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Femtosecond time-resolved coherent anti-Stokes Raman spectroscopy (CARS) was used to study the dynamics of the vibrational modes of liquid chloroform. The vibrational modes were selectively excited and their coherent vibrational dynamics were obtained. Some subtle features that are difficult to distinguish in the ordinary spontaneous Raman spectrum, such as overtones and combinations of some fundamental vibrational modes, were recognized from the CARS transients. Combined with theoretical calculations, the contributions of chlorine isotopes were also confirmed from the CARS transients of the vibrational modes involving the motion of chlorine atoms.

In this paper, under the peak-to-average power ratio (PAR) and energy constraints on the transmit waveforms, a new efficient weighted $\ell _p$ -norm metric (with $p\geq 2$ ) is introduced for colocated wideband/narrowband multiple-input multiple-output radar transmit beampattern matching design. The proposed metric can achieve quasi-equiripple beampattern, which is desirable to improve the robustness of radar systems with imprecise direction information of targets and reduce radar clutter. In addition, mainlobe ripple control, peak sidelobe suppression, and deep null/notch formation can be obtained. On the other hand, the PAR and energy constraints cause the radar waveform losing the degree of freedom to match arbitrary pattern shape, and the desired pattern usually lies in matching an appropriately scaled version of the shape. Thus, a scaling factor is introduced into the proposed metric to tackle the scaling ambiguity problem. The resultant optimization problems are difficult to solve due to the higher order polynomial objective function with coupled scaling factor and waveform matrix, and also the nonconvex PAR constraint. To develop a fast and efficient algorithm, by utilizing the majorization–minimization technique, we decouple the scaling factor and radar waveform matrix variables by a majorization step, derive a new quadratic majorizer to tackle the quadratic function with the matrix variable, and surrogate the nonsmooth nonconvex functions by convex functions. Numerical examples show that the devised approach can synthesize beampattern with lower peak sidelobe level and more flat mainlobe compared to the existing algorithms.

The topic of designing sequence set with good correlation properties has received considerable attention due to its numerous applications in many active sensing and communication systems. For example, a multiple-input multiple-output radar system transmits orthogonal waveforms or zero correlation zone sequences to achieve a long virtual aperture, which results in enhanced parameter identifiability and improved resolution. However, it is difficult to design sequence set with accurately controlled correlation properties due to numerous nonconvex and highly nonlinear inequality constraints. In this paper, new sequence set design methods, based on the local integrated sidelobe level and the local peak sidelobe level metrics, are proposed to tackle the optimization problems under the nonconvex and nonlinear inequality constraints to meet the correlation requirements. Numerical examples are provided to show the effectiveness of the proposed methods.

Vibrational energy redistribution (VER) of energetic materials plays an important role in transferring the injected energy to the hot spots, but it is extremely challenging to understand the mechanism of VER from experimental or theoretical studies. Here, we combined nonequilibrium molecular dynamics with density functional theory to study the processes of VER for solid nitromethane after the selective excitation of the C-H stretching vibration. The VER processes are traced by monitoring the normal-mode kinetic energies of both excited and unexcited vibrations. To explore the underlying VER mechanism, we also analyzed the spectral energy density for the normal mode, obtained from the squared modulus of the short-time Fourier transition of their normal mode momentum. The results showed that the simulated VER progress was reproduced well compared with the previous 3D IR-Raman experiments of liquid nitromethane. Interestingly, the symmetric dependence of the coupling mechanism between the normal modes has been found.

This paper proposes two methods for robust Capon beamforming. One is for the doubly constrained robust Capon beamforming problem, where the unit modular constraints on the elements of the steering vector of interest are enforced to circumvent the look direction error or phase perturbations of the signal-of-interest; and another addresses robust beamforming in impulsive noise environment, where we consider the l p -norm minimization (0 < p < 2) of the output while constraining the mainlobe response ripple term. We apply the splitting technique to simplify the resultant nonconvex optimization problem and solve it using alternating direction method of multipliers. The performance of the proposed methods is demonstrated via numerical examples.

This paper develops a novel ellipse fitting algorithm by recovering a low-rank generalized multidimensional scaling (GMDS) matrix. The main contributions of this paper are: i) Based on the derived Givens transform-like ellipse equation, we construct a GMDS matrix characterized by three unknown auxiliary parameters (UAPs), which are functions of several ellipse parameters; ii) Since the GMDS matrix will have low rank when the UAPs are correctly determined, its recovery and the estimation of UAPs are formulated as a rank minimization problem. We then apply the alternating direction method of multipliers as the solver; iii) By utilizing the fact that the noise subspace of the GMDS matrix is orthogonal to the corresponding manifold, we determine the remaining ellipse parameters by solving a specially designed least squares problem. Simulation and experimental results are presented to demonstrate the effectiveness of the proposed algorithm.

Vibrational coupling between different kinds of molecules in liquid mixture is studied by multiplex coherent anti-Stokes Raman spectroscopy (CARS). To identify vibrational coherence, fs-probe with high time resolution and narrowband-probe with high spectral resolution are adopted in CARS experiments. Using liquid nitromethane (NM) mixed with organic dye IR780 perchlorate as the sample, we can clearly observe the interference between different vibrational modes. The intermolecular vibrational interaction between NM and IR780 molecules results in the vibrational coherence transfer (VCT) in the form of a change of phase correlation. Compared with symmetric bending vibration of NO2, coherence transfer is found to be easier to take place between C―N bond of NM and vibrations of IR780, which indicates the selectivity of intermolecular vibrational interaction. The selectivity is deduced to be related to the coordination between intramolecular and collective motion of molecules. Copyright © 2016 John Wiley & Sons, Ltd.

Femtosecond time-resolved transient grating (TG) technique was employed to get insight into the photodissociation mechanism of liquid nitromethane (NM). Broadband white-light continuum was introduced as the probe to observe the evolution of electronic excited states of NM molecules and the formation of photodissociation products simultaneously. The reaction channel of liquid NM under 266 nm excitation was obtained that NM molecules in excited state S2 relax through two channels: about 73% relax to low lying S1 state through S2/S1 internal conversion with a time constant of 0.24 ps and then go back to the ground state through S1/S0 internal conversion; the other 27% will dissociate with a time constant of 2.56 ps. NO2 was found to be one of the products from the experimental TG spectra, which confirmed that C–N bond rupture was the primary dissociation channel of liquid NM.

The problem of designing waveforms with specified aperiodic/periodic correlation and spectral properties has received much attention because of its wide applicability in sonar, radar, and communications. In particular, a waveform with low peak sidelobe level (PSL) of autocorrelation can improve the detection performance of a weak target masked by a nearby strong target in range compression radar systems, and that with adaptive spectrum nulls/notches can make the sensing systems not be interfered by other electromagnetic equipment. On the other hand, a constant modulus (CM) waveform can maximize the transmit power efficiency of the system. Whereas, it is difficult to design a CM waveform with minimum local PSL and specified autocorrelation and/or spectral properties due to numerous nonconvex inequality and equality constraints. To develop efficient algorithms for this problem, we first express the waveform autocorrelation in the frequency domain, then use the proximal method of multipliers to tackle the resultant nonconvex and nonsmooth constrained optimization. Numerical examples have shown that the proposed methods can produce the waveforms satisfying these challenging requirements.

Selective excitation of C–H stretching vibrational modes, detection of intramolecular vibrational energy redistribution (IVR), and vibrational modes coupling in the electronic ground state of benzene are performed by using femtosecond time- and frequency-resolved coherent anti-Stokes Raman scattering (CARS) spectroscopy. Both of the parent modes in the Raman-active bands are coherently excited by an ultrafast stimulated Raman pump, giving initial excitations of 3056 cm–1 (A1g) and 3074 cm–1 (E2g) and subsequent IVR from the parent modes to daughter modes of 1181 and 992 cm–1, and the coherent vibrational coupling of the relevant modes is tracked. The directionality and selectivity of IVR and coherent coupling among all of the relevant vibrational modes are discussed in the view of molecular symmetry.

The femtosecond time-resolved multiplex coherent anti-Stokes Raman scattering (CARS) technique has been performed to investigate intramolecular vibrational redistribution (IVR) through vibrational couplings in 1,3,5-trinitro-1,3,5-triazacyclohexane (RDX) molecules. In the multiplex CARS experiment, the supercontinuum (SC) was used as broad-band Stokes light to coherently and collectively excite multiple vibrational modes, and quantum beats arising from vibrational couplings among these modes were observed. The IVR of RDX is visualized by a topological graph of these vibrational couplings, and with analysis of the topological graph, two vibrational modes, both of which are assigned to ring bending, are confirmed to have coupling interactions with most of the other vibrational modes and are considered to have a tendency of energy transfer with these vibrational modes. We suggest that the mode at 466 cm–1 is a portal of energy transfer from outside to inside of the RDX molecule and the mode at 672 cm–1 is an...

A modified photon echo (PE) technique, the supercontinuum probing photon echo (SCPPE), is introduced and performed to investigate the vibrational coherence in organic dye IR780 perchlorate doped polyvinyl alcohol (PVA) film. The coherences of multiple vibrational states which belong to four vibrational modes create complex oscillations in SCPPE signal. The frequencies of vibrational modes are confirmed from the results of Raman calculation which accord fairly well with the results of Raman scattering experiment. Compared with conventional one-color PE, the SCPPE technique can realize broadband detection and make the experiment about vibrational coherence more efficient.

An accurate and simple method, Raman peak-shift simulation, is proposed to determine the characteristics of a laser-driven shock wave. Using the principle of the Raman peaks shifting at high pressure and the pressure distribution in the gauge layer, the profile of the Raman peak can be numerically simulated. Combined with time-resolved Raman spectroscopy, some main characteristics of the shock wave were determined. In the experiment, polycrystalline anthracene was used as the pressure gauge. The pump–probe technique was used to obtain the time-resolved Raman spectra of anthracene under shock loading. The velocity of the shock wave, the peak pressure and the rise time of the shock front were determined by simulating the experimental spectra numerically. The result shows that the method of Raman peak-shift simulation is effective in obtaining the characteristics of a laser-driven shock wave. Copyright © 2010 John Wiley & Sons, Ltd.

The poly-[2-methoxy, (5-2′-ethyl-hexyloxy)-p-phenylene vinylene] (MEH-PPV) film is investigated by means of the Multi-Color Photon Echo (MCPE) technique. Under the three-order nonlinear response theory, the reason for the occurrence of the quantum beats in the time domain and the relations among signal wavevector, pulse sequence and response function are discussed. The analysis of the Raman spectrum of MEH-PPV and the fast Fourier transformation (FFT) results of photon echo (PE) signal dynamics demonstrate the coherent coupling between the C=C bonds and the CC-H bonds and the coherent coupling between the C-C stretching of the phenyl group and the C=C stretching of the vinylene group.

A 3-D numerical model for pulsed laser transformation hardening (LTH) is developed using the finite element method. In this model, laser spatial and temporal intensity distribution, temperature-dependent thermophysical properties of material, and multi-phase transformations are considered. The influence of laser temporal pulse shape on connectivity of hardened zone, maximum surface temperature of material and hardening depth is numerically investigated at different pulse energy levels. Results indicate that these hardening parameters are strongly dependent on the temporal pulse shape. For the rectangular temporal pulse shape, the temperature field obtained from this model is in excellent agreement with analytical solution, and the predicted hardening depth is favorably compared with experimental one. It should be pointed out that appropriate temporal pulse shape should be selected according to pulse energy level in order to achieve desirable hardening quality under certain laser spatial intensity distribution.

Using an improved two-dimensional hybrid model with microwave absorption and neutral gas transport, the properties of an electron cyclotron resonance (ECR) plasma have been investigated. Results indicate that there are two typical profiles of microwave power deposition and neutral gas density over the entire calculated parameter range: one is concentrated near the centre, and decreases with radial positions; another is a hollow distribution. The average value of neutral gas density in the source area is about 50-60% of that in the downstream region. The dependence of plasma parameters such as plasma density, electron temperature and ionization rate, and the ion angle distribution function at the substrate surface on the position and pressure were discussed. Results of our simulation can be compared qualitatively with many experimental measurements.

Using a refined two-dimensional hybrid-model with self-consistent microwave absorption, we have investigated the change of plasma parameters such as plasma density and ionization rate with the operating conditions. The dependence of the ion current density and ion energy and angle distribution function at the substrate surface vs. the radial position, pressure and microwave power were discussed. Results of our simulation can be compared qualitatively with many experimental measurements.

Microwave fields in an extended electron cyclotron resonance (ECR) plasma reactor were analyzed in the resonant cavity from Maxwell’s equations, and a formalism of microwave absorption was investigated at the resonance zone with plasma kinetic theory and simulated using a two-dimension computer hybrid-code. The results indicated that there are three typical power deposition profiles over the entire parameter range: (1) the parabolic shape: peaking on axis and decreasing slowly along the radius, (2) the trumpet shape: peaking on axis and decreasing quickly along the radius, and (3) the camel shape, i.e., a hollow distribution. The spatial distributions of plasma parameters (plasma density, electron temperature, ionization rate, etc.) would be significantly influenced by the microwave power absorption profile.

A two-dimensional hybrid model for an electron cyclotron resonance (ECR) plasma reactor is improved by taking into account the microwave absorption. With this code, the velocity and angle distribution function of ions in the source and downstream areas, and the ion current density at the substrate surface in an extended ECR plasma reactor were investigated. The spatial profiles as well as the dependence of these distribution functions on pressure and microwave power were discussed. It is shown that the results are in good agreement with many experimental measurements.

A nanosecond time-resolved shadowgraphy is performed to observe a laser-induced breakdown in nitromethane. The digital delays are introduced between a pump beam and an illumination light to achieve a measuring range from 40 ns to 100 ms, which enable us to study the shock wave propagation, bubble dynamics, and other process of the laser-induced breakdown. Compared with distilled water, there are two obvious differences observed in nitromethane: (1) the production of a non-evaporative gas at the final stage, and (2) an absence of the secondary shock wave after the first collapse of the bubble. We also calculated the bubble energy in nitromethane and distilled water under a different incident energy. The results indicate that the bubble energy in nitromethane is more than twice as large as that in water. It is suggested that chemical reactions contribute to the releasing of energy.

Femtosecond time-resolved transient grating (TG) technique is used to study the intermolecular dynamics in liquid phase. Non-resonant excitation of the sample by two crossing laser pulses results in a transient Kerr grating, and the molecular motion of liquid can be detected by monitoring the diffraction of a third time-delayed probe pulse. In liquid nitrobenzene (NB), three intermolecular processes are observed with lifetimes of 37.9±1.4 ps, 3.28±0.11 ps, and 0.44±0.03 ps, respectively. These relaxations are assigned to molecular orientational diffusion, dipole/induced dipole interaction, and libration in liquid cage, respectively. Such a result is slightly different from that obtained from OKE experiment in which the lifetime of the intermediate process is measured to be 1.9 ps. The effects of electric field on matter are different in TG and optical Kerr effect (OKE) experiments, which should be responsible for the difference between the results of these two types of experiments. The present work demonstrates that TG technique is a useful alternative in the study of intermolecular dynamics.

The collision‐frequencies between pairs of particles in a steady state for both smooth rigid elastic and Coulomb interaction are obtained by a uniform method. In addition, the results for smooth rigid elastic interaction are the same as that obtained previously [S. Champman and T. C. Cowling, in The Mathematical Theory of Non‐Uniform Gas (Cambridge U.P., Cambridge, 1970), Chap. 5]. A momentum change distribution for collision frequency is obtained. It cannot only be used as an exact means for the description of Coulomb collision frequency but can also be used for calculating reaction rates. In the case of smooth rigid elastic interaction, the reaction rate proved to be of Arrhenius formula [D. H. Charles and G. A. Rayford, Fundamentals of Chemical Reaction Engineering (Prentice–Hall, Englewood Cliffs, NJ, 1979), Chap. 12]. For Coulomb interaction, a formula for the fusion reaction rate between deuterium and tritium as 〈σvDT〉 = 0.24T−3/2Γ(−1, 17.3/T) cm3/sec (with T in keV) is obtained, which is in agreeme...

The microwave power absorption in electron cyclotron resonance plasma reactor was investigated with a two-dimensional hybrid-code. Simulation results indicated that there are two typical power deposition profiles over the entire parameter region: (1) microwave power deposition peaks on the axis and decreases in radial direction, (2) microwave power deposition has its maximum at some radial position, i.e., a hollow distribution. The spatial distribution of electron temperature resembles always to the microwave power absorption profile. The dependence of plasma parameter on the gas pressure is discussed also.

An improved nanosecond time-resolved Raman spectroscopy is performed to observe laser-driven shock propagation in the anthracene/epoxy glue layer. The digital delay instead of optical delay line is introduced for sake of unlimited time range of detection, which enables the ability to observe both shock loading and shock unloading that always lasts several hundred nanoseconds. In this experiment, the peak pressure of shock wave, the pressure distribution, and the position of shock front in gauge layer were determined by fitting Raman spectra of anthracene using the Raman peak shift simulation. And, the velocity of shock wave was calculated by the time-dependent position of shock front.

A modified model, a set of rate equations based on time-dependent correlation function, is used to study vibrational relaxation dynamics in transient grating spectroscopy. The dephasing, the population dynamics, and the vibrational coherence concerning two vibrational states are observed respectively in organic dye IR780 perchlorate molecules doped polyvinyl alcohol matrix. The result shows that in addition to the information concerning system-environment interaction and vibrational coherence, the vibrational energy transfer can be described by this modified model.

The shock wave driven by short laser pulse is used to study the damage of brittle material K9 glass. The damage morphology of K9 glass surface indicates that the material has experienced different loading modes, respectively, at the central area and the surrounding area of the shock wave. At the central area of shock wave, the wavefront is plane and has a uniform pressure distribution, the material mainly suffers a longitudinal shock pressure; but on the edge the shock wave, the wavefront is approximately spherical, besides longitudinal pressure, transverse tensile stress will emerge inside the material. In the latter case, the damage threshold of the material is much smaller than that in the case of compressing by longitudinal pressure only. According to the relationship between damage area and shock pressure, an experimental method is proposed to measure the damage threshold of materials under shock loading. The damage threshold of K9 glass under spherical shock wave is measured to be about 1.12 GPa; an...

The impact of noble metal nanostructures on the surface trapping state of semiconductor quantum dots was investigated by taking nanosecond time resolved photoluminescence measurements. It was observed that nonradiative resonant energy transfer from the excitonic state of quantum dots to Au nanofilm reduced the lifetime and intensity of excitonic state emission. No nonradiative resonant energy transfer from surface trapping state to Au nanofilm was observed. It is possibly implied that noble metal nanostructures have no impact on the surface trapping state of semiconductor quantum dots.