recrystallization and/or phase transformation processes induced by temperature and the presence of hydrogen.

Photoinduced coherent adsorbate dynamics on a metal surface: Nuclear wave-packet simulation with quasi-diabatic potential energy curves using an open-boundary cluster model approach

We present a nuclear wave-packet simulation of photoinduced coherent adsorbate dynamics on a metal surface with quasi-diabatic potential energy curves obtained from our recently developed open-boundary cluster model approach. Photoexcitation to the resonant adsorbate state and the subsequent ultrafast decay to the electronically excited substrate states were found to cause a coherent vibration of the adsorbate on the metal surface. This process competes with a Raman scattering process, which is generally believed to explain the coherent adsorbate vibration. These two mechanisms induce vibrations with a common frequency, and therefore cannot be distinguished from each other in a frequency-domain experiment. However, they can be distinguished by determining the initial vibrational phase through a time domain experiment such as ultrafast pump-probe spectroscopy. We further demonstrate that for near-resonant excitation the oscillation amplitude induced by our proposed mechanism largely exceeds the amplitude due to the Raman mechanism.

So many factors, so little time…Simulation experiments in the frequency domain

We illustrate how an efficient methodology called frequency domain experimentation can be used to gain better insight into the behavior of production systems. With the full factorial designs commonly used for simulation experiments, the number of runs grows exponentially with the number of factors involved, while the run length remains constant. In frequency domain experiments, the number of runs is independent of the number of factors, while the required run lengths increase relatively slowly. We describe the method, illustrate its effectiveness at identifying important main effects, two-way interactions, and quadratic terms for a known model, demonstrate the approach by evaluating a kanban system involving 34 factors, and provide links to software. We also present computational requirements for running simulation experiments that combine a batch means approach with efficient run-oriented designs for a variety of systems. The results indicate that frequency domain experiments perform very favorably for systems, such as queueing networks, where the simulation's output stream exhibits high positive autocorrelation.

Evaluation of photon migration using a two speed model for characterization of packed powder beds and dense particulate suspensions

A two-speed photon diffusion equation is developed for light propagation in a powder bed of high volume fraction or dense particulate suspension, whereby the light speed is impacted by the refractive index difference between particles and the suspending medium. The equation is validated using Monte Carlo simulation of light propagation coupled with dynamic simulation of particle sedimentation for the non-uniform arrangement of powder particles. Frequency domain experiments at 650 nm for a 77-microm-diameter resin-powder and 50-microm-diameter lactose-powder beds as well as resin-water and lactose-ethanol suspensions confirm the scattering and absorption coefficients derived from the two-speed diffusion equation.

2H nuclear magnetic resonance study of the molecular motion in cyanoadamantane. I. Supercooled plastically crystalline phase.

The supercooled plastically crystalline phase (glassy crystal) of cyanoadamantane was investigated by multidimensional 2H NMR (T>Tg). Although the orientationally disordered crystalline phase always coexisted with the orientationally ordered crystalline phase, we were able to single out the signal from the glassy crystal by selective excitation and it was possible to carry out line shape measurements and two-dimensional (2D) experiments (in frequency and time domain). The latter directly reveal sixfold jumps with an reorientation of the molecular C3 axis via 90 degrees angles, thus reflecting the symmetry of the lattice. The motion around the C3 axis is found to be always fast. We can reproduce the line shape by random walk simulations properly taking into account these molecular motions. Both methods (line shape and 2D experiments) yield time constants which agree with those reported by other techniques. Refining the analysis a narrow distribution of correlation times is introduced to account for a weak stretching of the correlation function. We did not find any indication of a small angle process usually found in structural glasses. Thus, the motional process in the glassy crystal appears to be simple and quite different from that in structural glasses.

Wideband measurement of transition duration and frequency spectra due to small gap discharge as low-voltage ESD

Very fast transition duration due to small gap discharge as the low-voltage ESD was investigated in time and frequency domains. The characteristics of rising time of the transition duration were already cleared in 4.5 GHz bandwidth system. In this paper, the measurement system with a tapered coaxial electrode was improved for higher frequency. The insertion loss of a new coaxial electrode was within −3 dB in frequency range below about 6 GHz. As a consequence of the experiment using the new system, the voltage rise time was under 90 ps in discharging voltage of below 600 V. In addition, a relationship between maximum frequency range of the transients and discharging voltage were cleared by experiments in frequency domain using the 4.5 GHz system.

Wideband measurement of transition duration and frequency spectra due to small gap discharge as low-voltage ESD

Very fast transition duration due to small gap discharge as the low-voltage ESD was investigated in time and frequency domains. The characteristics of rising time of the transition duration were already cleared in 4.5 GHz bandwidth system. In this paper, the measurement system with a tapered coaxial electrode was improved for higher frequency. The insertion loss of a new coaxial electrode was within −3 dB in frequency range below about 6 GHz. As a consequence of the experiment using the new system, the voltage rise time was under 90 ps in discharging voltage of below 600 V. In addition, a relationship between maximum frequency range of the transients and discharging voltage were cleared by experiments in frequency domain using the 4.5 GHz system.

Primary Charge-Separation Rate at 5 K in Isolated Photosystem II Reaction Centers Containing Five and Six Chlorophyll a Molecules

Although the primary charge-separation rate in the photosystem II (PSII) reaction center (RC) has been the subject of many time- and frequency-domain experiments, its value is still an unsettled issue, especially at or near room temperature. As a first step toward understanding its temperature dependence, it is important to have a reliable value for the rate at liquid helium temperatures. Presented are results from triplet bottleneck hole-burning (TBHB) experiments at 5.0 K on RCs isolated from spinach that contain either six or five chlorophyll a (Chl) molecules per two pheophytin (Pheo) molecules (referred to as RC-6 and RC-5). RC-5 possesses only one of the two peripheral Chls of the RC. The triplet state that serves as the population bottleneck is formed by charge recombination of the radical ion pair P680 + Pheo1- , where P680 is the primary electron donor and Pheo1 is the acceptor on the active (D1) branch. In the TBHB experiments, the laser burn intensity was varied over 2 orders of magnitude to assess the contribution from fluence broadening to the width of the zero-phonon hole (ZPH) burned into the P680 absorption band. The widths of the ZPH were also corrected for interference from a weak and sharp hole contribution from a state(s) that is unlikely to be involved in efficient charge-separation. The corrected ZPH widths for RC-5 and RC-6 were the same (2.3 ± 0.2 cm-1) and correspond to a charge-separation rate of (4.6 ± 0.4 ps)-1, in good agreement with a value recently measured for RC-6 by femtosecond pump−probe spectroscopy at 7 K (Greenfield, S. R.; Seibert, M.; Wasielewski, M. R. J. Phys. Chem. B 1999, 103, 8364). It appears that the removal of a peripheral Chl does not lead to structural changes in the core region of the RC that affect primary charge separation.

Predissociation of the 4pπL 1Π Rydberg state of carbon monoxide

Time-domain and frequency-domain spectroscopic experiments have been performed on the (4pπ)L 1 Π, v=0 Rydberg state in three isotopomers of carbon monoxide. Accurate values for the excited state lifetimes of the f-parity components have been determined: τ( 12 C 16 O)=1.08±0.05 ns , τ( 13 C 16 O)=72±10 ps and τ( 13 C 18 O)=29±6 ps . The spectral resolution in the frequency-domain experiment goes as far as the limit imposed by the natural lifetime; Q-branch lines, or f-parity components of the heavier isotopes, are resolved for the first time. Highly accurate transition frequencies are determined in a molecular beam experiment using comparison and interpolation with a saturated iodine reference standard. The results reveal a number of perturbations and predissociation mechanisms, displaying a high degree of complexity in the energetic region of the 4p Rydberg states of CO with strong isotopic effects.

Efficient data allocation for frequency domain experiments

Frequency domain experiments are efficient screening procedures for identifying important factors in simulation models. An empirical investigation involving a simple autoregressive system and a complex queueing system shows that allocating data unequally among the signal and noise runs may be more effective when the total data collection effort is limited.

Femtosecond CARS on H2

Abstract Time-resolved non-resonant CARS (coherent anti-Stokes Raman spectroscopy) is applied to probe H2 in the gas phase with femtosecond time resolution. The experimental transients show detailed beating structures, reflecting the rotational dynamics of all thermally populated ground state J levels with high fidelity. Modelling of the experimental data yields temperatures and pressures with high resolution. The good agreement with values determined by frequency-domain experiments demonstrates the suitability of this method for the determination of J-dependent collisional-broadening and line-shifting coefficients in gaseous mixtures at high pressures and temperatures, where frequency-domain CARS spectra are difficult to model.

H ∞ -Based Robust Torque Control of Harmonic Drive Systems

In this paper, the torque control of a harmonic drive system for constrained-motion and free-motion applications is examined in detail. A nominal model for the system is obtained in each case from experimental frequency responses of the system, and the deviation of the system from the model is encapsulated by a multiplicative uncertainty. Robust torque controllers are designed using this information in an H∞-framework, and implemented on two different setups. From time and frequency domain experiments, it is shown that the closed-loop system retains robust stability, while improving the tracking performance exceptionally well. To further improve the performance of the system for free-motion case, a feedforward friction-compensation algorithm is implemented in addition to the robust torque control. It is shown that friction-compensation will shrink the model uncertainty at low frequencies and hence, the performance of the closed-loop system will be improved at those frequencies.