Ultrafast atomic view of laser-induced melting and breathing motion of metallic liquid clusters with MeV ultrafast electron diffraction.

Under the irradiation of an ultrafast intense laser, solid materials can be driven into nonequilibrium states undergoing an ultrafast solid-liquid phase transition. Understanding such nonequilibrium states is essential for scientific research and industrial applications because they exist in various processes including laser fusion and laser machining yet challenging in the sense that high resolution and single-shot capability are required for the measurements. Herein, an ultrafast diffraction technique with megaelectron-volt (MeV) electrons is used to resolve the atomic pathway over the entire laser-induced ultrafast melting process, from the initial loss of long-range order and the formation of high-density liquid to the progressive evolution of short-range order and relaxation into the metastable low-density liquid state. High-resolution measurements using electron pulse compression and a time-stamping technique reveal a coherent breathing motion of polyhedral clusters in transient liquid aluminum during the ultrafast melting process, as indicated by the oscillation of the interatomic distance between the center atom and atoms in the nearest-neighbor shell. Furthermore, contraction of interatomic distance was observed in a superheated liquid state with temperatures up to 6,000 K. The results provide an atomic view of melting accompanied with internal pressure relaxation and are critical for understanding the structures and properties of matter under extreme conditions.

Multiple coherent amplitude modes and exciton-phonon coupling in quasi-one-dimensional excitonic insulator Ta2NiSe5.

An excitonic insulator (EI) is an intriguing correlated electronic phase of condensed excitons. Ta2NiSe5 is a model material for investigating condensed excitonic states. Herein, femtosecond pump-probe spectroscopy is used to study the coherent phonon dynamics and associated exciton-phonon coupling in single-crystal Ta2NiSe5. The reflectivity time series consists of exponential decay due to hot carriers and damped oscillations due to the Ag phonon vibration. Given the in-plane anisotropic thermal conductivity of Ta2NiSe5, coherent phonon oscillations are stronger with perpendicular polarization to its quasi-one-dimensional chains. The 1-, 2-, and 4-THz vibration modes show coherent amplitude responses in the EI phase of Ta2NiSe5 with increasing temperature, totally different from those of normal coherent phonons (the 3- and 3.7-THz modes). The amplitude modes at higher frequencies decouple with the EI order parameter at lower temperatures, as supported by theoretical analysis with a model Hamiltonian of the exciton-phonon coupling system. Our work provides valuable insights into the character of the EI order parameter and its coupling to multiple coherent amplitude modes.

Resonance-enhanced excitation and relaxation dynamics of coherent phonons in Fe1.14Te.

Lattice dynamics plays a significant role in manipulating the unique physical properties of materials. In this work, femtosecond transient optical spectroscopy is used to investigate the generation mechanism and relaxation dynamics of coherent phonons in Fe1.14Te-a parent compound of chalcogenide superconductors. The reflectivity time series consist of the exponential decay component due to hot carriers and damped oscillations caused by the A1g phonon vibration. The vibrational frequency and dephasing time of the A1g phonons are obtained as a function of temperature. With increasing temperature, the phonon frequency decreases and can be well described with the anharmonicity model. Dephasing time is independent of temperature, indicating that the phonon dephasing is dominated by phonon-defect scattering. The impulsive stimulated Raman scattering mechanism is responsible for the coherent phonon generation. Owing to the resonance Raman effect, the maximum photosusceptibility of the A1g phonons occurs at 1.590 eV, corresponding to an electronic transition in Fe1.14Te.

Abnormal Hot Carrier Decay via Spin-Phonon Coupling in Intercalated van der Waals Ferromagnetic Fe1/3TaS2.

Spin-phonon coupling is a fundamental interaction in ferromagnets/antiferromagnets and plays a key role in hot carrier decay. Normally, spin transfers its excess energy to a lattice via spin-phonon coupling in hot carrier decay in ferromagnets/antiferromagnets. However, the reverse energy transfer process (i.e., from lattice to spin) is feasible in principle but rarely reported. Here, we observe an abnormal hot carrier decay with a slow fall (80 ps) in ΔR(t)/R0 time series in ferromagnet Fe1/3TaS2, which is a result of the lattice of TaS2 vdW layer transfering its energy to spin via spin-phonon coupling. The Fe ions inserted between TaS2 vdW layers with very weak bonding with TaS2 vdW layer, are the origin of the ferromagnetism and give rise to its weak electron-spin and spin-phonon couplings which in turn lead to the observed abnormal hot carrier decay in the ferromagnetic phase Fe1/3TaS2.

Effects of Anharmonicity, Recrossing, Tunneling, and Pressure on the H-Abstractions from Dimethylamine by Triplets O and O2.

Rate constants of the H-abstraction reactions from dimethylamine (DMA) by triplets O and O2 are theoretically determined with the canonical variational transition-state theory (CVT). By comparing the barrier heights and reaction energies obtained from different density-functional theory methods to those computed from the gold-standard method CCSD(T)/CBS(T-Q), we identify the M08-HX/ma-TZVP method as the best with a mean unsigned deviation of 1.0 kcal mol-1. On the basis of the optimized geometries and frequencies with the selected method, the rate constants are calculated using the CVT method combined with the multistructural torsional anharmonicity and small-curvature tunnelling (MS-CVT/SCT) options in the temperature range 200-2000 K. The calculations show that OH and HO2 are mainly produced from the direct abstraction from the C-H bond. The multistructural torsional anharmonicity has a large contribution to the rate constants, and the effects of recrossing and tunneling at the N-site are more important than those at C-site. Additionally, given the formation of reactant complex between DMA and triplet O, the H-abstraction channel is not favored at high pressure. Our calculations with both the Polyrate and MESS codes agree with the reported data within the uncertainty.

Low-threshold upconverted single-mode lasing from CdS hexagonal microcavities.

Upconversion micro/nanolasers are promising in fundamental physics research and practical applications. However, due to the limitation of gain medium and cavity quality, such lasers still suffer from a high lasing threshold (P th). Herein, upconverted whispering-gallery-mode lasing by two-photon absorption is achieved from CdS microplatelets with single-mode emission and low threshold (∼1.2 mJ cm-2). The threshold is three times lower than the best reported value in previous CdS upconversion lasers. Moreover, wavelength-tunable upconverted single-mode lasing is demonstrated from 510.4 to 518.9 nm with narrow linewidths around 0.85 nm, which is further verified through numerical simulations. In addition, the size-dependent lasing behavior is realized from single-mode to multimode oscillation; the corresponding lasing threshold decreases with increasing cavity edge length (L), following a P th ∝ 1/L 2 relationship. These results underscore the promise of CdS microplatelets for developing chip-level frequency upconversion lasers.

Solution-processed whispering-gallery-mode microsphere lasers based on colloidal CsPbBr3perovskite nanocrystals.

Perovskite nanocrystals (NCs) have emerged as attractive gain materials for solution-processed microlasers. Despite the recent surge of reports in this field, it is still challenging to develop low-cost perovskite NC-based microlasers with high performance. Herein, we demonstrate low-threshold, spectrally tunable lasing from ensembles of CsPbBr3NCs deposited on silica microspheres. Multiple whispering-gallery-mode lasing is achieved from individual NC/microspheres with a low threshold of ∼3.1µJ cm-2and cavity quality factor of ∼1193. Through time-resolved photoluminescence measurements, electron-hole plasma recombination is elucidated as the lasing mechanism. By tuning the microsphere diameter, the desirable single-mode lasing is successfully achieved. Remarkably, the CsPbBr3NCs display durable room-temperature lasing under ∼107shots of pulsed laser excitation, substantially exceeding the stability of conventional colloidal NCs. These CsPbBr3NC-based microlasers can be potentially useful in photonic applications.

Multistructural Variational Reaction Kinetics of the Simplest Unsaturated Methyl Ester: H-Abstraction from Methyl Acrylate by H, OH, CH3, and HO2 Radicals.

The H-abstraction reaction kinetics of methyl acrylate (MA) + H/OH/CH3/HO2 radicals have been investigated theoretically in the present work. For these reactions, the reaction energies and barrier heights are first computed using several density functionals and compared to the coupled cluster CCSD(T)-F12/jun-cc-pVTZ benchmark calculations. The M062X/maug-cc-pVTZ method shows the best performance with the smallest mean unsigned deviation (MUD) of 0.42 kcal mol-1. Combined with the electronic structure calculations using the M062X/maug-cc-pVTZ method, the multistructural canonical variational transition-state theory (MS-CVT) with small-curvature tunneling (SCT) is employed to calculate the reaction rate constants at 500-2000 K. The variational effect is between 0.56 and 1.0, the multistructural torsional anharmonicity factor ranges from 0.004 to 4.57, and the tunneling coefficient is in the range of 0.5-4.70. Notably, given the existence of reactant complexes (RCs) between reactants and transition states for the reaction systems MA + OH/HO2, we further compare the rate constants under the low-pressure limit (LPL) kinetic model, which treats the reaction as a single-step process and neglects RCs, and the pre-equilibrium model, which takes RCs into account in the reaction and treats the reaction as a two-step process. The rate constants calculated by these two models are similar within the combustion temperature range, and apparent differences occur at lower temperatures. In addition, we determine the branching ratios as a function of temperature and find that the methyl site (S3) abstractions by OH and H radicals are dominant in the low- and high-temperature ranges, respectively. Moreover, we update the kinetic model with the calculated H-abstraction rate constants to simulate the ignition delay times of MA. The simulations of the updated model are in good agreement with experimental results. The accurate reaction kinetics determined in this work are useful for the understanding and prediction of consumption branching fractions and ignition properties of the unsaturated methyl esters.

Kinetic modeling of methyl pentanoate pyrolysis based on ab initio calculations.

Recently, methyl pentanoate (MP) was proposed as a viable biodiesel surrogate to petroleum-based fuels. To better understand the pyrolysis chemistry of MP, the unimolecular decomposition kinetics of MP is theoretically investigated on the basis of ab initio calculations; ten primary channels, including four intramolecular H-shifts and six C-C and C-O bond fissions, are identified. The geometries are optimized at the M06-2X/cc-pVTZ level of theory, and accurate barrier heights are determined using the DLPNO-CCSD(T)/CBS(T-Q) method, which shows a good performance against the CCSD(T)/CBS(T-Q) method with an uncertainty of 0.5 kcal mol-1 for small methyl esters. The atomization enthalpy method is adopted to obtain the thermodynamics of involved species. The Rice-Ramsperger-Kassel-Marcus/master equation theory coupled with one-dimensional hindered rotor approximation is employed to calculate the phenomenological rate constants at 500-2000 K and 0.01-100 atm. The branching ratio analysis indicates that two reactions, MP ↔ CH3OC([double bond, length as m-dash]O)CH3 + CH2CHCH3 and MP ↔ CH3OC([double bond, length as m-dash]O)CH2 + CH2CH2CH3, are the dominant channels at low and high temperatures, respectively. The model from Diévart et al. [Proc. Combust. Inst., 2013, 34(1), 821-829] is updated with our calculations, and the modified model can yield a better prediction in reproducing the ignition delay times of MP at high temperatures. This work provides a comprehensive investigation of MP unimolecular decomposition, and can serve as a prototype for understanding the pyrolysis of larger alkyl esters.

Effects of temperature and grain size on deformation of polycrystalline copper-graphene nanolayered composites.

The effects of temperature and grain size on mechanical properties of polycrystalline copper-graphene nanolayered (PCuGNL) composites are investigated by analytical mechanical models and molecular dynamics simulations. The yield of PCuGNL composites under tension depends on temperature, copper grain size, and repeat layer spacing. Graphene-copper interfaces play the dominant role in the ultimate tensile strength of PCuGNL composites. The optimal range for strengthening of repeat layer spacing is 2-10 nm, and the failure stress of PCuGNL composites is weakly dependent on temperature. An analytical model is proposed to accurately characterize the mechanical behaviors of PCuGNL composites.

Reaction pathways and kinetics study on a syngas combustion system: CO + HO2 in an H2O environment.

The reaction between CO and HO2 plays a significant role in syngas combustion. In this work, the catalytic effect of single-molecule water on this reaction is theoretically investigated at the CCSD(T)/aug-cc-pV(D,T,Q)Z and CCSD(T)-F12a/jun-cc-pVTZ levels in combination with the M062X/aug-cc-pVTZ level. Firstly, the potential energy surface (PES) of CO + HO2 (water-free) is revisited. The major products CO2 + OH are formed via a cis- or a trans-transition state (TS) channel and the formation of HCO + O2 is minor. In the presence of water, the title reaction has three different pre-reactive complexes (i.e., RC2: COHO2 + H2O, RC3: COH2O + HO2, and RC4: HO2H2O + CO), depending on the initial hydrogen bond formation. Compared to the water-free process, the reaction barriers of the water-assisted process are reduced considerably, due to more stable cyclic TSs and complexes. The rate constants for the bimolecular reaction pathways CO + HO2, RC2, RC3, and RC4 are further calculated using conventional transition state theory (TST) with Eckart asymmetric tunneling correction. For reaction CO + HO2, our calculations are in good agreement with the literature. In addition, the effective rate constants for the water-assisted process decrease by 1-2 orders of magnitude compared to the water-free one at a temperature below 600 K. In particular, the effective rate constants for the water-assisted and water-free processes are 1.55 × 10-28 and 3.86 × 10-26 cm3 molecule-1 s-1 at 300 K, respectively. This implies that the contribution of a single molecule water-assisted process is small and cannot accelerate the title reaction.

Resolving dynamic fragmentation of liquids at the nanoscale with ultrafast small-angle X-ray scattering.

High-brightness coherent ultrashort X-ray free-electron lasers (XFELs) are promising in resolving nanoscale structures at the highest temporal resolution (∼10 fs). The feasibility is explored of resolving ultrafast fragmentation of liquids at the nanoscale with single-shot small-angle X-ray scattering (SAXS) on the basis of large-scale molecular dynamics simulations. Fragmentation of liquid sheets under adiabatic expansion is investigated. From the simulated SAXS patterns, particle-volume size distributions are obtained with the regularization method and average particle sizes with the weighted Guinier method, at different expansion rates. The particle sizes obtained from simulated SAXS are in excellent agreement with direct cluster analysis. Pulse-width effects on SAXS measurements are examined. The results demonstrate the feasibility of resolving the nanoscale dynamics of fragmentation and similar processes with SAXS, and provide guidance for future XFEL experiments and data interpretation.

Collision-mediated ultrafast decay of N2 fluorescence during fs-laser-induced filamentation.

We investigate experimentally spatiotemporal characteristics of fluorescence emission from fs-laser-induced filaments in air. Emissions accompanying the transitions of N2 (C3Πu-B3Πg) and N 2+ (B2Σu+-X2Σg+) are dominant. The decay dynamics of fluorescence from different radial positions and longitudinal sections of a filament column are obtained along with high resolution spectra. A decay curve contains two exponential components: a fast one (with a decay time constant ∼10s ps), and a slow one (∼sub-ns). The lifetime of the N 2 fluorescence is about three orders shorter than its spontaneous emission lifetime, indicating that most of the N 2 molecules in the excited state (C3Πu) are de-excited through collision. Different de-excitation mechanisms of N 2 (C3Πu) molecules contributing to fluorescence decay constants, e.g., the e --N2, N 2-N2, and O 2-N2 collisions, are elucidated. We analyze the variations of decay constants together with corresponding fluorescence intensities, and obtain temperature distributions by fitting band spectra of N 2 molecules and N 2+ ions with a synthetic spectral model. Our results suggest that the fast and slow decay processes originate from the e --N2 and O 2-N2 collisions, respectively.

Benchmarking dual-level MS-Tor and DLPNO-CCSD(T) methods for H-abstraction from methyl pentanoate by an OH radical.

Methyl pentanoate (MP) was recently proposed as a potential biodiesel surrogate due to its negative temperature coefficient region. To provide a basis for constructing an accurate mechanism, chemical kinetics of H-abstraction from MP by an OH radical are investigated theoretically at 200-2000 K. M06-2X/cc-pVTZ is applied for geometry optimizations and frequency calculations. Given the long alkyl-chain in MP, the multi-structural torsional anharmonicity is characterized by using the dual-level MS-T method due to its relatively low computational cost and established accuracy. In particular, AM1 and M06-2X/cc-pVTZ are adopted as low-level and high-level methods in dual-level MS-T, respectively. The results of dual-level MS-T are further used to benchmark against the full high-level method (M06-2X/cc-pVTZ), leading to an uncertainty of less than 30% in the high temperature range. For the single-point energy calculations, the lower computational cost DLPNO-CCSD(T) method is first used to benchmark against the gold-standard method CCSD(T) for small methyl ester (C2-C4) + OH reaction systems, yielding an overestimation of less than 1.1 kcal mol-1 for barrier height; it is then used to refine the electronic energies for the present reaction system MP + OH. Phase-space theory and conventional transition state theory are used to calculate the H-abstraction rate constants. After compensating the uncertainty of barrier height, the calculated phenomenological H-abstraction rate constants agree well with the experimental data at 263-372 K. Branching ratio analysis indicates that the ß-site H abstraction is the dominant channel at 200-1200 K due to its lowest barrier height.

Unusually high flexibility of graphene-Cu nanolayered composites under bending.

The mechanical properties of graphene-Cu nanolayered (GCuNL) composites under bend loading are investigated via an energy-based analytical model and molecular dynamics (MD) simulations. For an anisotropic material, if it has a weak strength in a certain direction, improving the mechanical properties along this direction is normally difficult for its composites. Here, we find that the flexibility of GCuNL composites can be improved considerably by graphene interfaces, despite graphene's small bending stiffness. The graphene interfaces can delocalize slip bands in the inner Cu layers of GCuNL composites, and impede local nucleation of dislocations, thus greatly increasing the yield and failure bend angles. As the thickness decreases, the flexibility of GCuNL nanofilms increases. However, the GCuNL nanofilms are thermodynamically unstable due to interface instability when the repeat layer spacing is less than 2 nm. The energy-based analytical model for large deformation can accurately characterize the bending response of GCuNL nanofilms.

Crack propagation in graphene monolayer under tear loading.

Crack propagation in graphene monolayer under tear loading is investigated via an energy-based analytical model and molecular dynamics (MD) simulations. The classical mechanics-based model describes steady-state crack propagation velocity as a function of applied stress, lateral dimension and loading geometry, as well as the critical stress and critical size for initiating steady crack propagation. MD simulations reveal that cracks propagate along the zigzag direction but yield different "fracture surface" roughnesses for different loading geometries. MD simulations and the predictions of the analytical model are in excellent agreement. Our findings lead to an improved fundamental understanding of the mode-III crack of monolayer graphene necessary for the design and fabrication of graphene-based devices.

Chemical kinetics of H-abstractions from dimethyl amine by H, CH3, OH, and HO2 radicals with multi-structural torsional anharmonicity.

Dimethyl amine (DMA) is identified as a promising nitrogen-containing fuel candidate. To better understand the atmospheric and combustion chemistry of aliphatic amines, we systematically investigate the reaction kinetics of H-abstractions from DMA by H, CH3, OH, and HO2 radicals in a broad temperature range (100-2000 K). The BHandHLYP/cc-pVTZ method is adopted to determine the optimized geometries and frequencies, and the multi-structural torsional anharmonicity method (MS-T) is employed to characterize the effects of multi-conformer and torsional coupling for the involved species. High-level methods CCSD(T) and CCSD(T)-F12 combined with cc-pVXZ (X = D, T, Q), cc-pVXZ-F12 (X = D, T), and jun-cc-pV(T+d)Z basis sets are used to refine the electronic energies. The results of the gold standard method CCSD(T)/CBS(D-T-Q) with the zero point energy correction are adopted for the kinetic calculations. For the DMA + H/CH3 reactions, the conventional transition state theory (cTST) as well as one-dimensional Eckart tunneling correction is adopted. But for the DMA + OH/HO2 reactions, the reactant-complex (RC) is formed with a deep well (-6.4 and -11.7 kcal mol-1 for RC3 and RC4, respectively), due to the strong hydrogen bonding between the reactants. Hence, the variational transition state theory (VTST) combined with cTST is used to calculate the rate constants. The Rice-Ramsperger-Kassel-Marcus/master equation method is employed to determine the pressure-dependent rate constants in the pressure range of 0.001-100 atm. Our calculations are in agreement with previous experimental measurements and show well the trend in a broad temperature range. In addition, a pronounced pressure-dependence is observed under 400 K, indicating that pressure impacts the reaction mechanisms especially at atmospheric or interstellar temperatures.

Small-angle scattering of polychromatic X-rays: effects of bandwidth, spectral shape and high harmonics.

Polychromatic X-ray sources can be useful for photon-starved small-angle X-ray scattering given their high spectral fluxes. Their bandwidths, however, are 10-100 times larger than those using monochromators. To explore the feasibility, ideal scattering curves of homogeneous spherical particles for polychromatic X-rays are calculated and analyzed using the Guinier approach, maximum entropy and regularization methods. Monodisperse and polydisperse systems are explored. The influence of bandwidth and asymmetric spectra shape are explored via Gaussian and half-Gaussian spectra. Synchrotron undulator spectra represented by two undulator sources of the Advanced Photon Source are examined as an example, as regards the influence of asymmetric harmonic shape, fundamental harmonic bandwidth and high harmonics. The effects of bandwidth, spectral shape and high harmonics on particle size determination are evaluated quantitatively.

Interfacial anti-fatigue effect in graphene-copper nanolayered composites under cyclic shear loading.

Low-cycle fatigue behaviors of graphene-copper nanolayered (GCuNL) composites are explored at different interface configurations and repeat layer spacings. The graphene interfaces can trap dislocations through impeding the propagation of dislocations in copper layers, giving rise to the absence of softening, and an increase in the fatigue strength of GCuNL composites (up to 400% that of pure copper). This anti-fatigue effect is independent of the crystal orientation of copper or the chirality of graphene due to interfacial constraints and can be controlled by tailoring the repeat layer spacing. Low repeat layer spacing increases the instability and nonlinearity of the composites, while high repeat layer spacing decreases the anti-fatigue effect. The optimum value of the repeat layer spacing for the GCuNL composites is 3-7 nm, in order to achieve a balanced anti-fatigue capability and interface stability.

Competing roles of interfaces and matrix grain size in the deformation and failure of polycrystalline Cu-graphene nanolayered composites under shear loading.

The roles of interfaces and matrix grain size in the deformation and failure of polycrystalline Cu-graphene nanolayered (PCuGNL) composites under shear loading are explored with molecular dynamics simulations for different repeat layer spacings (λ), Cu grain sizes (D) and graphene chiralities, and an analytical model is proposed to describe the shear behavior. At the yield stage, the yield stress of the PCuGNL composites is mainly controlled by λ for λ ≤ 15 nm but mainly by D for λ > 15 nm; the yield strain of the composites is approximately a constant value of 0.056, weakly dependent on λ, D and graphene chirality. The shear failure strain and failure stress are determined only by the Cu-graphene interfaces. Small λ reduces the stability of the composites, while large λ decreases their shear failure strength. Considering the yield, failure and interface stability, the optimum λ value for the PCuGNL composites is 2-15 nm. In this optimum λ range, PCuGNL composites can be designed by tailoring Cu-graphene interfaces, regardless of the microstructures of polycrystalline Cu.