Tracking the relaxation pathway of photo-excited electrons in 1T-TiSe2

The ultrafast dynamics of excited electrons in 1T-TiSe₂ after absorption of a 390 nm light pulse is probed by time- and angle-resolved photoemission spectroscopy using femtosecond XUV pulses. It is demonstrated that the experimental approach can provide a comprehensive view of hot carrier motion in momentum space during relaxation back to equilibrium. This capability opens a new avenue in the investigation of energy dissipation processes in solids after intense optical excitation.     

Vibrational properties of Cu<Subscript>3</Subscript>XY<Subscript>4</Subscript> sulvanites (X = Nb,Ta, and V; and Y = S,and Se) by ab initio molecular dynamics

In this work, we present a structural and dynamic characterisation of six different types of sulvanites Cu₃ X Y ₄ with X = Nb, V and Ta, and Y = S and Se. These materials have been the subject of intense study in recent times primarily as potential candidates for solar cell devices, as well as for their enhanced opto-electrical properties. Here, by means of first-principles calculations, we study the structural and dynamic behaviour of these materials at different temperatures, which is important for use of these materials in high-temperature conditions. In this work the dynamic and structural properties are studied using the Density Functional Theory technique. The simulations were performed at four different temperatures, ranging from room temperature to ~1500 K. By using first-principles molecular dynamics in the microcanonical ensemble, we are able to determine the vibrational spectra of these sulvanites. With this information we report for the first time the partial vibrational density of states of these structures at different temperatures. With these results we determine the vibrational properties of the basic building blocks of those sulvanites and their dynamic behaviour under temperature effects. We also show that the building blocks that which make up these structures, remain stable as the temperature increases.     

Structural stabilities and optical properties of SixGeyHz nanocrystals

Based on the high-throughput first-principles calculations with structural recognition, we have ystematically investigated the structural stabilities and optical properties of Si_xGe_yH_z nanocrystals(H-SiGeNCs), including various sizes, shapes and compositions. The total energies of H-SiGeNCs can be simply estimated by the bond energy model in high accuracy, where the error of test set is less than 0.5 meV per atom. According to the energy difference of Si/Ge in various bonding environments, we have determined the ground state structures by the geometry analysis, as is confirmed the convex hulls of formation enthalpy from the first-principles calculations. In addition, the energy gaps of H-SiGeNCs are modulated by the atomic distributions, as well as the vibrations of Si-H and Ge-H bonds at room temperature which is revealed by ab initio molecular dynamics simulations.     

Collisionally pumped hybrid soft X-ray laser in Ne-like sulphur

We describe an experiment demonstrating XUV amplification following collisional excitation in a capillary discharge plasma irradiated by a picosecond IR laser pulse. Guiding and temporally resolved transmission of the pump laser beam are also demonstrated and analysed. The short pump laser pulse heated rapidly the electrons producing amplification in the 3p¹S₀–3s¹P₁ transition of Ne-like sulphur at 60.84 nm. The estimated gain–length product was equal to 6.8, while the beam divergence reached 2.5 mrad for 30 mm capillary. This new, hybridly pumped collisional soft X-ray laser with the transient gain offers a new way towards efficient table-top XUV sources.     

Selective cap opening in carbon nanotubes driven by laser-induced coherent phonons

We demonstrate the possibility of a selective nonequilibrium cap opening of carbon nanotubes as a response to femtosecond laser excitation. By performing molecular dynamics simulations based on a microscopic electronic model we show that the laser-induced ultrafast structural changes differ dramatically from the thermally induced dimer emission. Ultrafast bond weakening and simultaneous excitation of two coherent phonon modes of different frequencies, localized in the spherical caps and cylindrical nanotube body, are responsible for the selective cap opening.     

Classial and quantum solitons in the transverse field Ising model

We investigate the shape and the dynamics of domain walls in the one-dimensional Ising model with spin S, exchange constant J and external transverse field Γ using numerical calculations up to S = 20 and analytical approximations. For $\tfrac{\Gamma } {{JS}}$ \] we describe classical domain walls as strongly localized excitations, which have either central spin or central bond symmetry. These symmetries are identified also in the quantum case, when solitary excitations develop into energy bands. In the classical limit S → ∞ localization results from the exponential vanishing of the bandwidth for the lowest bands. We describe the relation between the spectrum of moving classical solitons and the quantum band structure.     

Effects of potassium on the adsorption of methanol on β-Mo2C(001) surface

We have studied the effect of K on the adsorption of methanol on the β-Mo₂C(001) surface and compared our experimental data with theoretical calculations. We have also performed high resolution electron energy loss spectroscopy (HREELS) (LK, ELS3000). For calculations we used the density functional theory under the VASP implementation. The most favorable sites for methanol adsorption are on top of a Mo atom in the clean surface and on top of a K atom in the pre-dosed surface. The changes in the work function fit our model as the surface withdraws charge from the adsorbate. The changes in the computed vibrational frequencies also agree with the HREELS results at very low coverage. The C–O bond distance increases while the O–H bond decreases making a C–O bond breakage a possibility on K covered surfaces.     

High Pressure Studies on the Optical and Electronic Properties of Para -Terphenyl

Based on X-ray powder diffraction experiments of p -terphenyl that yield pressure dependent lattice parameters, we performed geometry optimizations of all structural degrees of freedom including bond lengths and bond angles as well as molecular orientation using ab initio bandstructure methods. For these optimized structures we calculated the dielectric tensors. Upon increasing pressure we find a spectral red shift of the lowest energy absorption feature that is significantly smaller than that of the second peak at higher energies. We attribute this effect to intermolecular interactions that affect the bands involved in the second transition more than the bands giving raise to the first transition. This picture is confirmed by quantum chemical calculations for single molecules with the internal geometry extracted from the optimized structures.     

Structural and electronic properties of C6 cluster

First-principles total-energy calculations of structural properties of the carbon cluster C₆ have been made using the full-potential augmented plane-waves plus local orbital (APW+LO) method with the generalized gradient approximation (GGA). Initiated from a hexagonal configuration, we performed geometry optimization with damped Newton dynamics. The computed ground state atomic configuration for C₆ belongs to a monocyclic D_3h structure. The average bond length is 1.52 a.u. and the bond angle is 90.2^○, respectively, which are in agreement with the reported results.     

Laplace transform approach for the dynamics of N qubits coupled to a resonator

An approach to use the method of Laplace transform for the perturbative solution of the Schrödinger equation at any order of the perturbation for a system of N qubits coupled to a cavity with n photons is suggested. We investigate the dynamics of a system of N superconducting qubits coupled to a common resonator with time-dependent coupling. To account for the contribution of the dynamical Lamb effect to the probability of excitation of the qubit, we consider counter-rotating terms in the qubit-photon interaction Hamiltonian. As an example, we illustrate the method for the case of two qubits coupled to a common cavity. The perturbative solutions for the probability of excitation of the qubit show excellent agreement with the numerical calculations.     

Modification of laser-induced state in atomic attosecond transient absorption by the XUV pulse pair

Attosecond transient absorption(ATA) has been developed as an all-optical technique for probing electron dynamics in matter.Here we present a scheme that can modify the laserinduced state and the corresponding ATA spectrum via excitation by a pair of XUV attosecond pulses and by a time-delayed mid-infrared(MIR) laser probe.Different from the scheme of the electronic excitation by a single XUV attosecond pulse,the application of a pair of XUV pulses provides extra degrees of freedom,such as the t...     

Examination of high-throughput hybrid calculations using coarser reciprocal space meshes

High-throughput density functional theory calculations have been typically performed with reduced accuracy and notable error in the band gap. Here we suggest several approaches to calculate the optoelectronic properties by using coarser k-point meshes for the Fock exchange potential. In our benchmark calculations, we were able to obtain the optical properties of zinc-blende and wurtzite materials with reasonable accuracy. We also propose an approach of high-throughput calculations using a pre-converged wavefunction by the reduced k-point meshes for the Fock exchange and performing the subsequent non-self-consistent-field calculations.     

Hydrocarbon film growth by energetic CH3 molecule impact on SiC (0 0 1) surface

Classic molecular dynamics (MD) calculations were performed to investigate the deposition of thin hydrocarbon film. SiC (1 0 0) surfaces were bombarded with energetic CH₃ molecules at impact energies ranging from 50 to 150 eV. The simulated results show that the deposition yield of H atoms decreases with increasing incident energy, which is in good agreement with experiments. During the initial stages, with breaking Si-C bonds in SiC by CH₃ impacting, H atoms preferentially reacts with resulting Si to form Si-H bond. The C/H ratio in the grown films increases with increasing incident energy. In the grown films, CH species are dominant. For 50 eV, H-Csp3 bond is dominant. With increasing energy to 200 eV, the atomic density of H-Csp2 bond increases.     

Quantum Darwinism in a Composite System: Objectivity versus Classicality

We investigate the implications of quantum Darwinism in a composite quantum system with interacting constituents exhibiting a decoherence-free subspace. We consider a two-qubit system coupled to an N-qubit environment via a dephasing interaction. For excitation preserving interactions between the system qubits, an analytical expression for the dynamics is obtained. It demonstrates that part of the system Hilbert space redundantly proliferates its information to the environment, while the remaining subspace is decoupled and preserves clear non-classical signatures. For measurements performed on the system, we establish that a non-zero quantum discord is shared between the composite system and the environment, thus violating the conditions of strong Darwinism. However, due to the asymmetry of quantum discord, the information shared with the environment is completely classical for measurements performed on the environment. Our results imply a dichotomy between objectivity and classicality that emerges when considering composite systems.     

Decay Dynamics of 3‐methyl‐3‐pentene‐2‐one from the light absorbing S2(ππ*) state ‐ Resonance Raman Spectroscopy and CASSCF Study

The photophysics of 3‐methyl‐3‐pentene‐2‐one (3M3P2O) after excitation to the S₂(ππ*) electronic state were studied using the resonance Raman spectroscopy and complete active space self‐consistent field (CASSCF) method calculations. The A‐band resonance Raman spectra were obtained in cyclohexane, acetonitrile, and methanol with excitation wavelengths in resonance with the first intense absorption band to probe the structural dynamics of 3M3P2O. The B3LYP‐TD/6‐31++G(d, p) computation was carried out to determine the relative A‐band resonance Raman intensities of the fundamental modes, and the result was used to reproduce the corresponding fundamental band intensities of the 223.1 nm resonance Raman spectrum and thus to examine whether the vibronic‐coupling existed in Franck‐Condon region or not. CASSCF calculations were carried out to determine the minimal singlet excitation energies of S_{1, FC}, S_1,min (nπ*), S_{2, FC}, S_2,min (ππ*), the transition energies of the conical intersection points S_n/S_π, S_n/S₀, and the optimized excited state geometries as well as the geometry structures of the conical intersection points. The A‐band short‐time structural dynamics and the corresponding decay dynamics of 3M3P2O were obtained by the analysis of the resonance Raman intensity pattern and CASSCF computations. It was revealed that the initial structural dynamics of 3M3P2O was towards the simultaneous C₃=C₄ and C₂=O₇ bond elongation, with the C₃=C₄ bond length lengthening greater at the very beginning, whereas the C₂=O₇ bond length changing greater at the later evolution time before reaching the CI(S₂/S₁) conical intersection point. The decay dynamics from S₂(ππ*) to S₁(nπ*) via S₂(ππ*)/S₁(nπ*) in singlet realm and from S₁(nπ*) to T₁(nπ*) via ISC[S₁(nπ*)/T₂(ππ*)/T₁(nπ*)] in triplet realm are proposed. Copyright © 2014 John Wiley & Sons, Ltd.     

Oriented dynamics in van der Waals complexes

The effects of relative orientation on collision and reaction dynamics can be examined by characterizing the unimolecular decay of van der Waals complexes. Most commonly, decomposition is initiated by exciting one of the monomers within a complex, and the relative orientation is defined by the zero-point motions for the intermolecular degrees of freedom. However, if simultaneous excitation of the intermolecular degrees of freedom is achieved, a considerable range of starting configurations may be accessed. We are currently studying I₂-Rg and CN-H₂/D₂ complexes with the goal of using these systems to examine oriented dynamics. For I₂(B)-Ne we have observed excited intermolecular vibrations and the effects of these motions on the predissociation dynamics. The correlation between structure and dynamics suggested by these results can be understood using classical mechanics, but the implications of this model are at variance with accepted ideas about the topology of I₂(B)-Rg potential surfaces. Spectroscopic and theoretical studies of CN-H₂/D₂ indicate that this pre-reactive complex may be used to examine steric effects in the H₂+CN→H+HCN reaction. The complex was characterized using the A-X and B-X electronic transitions. As is often the case for weakly bound systems, the insights provided by high-level theoretical calculations were essential for interpretation of the spectra. The properties of excited intermolecular vibrations of CN-H₂ were predicted as a prelude to studies of the intra-cluster reaction dynamics.     

High-lyingM 1-states of spherical nuclei

The cross section of inelastic scattering of slow electrons at large angles with excitation of high-lying 1⁺-states in²⁰⁸Pb are calculated in DWBA. The calculations are carried out both in the random-phase approximation and with the interaction of one-and two-phonon states in the framework of semi-microscopic quasi-particle-phonon model. A group of noncollective 1⁺-states with a large excitation probability in backward (e, é)-scattering is found in the excitation energy region of 17–21 MeV. We discuss also the problem of existence, properties, and possibility of discovery the high-lying collective 1⁺-states (2?ω M 1-resonance) predicted by Speth et al.     

Comparative study of photoionization of atomic hydrogen by solving the one- and three-dimensional time-dependent Schrödinger equations

We develop a numerical scheme for solving the one-dimensional (1D) time-dependent Schrödinger equation (TDSE), and use it to study the strong-field photoionization of the atomic hydrogen. The photoelectron energy spectra obtained for pulses ranging from XUV to near infrared are compared in detail to the spectra calculated with our well-developed code for accurately solving the three-dimensional (3D) TDSE. For XUV pulses, our discussions cover intensities at which the ionization is in the perturbative and nonperturbative regimes. For pulses of 400 nm or longer wavelengths, we distinguish the multiphoton and tunneling regimes. Similarities and discrepancies between the 1D and 3D calculations in each regime are discussed. The observed discrepancies mainly originate from the differences in the transition matrix elements and the energy level structures created in the 1D and 3D calculations.