Multi-pulse Experiment Research Articles

Comparison of Submillimeter Spot Ablation of Copper and Nickel by Multipulse Picosecond and Femtosecond Laser

The current transmission and reflection laser ablation micropropulsion modes have the problem of a complex working medium supply system in engineering. Therefore, we propose large-spot laser ablation with a one-dimensional supply mode. In order to verify this ablation mode, a multipulse ablation experiment of submillimeter-scale light spots was carried out on the surface of pretreated copper and nickel under the atmosphere using an ultrafast laser with a pulse width of 290 fs and 10 ps. The results show that femtosecond laser multipulse ablation (FLMA) leads to the grain refinement of copper, the crater quality of the two metals under FLMA is better, and picosecond laser multipulse ablation (PLMA) causes the crater of nickel to form a dense remelting bulge that affects laser absorption; both metals have obvious heat-affected zones after FLMA and PLMA, the heat-affected zones of nickel are 5–10% larger than those of copper, and the ablation depth of copper is deeper. Under the same conditions, the ablation mass of copper is smaller than that of nickel, and the specific impulse performance of laser ablation micropropulsion is better.

Free Induction Decay in Zigzag Spin Chains in a Multi-pulse NMR Experiment

The free induction decay (FID) is investigated for zigzag (alternating) spin chains in the approximation of the nearest neighbor interactions. The dependence of the FID on the ratio of dipolar coupling constants (the parameter of dimerization) is also studied. Possible comparison of the obtained results with the experimental data for proton zigzag chains in a single crystal of hambergite is discussed.

Controlling Intermolecular H-atom Abstraction with Ultrafast Pump-Push-Probe Spectroscopy

We utilize ultrafast multi-pulse pump-push-probe transient absorption spectroscopy and time-resolved photoluminescence to monitor excited-state H-atom transfer from hydroxylic compounds to the heptazine derivative 2,5,8-tris(4-methoxyphenyl)-1,3,5,6,7,9,9b-heptaazaphenalene (TAHz). The heptazine moiety is structurally related to the monomer unit of the ubiquitous organic polymeric photocatalyst, carbon nitride. We show that TAHz can photochemically abstract an H-atom from water, in addition to generating H2 in aqueous suspensions with photocatalytic activity matching that of carbon nitride. In our multi-pulse experiment, we use resonant pump pulse to photoexcite TAHz to a bright high-lying excited state, and after a relaxation period of roughly 6 ps, we use a NIR (1150 nm) pulse to “push” the chromophore to a higher-lying excited state. When phenol is present, the push induces a persistent decrease (ΔΔOD) in the initial excited-state absorption, indicating the push pulse engenders a divergence in the photochemical branching ratios. In the presence of electron-donating substituted phenols, the magnitude of ΔΔOD diminishes markedly due to the increased excited-state reactivity of the complex accompanied by the cathodic shift in the phenol oxidation potential. Thus, the H-atom abstraction appears to proceed without aid from the additional energy of the push pulse. These results reveal new insight into the branching ratio among unreactive localized heptazine excited states and reactive intermolecular charge transfer states of H-bonded heptazine chromophores. More generally, this work provides new insight into molecular design strategies to control the excited-state photochemistry of aza-aromatic materials toward important reactions such as H-atom abstraction from water.

Inrush Current Effects on SiC-MOSFETs for LLC Converter

We prototyped an LLC converter using SiC-MOSFET and GaN-FET to study the device dependence of the resonant type converter characteristics and high current effects on the devices during the start-up mode.Under steady state operation, no observable difference exists between the SiC-MOSFET and GaN-FET in current and voltage waveforms and energy conversion efficiency is over 96% regardless of the device used.However, significant differences exist during the start-up mode. As for the SiC-MOSFET, the inrush current is at maximum in the first few cycles and gradually decreases to steady state. As for the effects on the device, there are no characteristic changes or current waveform fluctuation during the start-up period even after 50 repetitions.Alternately, in the case of GaN-FET, the maximum inrush current is at the first cycle and decreases rapidly to half only a few cycles later. The multi-pulse experiment shows that a single high current pulse degrades the I-V characteristics of the device and the subsequent pulse current decreases. It is also noted that the degradation could be overcome by a longer interval after the high current, however, no further improvement is seen above a 20 µs interval.

The exact solution for the free induction decay in a quasi-one-dimensional system in a multi-pulse NMR experiment

The exact solution for the free induction decay in a one-dimensional system in the multi-pulse experiment is obtained at both high and low temperatures in the approximation of nearest neighbor interactions. The experimental investigation is performed on a quasi-one-dimensional system of 19F nuclear spins in a single crystal of fluorapatite. The theoretical results are in a good agreement with the obtained experimental data.

A miniature multi-pulse series loading Hopkinson bar experimental device based on an electromagnetic launch.

Controlled multi-pulse loading has seldom been applied in classical split Hopkinson pressure bar (SHPB) research. Therefore, a new technique is proposed to realize a controlled multi-pulse loading SHPB experiment, and a miniature multi-pulse series reluctance coil launcher is developed. The micro-multi-pulse series reluctance coil emitter consists of two single-stage reluctance coils and two impact bars. Two loading pulses with the same amplitude are produced by driving the two impact bars to successively impact the incident bar. The distance between the impact bars controls the delay of the second pulse relative to the first one. The delay can be controlled by adjusting the distance between the impact bars and the launch speed. This precise multi-pulse loading technique is easy to implement and can be used to measure and study the dynamic response of various materials in loading SHPB experiments.

Experimental research on impulse coupling effect of a multi-pulse laser on an aluminum target

Impact and torsion pendulums are applied in impulse coupling experiments of high-energy laser irradiation of space debris. It is difficult to achieve a multi-pulse experiment and thus hard to analyze the multi-pulse impulse coupling effect. Here, we designed a new recoil impulse experimental measurement system of non-contact, multi-degrees of freedom, and multi-pulse irradiation. The system used a low-pressure and low-temperature vacuum chamber to simulate the space environment, the pinning effect of magnetic levitation to achieve aluminum target suspension, and high-speed cameras to record the displacement over time to calculate the impulse of the target. Then the impulse coupling experiment of multi-pulse laser irradiation on the aluminum target was performed. The result shows that the multi-pulse impulse coupling effect is not the linear accumulation of coupling results by every single-pulse and multi-pulse coefficient that decreases with the increase of the number of pulses, and eventually stabilizes as the decrease gets smaller.

Influence of treatment conditions on decomposition activity of N2O over FeZSM-5 with high iron content

The influence of treatment on formation and activity of surface oxygen over FeZSM-5 with high iron content was studied by using a transient multipulse technique combined with temperature-programmed desorption (TPD) and steady-state kinetics. All samples after various treatments were characterised by N2 sorption, NH3-TPD, H2-TPR, and DR-UV–vis. Multipulse experiment results showed that the reductive and inert treatments increase the amount of surface oxygen species formed from N2O. However, the oxidative treatment inhibits almost half of the oxygen formation. High temperature treatment increases not only the surface oxygen amount but also steady-state decomposition of N2O. Based on TPD and CO oxidation with surface oxygen, the active oxygen species formed from N2O are desorbed at low temperature and their amounts increase with oxidative and vacuum treatments and decrease with high temperature treatment. Vacuum treatment in a batch system increases the amount of surface oxygen and prevents transformation of active oxygen species to inactive oxygen species.

Multipulse laser damage in potassium titanyl phosphate: statistical interpretation of measurements and the damage initiation mechanism

Multipulse laser-induced damage is an important topic for many applications of nonlinear crystals. We studied multipulse damage in X-cut KTiOPO4. Using a 6-ns Nd:YAG laser with a weakly focused beam, a fatigue phenomenon was observed. We addressed whether this phenomenon necessarily implies material modifications. Two possible models were tested, both of them predicting increasing damage probability with increasing pulse number while all material properties are kept constant. The first model, pulse energy fluctuations and depointing, increases the probed volume during multiple pulse experiments. The probability to cause damage thus increases with increasing pulse number; however, this effect is too small to explain the observed fatigue. The second model assumes a constant single-shot damage probability p1, so a multipulse experiment can be described by statistically independent resampling of the material. Very good agreement was found between the 2000-on-1 volume damage data and this statistical multipulse model. Additionally, the spot size dependency of the damage probability is well described by a precursor presence model. Supposing that laser damage precursors are transient, the presented data explain the experimental results without supposing material modifications.

フラッシュランプアニールによる太陽電池用多結晶シリコン薄膜形成

Flash lamp annealing (FLA) is a promising technique to form polycrystalline silicon (poly-Si) films for solar cells, because of its ms-order proper annealing duration which enables to sufficiently heat μm-order-thick a-Si films without inducing thermal damage onto whole glass substrates. The flash-lamp-induced crystallization of amorphous Si (a-Si) takes place laterally triggered by the release of latent heat, generally referred to as explosive crystallization (EC). The rapid lateral crystallization results in the suppression of hydrogen desorption out of Si films and the preservation of original dopant profiles, which are of great advantages to the realization of high-efficiency solar cells. We can obtain poly-Si films both consisting only of μm-order-long large grains and containing 10-nm-sized fine grains, depending on the method of preparing precursor a-Si films. Based on the results of multi-pulse FLA experiment, this can be explained as the emergence of different EC modes.

Patterns of intraparticle distribution of surface concentration due to irreversible adsorption in TAP multi-pulse experiments

Information on non-uniform distribution of catalyst surface concentration of active species or catalyst activity during a TAP multi-pulse experiment is required for accurate interpretation of TAP response data. In this work, characteristics of intraparticle distribution of surface concentration due to first-order irreversible adsorption during a TAP multi-pulse experiment were theoretically investigated. The profiles of occupied fractional surface coverage ( θ) in each spherical catalyst pellet were numerically simulated from the first pulse experiment until the catalyst was saturated. Distribution of θ with gradual change after each consecutive pulse experiment was examined. It was found that the profiles of θ in all catalyst pellets at any axial coordinates in the catalyst bed are the same when compared at the same amount of gas uptake in the pellet although more gas pulses are required to reach the same amount of gas uptake for the pellet closer to the reactor exit. The development of distribution of θ therefore follows the same pattern for all catalyst pellets in the reactor. Calculation results for different reactor configurations and different system parameters show that the distribution pattern of θ developed in the catalyst pellets during the multi-pulse experiment is fixed by the Thiele modulus or effectiveness factor of fresh catalyst in the first pulse experiment. Distribution patterns showing very steep profiles of θ for small effectiveness factors indicate that intraparticle non-uniformity of change in catalyst activity should be concerned when interpreting TAP response data during the multi-pulse experiment. Calculation examples showing the effect of the non-uniformity on the response data are provided.

CO Oxidation over Pd Catalysts Supported on Different Supports: A Consideration of Oxygen Storage Capacity of Catalyst

A series of Pd catalysts supported on different materials (SiO2, γ-Al2O3, CeO2, TiO2) were synthesized through precipitation reduction of palladium ions with H2. They exhibited different activity during catalytic oxidation of CO to CO2. With the characterization of N2 physisorption experiment, X-ray diffraction, transmission electron microscopy, temperature-programmed reduction, and CO and O2 multi-pulse experiment, the correlation of catalytic activity and oxygen storage capacity (OSC) of catalyst has been investigated. It was found that the reactivity is largely controlled by the OSC of the catalysts, and thus Pd/CeO2 and Pd/TiO2 were more active than Pd/SiO2 and Pd/Al2O3 for CO oxidation.

Increasing the efficiency of the ring-opening reaction of photochromic indolylfulgides by optical pre-excitation

For three indolylfulgides the quantum efficiency of the ring-opening reaction upon pre-excitation is investigated in a multipulse experiment. The quantum efficiency grows by factor of up to 3.4, when the pre-excitation pulse immediately precedes the excitation process. The change in quantum efficiency after pre-excitation is discussed as a function of reaction time, steady-state quantum efficiency and energetic barriers in the excited electronic state. The observed differences can be explained by the molecular properties of the investigated indolylfulgides.

Product operator descriptions of the 2D DEPT J-resolved NMR experiment for weakly coupled ISn (, ; n=1, 2, 3) spin systems

There are a variety of multi-pulse nuclear magnetic resonance (NMR) experiments for spectral assignment of complex molecules in a solution. The two-dimensional (2D) distortionless enhancement by polarization transfer (DEPT) J-resolved NMR experiment is a 13C-detected, spectral editing polarization transfer technique. The product operator theory is widely used for an analytical description of the multi-pulse NMR experiment for weakly coupled spin systems. In this study, analytical descriptions of the 2D DEPT J-resolved NMR experiment for weakly coupled ISn (, ; n=1, 2, 3) spin systems using the product operator theory have been introduced for the first time. The calculated intensities and positions of the observable signals are simulated for molecules containing [13C , 81Br )] nuclei by using a MAPLE program on a computer. Finally, we present a theoretical discussion and experimental suggestions.

UNIFORMITY OF CHANGE IN POROUS CATALYST ACTIVITY DURING A MULTI-PULSE TAP EXPERIMENT

Uniformity of change in catalyst activity during a multi-pulse TAP experiment with porous catalyst was theoretically analyzed for a TAP reactor with the ratio of the catalyst bed thickness to the reactor length of 1/30. The analysis was performed by simulation of an irreversible adsorption process. The catalyst change is described by the change in the occupied/unoccupied fractional surface coverages. The intraparticle uniformity is indicated by a small magnitude of Δθ p,max , the maximum difference between the occupied fractional surface coverages at the outermost and the innermost of the catalyst pellet during the multi-pulse experiment. In the interparticle region, the indicating quantity is Δθ b,max , the maximum difference between the pellet-outermost fractional surface coverages at the inlet and the outlet of the catalyst bed. It was found that Δθ p,max generally depends only on the effectiveness factor in the first pulse experiment, η. For η ≥ 0.94, Δθ p,max ≤ 0.05. In addition, Δθ b,max depends only on the gas conversion in the first pulse experiment, X. For X ≤ 0.7, Δθ b,max ≤ 0.05.

A complete product operator theory for IS ( I = ½, S = 1) spin system and application to DEPT–HMQC NMR experiment

There exist a variety of multi-pulse NMR experiments for spectral assignment of complex molecules in solution. The DEPT−HMQC NMR experiment is a combination of DEPT and HMQC experiments. The product operator theory is widely used for analytical description of multi-pulse NMR experiment for weakly coupled spin systems. In this study, a complete product operator theory for the IS (I = ½, S = 1) spin system is presented by obtaining the evolutions of some product operators under the spin–spin coupling Hamiltonian and the evolutions of some angular momentum operators under the chemical shift and radio frequency (r.f.) pulse Hamiltonians. As an application, product operator theory has been used for the analytical description of DEPT−HMQC NMR experiment for CDn groups. Theoretical discussions and experimental suggestions for the sub-spectral editing of CDn groups are presented for this experiment.

A Pulse Radiolysis Study of Catalytic Superoxide Radical Dismutation by a Manganese(II) Complex with an N‐Tripodal Ligand (Eur. J. Inorg. Chem. 14/2005)

AbstractThe cover picture shows an allegory of bioinorganic chemistry and biomimetic studies. Here, the metalloprotein (human Mn‐SOD, PDB‐code 1MSD) is posing for the painter. As a bioinorganic chemist, he is designing new molecules that would reproduce this protein activity; he is testing the activity of these molecules and deciphering mechanisms. Here, we show the structure of a small MnII complex that has been studied for its SOD‐like activity by pulsed radiolysis. In the painting, the UV/Vis trace of a multipulse experiment at 300 nm is shown, with successive spikes and plateaus. An overall increase in the absorbance of the successive plateaus is observed. The asterisks indicate irradiations. Simulations of the individual growth of the spikes, of the individual decrease down to the plateau and of the overall increase of the absorbance for the successive plateaus leads to the proposition of a mechanism. This is the triangular cycle shown in the painting, involving an {MnOO}6 adduct between MnII and superoxide. This label, {MnOO}6, is proposed by analogy with the nomenclature used for iron−nitroxide adducts. Each event indicated in the cycle has been proven. Moreover, this study shows unambiguously that the MnII compound displays a catalytic activity for superoxide dismutation. Details are presented in the article by J.‐P. Renault, C. Policar et al. on p. 2789 ff. In the original picture by Magritte, the inset in the lower right‐hand corner, is entitled “La clairvoyance” (clear‐sightedness) and was painted in 1936.

Uniformity in a thin-zone multi-pulse TAP experiment: numerical analysis

The thin-zone TAP reactor (TZTR) model of a multi-pulse experiment is computationally validated based on a more general three-zone reactor model. The analysis is focused on the uniformity of gaseous and surface concentrations in the catalyst zone, which is a key property of TZTR model. It is shown that if the TZTR model is valid for the first pulse in a multi-pulse experiment then it is valid for all subsequent pulses. For a typical reactor packing (the ratio of the thin-zone thickness to the length of reactor is 1/30) and with the first pulse conversion up to 97%, the gaseous and surface concentration profiles can be considered uniform and characterized by their spatial average values only. The reaction rate in the catalyst zone may also be characterized by its spatial average value and directly related to the spatial average gaseous and surface concentrations, in the same way as an elementary rate is related to concentrations. As a result of these unique characteristics, the TZTR may be considered a “perfectly-mixed” reactor even at high conversion.

Surface instability of multipulse laser ablation on a metallic target

Large scale wavelike patterns are observed on an aluminum surface after it is ablated by a series of KrF laser pulses (248 nm, 40 ns, 5 J/cm2). These surface structures have a wavelength on the order of 30 μm, much longer than the laser wavelength. We postulate that these wave patterns are caused by the Kelvin–Helmholtz instability at the interface between the molten aluminum and the plasma plume. A parametric study is given in terms of the molten layer’s thickness and of the spatial extent and kinetic energy density in the laser-produced plasma plume. Also included is an estimate of the cumulative growth in a multipulse laser ablation experiment. These estimates indicate that the Kelvin–Helmholtz instability is a viable mechanism for the formation of the large scale structures.

Counting echoes: Application of a complete reciprocal-space description of NMR spin dynamics

A complete reciprocal-space formalism for describing the spatial aspects of nuclear magnetic resonance (NMR) spin dynamics in the presence of hard radiofrequency (RF) pulses and linear-refocusing inhomogeneities is reviewed. The formalism demonstrates how the magnetization in a sample can be decomposed into a linear combination of simple basis functions consisting of helical phase modulations in the transverse plane and sinusoidal amplitude modulations along the principal axis of symmetry. It is shown that plotting the evolution of the spatial Fourier variable for each basis function provides a simple way to compute both the number of echoes resulting from any multipulse experiment and when the echoes will form. The maximum number of echoes possible for a sequence of n hard RF pulses with 90°; flip angles and with arbitrary flip angles, both under the action of a time-invariant linear Iz Hamiltonian, is computed using this formalism. A simple criterion for the delay time necessary between pulses to observe the maximum number of echoes is presented. Experimental results are shown for pulse sequences of up to four pulses. ©1998 John Wiley & Sons, Inc. Concepts Magn Reson 10: 331–341, 1998