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Dissociating diatomic molecules in ultrafast and intense light
摘要： An ab initio theory is devised for the quantum dynamics of molecules undergoing multiple ionization in ultrafast and intense light. Specifically, the intertwined problem of photoionization, radiative, and electronic transitions in the course of dissociation is addressed which arises, e.g., when molecules are exposed to xuv light or x rays from free electron lasers or attosecond light sources, but the approach is equally useful in optical strong-field physics. The coherent interaction of the molecule with the light in a specific charge state is also treated. I set out from an abstract formulation in terms of the quantum optical notion of system-reservoir interaction using a master equation in Lindblad form and analyze its short-time approximation. First, I express it in a direct sum rigged Hilbert space for an efficient solution with numerical methods for systems of partial differential equations. Second, I derive a treatment via quantum Monte Carlo wave packet (MCWP) propagation. The formalism is concretized to diatomic molecules in Born-Oppenheimer approximation whereby molecular rotation is disregarded. The numerical integration of the master equation is carried out with a suitably factored density matrix that exploits the locality of the Hamiltonian and the Lindblad superoperator with respect to the internuclear distance. The formulation of the MCWP for molecules requires a thorough analysis of the quantum jump process; namely, the dependence on the continuous distance renders a straight wave packet promotion useless and, instead, a projected outer product needs to be employed involving an integrated quantum jump operator.
关键词： Ultrafast and intense light,Born-Oppenheimer approximation,Dissociation of diatomic molecules,Quantum Monte Carlo algorithm,Master equation in Lindblad form
更新于2025-09-10 09:29:36
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Electronic Excitations in Complex Molecular Environments: Many-Body Green’s Functions Theory in VOTCA-XTP
摘要： Many-body Green’s functions theory within the GW approximation and the Bethe-Salpeter Equation (BSE) is implemented in the open-source VOTCA-XTP software, aiming at the calculation of electronically excited states in complex molecular environments. Based on Gaussian-type atomic orbitals and making use of resolution of identity techniques, the code is designed specifically for nonperiodic systems. Application to a small molecule reference set successfully validates the methodology and its implementation for a variety of excitation types covering an energy range from 2 to 8 eV in single molecules. Further, embedding each GW-BSE calculation into an atomistically resolved surrounding, typically obtained from Molecular Dynamics, accounts for effects originating from local fields and polarization. Using aqueous DNA as a prototypical system, different levels of electrostatic coupling between the regions in this GW-BSE/MM setup are demonstrated. Particular attention is paid to charge-transfer (CT) excitations in adenine base pairs. It is found that their energy is extremely sensitive to the specific environment and to polarization effects. The calculated redshift of the CT excitation energy compared to a nucelobase dimer treated in vacuum is of the order of 1 eV, which matches expectations from experimental data. Predicted lowest CT energies are below that of a single nucleobase excitation, indicating the possibility of an initial (fast) decay of such an UV excited state into a binucleobase CT exciton. The results show that VOTCA-XTP’s GW-BSE/MM is a powerful tool to study a wide range of types of electronic excitations in complex molecular environments.
关键词： GW approximation,aqueous DNA,Bethe-Salpeter Equation,complex molecular environments,charge-transfer excitations,VOTCA-XTP,Many-body Green’s functions theory,electronic excitations
更新于2025-09-10 09:29:36
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Raman Scattering by Plasma Oscillations in Quantum Rings
摘要： Resonant Raman scattering in quantum rings with a sufficiently large number of conduction electrons has been studied. The cross section for Raman scattering accompanied by the excitation of a one-dimensional plasmon in a ring has been determined in the self-consistent field approximation.
关键词： self-consistent field approximation,Raman scattering,quantum rings,plasma oscillations
更新于2025-09-10 09:29:36
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A Markovian Approach to Unsupervised Change Detection with Multiresolution and Multimodality SAR Data
摘要： In the framework of synthetic aperture radar (SAR) systems, current satellite missions make it possible to acquire images at very high and multiple spatial resolutions with short revisit times. This scenario conveys a remarkable potential in applications to, for instance, environmental monitoring and natural disaster recovery. In this context, data fusion and change detection methodologies play major roles. This paper proposes an unsupervised change detection algorithm for the challenging case of multimodal SAR data collected by sensors operating at multiple spatial resolutions. The method is based on Markovian probabilistic graphical models, graph cuts, linear mixtures, generalized Gaussian distributions, Gram–Charlier approximations, maximum likelihood and minimum mean squared error estimation. It bene?ts from the SAR images acquired at multiple spatial resolutions and with possibly different modalities on the considered acquisition times to generate an output change map at the ?nest observed resolution. This is accomplished by modeling the statistics of the data at the various spatial scales through appropriate generalized Gaussian distributions and by iteratively estimating a set of virtual images that are de?ned on the pixel grid at the ?nest resolution and would be collected if all the sensors could work at that resolution. A Markov random ?eld framework is adopted to address the detection problem by de?ning an appropriate multimodal energy function that is minimized using graph cuts.
关键词： Markov random ?elds (MRF),maximum likelihood (ML) estimation,synthetic aperture radar (SAR),Gram–Charlier approximation,multiresolution data fusion,multimodality data fusion,graph cuts,minimum mean squared error (MMSE) estimation,generalized Gaussian
更新于2025-09-10 09:29:36
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Energy levels and transition rates for Al-like Cu XVII
摘要： Energy levels and transition rates for electric-dipole, electric-quadrupole, magnetic-dipole, and magnetic-quadrupole transitions among the levels arising from the n ≤ 5 configurations in Al-like Cu XVII are calculated respectively by two state-of-the-art methods, namely, the Multi-Configuration Dirac–Hartree–Fock approach and the second-order many-body perturbation theory. Our results are compared with available experimental and other theoretical values. In addition, effective collision strengths of direct excitation between these levels are provided using the distorted wave approximation. Several line pairs are pointed out that might be useful for density diagnostics.
关键词： Cu XVII,Many-body perturbation,Energy levels,Transition probabilities,Direct excitation,Distorted wave approximation
更新于2025-09-10 09:29:36
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Insight into phonon scattering in Si nanowires through high-field hole transport: Impacts of boundary condition and comparison with bulk phonon approximation
摘要： The impact of how to model phonon scattering on hole transport in Si nanowires was studied based on Boltzmann’s transport equation. Boundary conditions for atomistic description of phonons in nanowires and approximation by bulk acoustic and optical phonons were analyzed in terms of their impacts on high-field hole transport. The boundary conditions for phonons influence the drift velocity and momentum relaxation time, especially at low electric field, but the energy relaxation time hardly depends on the boundary conditions. The impacts by the change of boundary conditions can be approximated by the change of the strength of acoustic phonon scattering in bulk phonon picture, though the behavior of energy relaxation and distribution function of holes can not be reproduced by bulk phonon approximation.
关键词： Si nanowires,boundary condition,bulk phonon approximation,phonon scattering,high-field hole transport
更新于2025-09-10 09:29:36
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I–V characteristics and conductance of strained SWCNTs
摘要： We present a new procedure to investigate the I–V characteristics and the conductance for strained SWCNTs. These electronic transport properties have been studied theoretically at zero temperature for zig-zag, armchair and chiral SWCNTs under the effect of the uniaxial tension and torsional strain. The analytical expression of the energy spectrum in the tight binding approximation has been used to calculate the induced current and the conductance through Landauer–Büttiker formalism. It is shown that the conductance for unstrained CNTs at initial values of the voltage can take discrete values which are equal to zero and 4 (e2/h) for semiconducting and conducting SWCNTs respectively. The emergence of the kinks in the I–V characteristics is due to the discrete electronic spectrum in the SWCNTs. The location and number of kinks are changeable under the effect of strain process. The conductance in a strained armchair (5, 5) CNT decreases to zero under torsional strain, consequently, it will transform the conducting SWCNTs at a threshold value of strain to a semiconducting SWCNT. In contrast, by applying the uniaxial tension on the armchair (5, 5) CNT, the conductance does not change absolutely. There is a different behavior can be observed by applying the strain on zig-zag (10, 0) CNT, where the conductance decreases rapidly and slightly under the in?uence of uniaxial tension and torsional strain, respectively. We found that the conductance of chiral (10, 9) CNT is not signi?cantly affected by applying the strain under consideration. More interestingly, the band structure of chiral (10, 9) CNT under uniaxial tension and torsional strain have been investigated within the tight binding approximation.
关键词： I–V characteristics,Mechanical strain,Tight binding approximation,SWCNT,Conductance
更新于2025-09-10 09:29:36
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Simple eigenvalue-self-consistent ΔˉGW0
摘要： We show that a rigid scissors-like GW self-consistency approach, labeled here ˉ?GW0, can be trivially implemented at zero additional cost for large scale one-shot G0W 0 calculations. The method significantly improves one-shot G0W 0 and for large systems is very accurate. ˉ?GW0 is similar in spirit to evGW 0 where the self-consistency is only applied on the eigenvalues entering Green’s function, while both W and the eigenvectors of Green’s function are held fixed. ˉ?GW0 further assumes that the shift of the eigenvalues is rigid scissors-like so that all occupied states are shifted by the same amount and analogously for all the unoccupied states. We show that this results in a trivial modification of the time-dependent G0W 0 self-energy, enabling an a posteriori self-consistency cycle. The method is applicable for our recent stochastic-GW approach, thereby enabling self-consistent calculations for giant systems with thousands of electrons. The accuracy of ˉ?GW0 increases with the system size. For molecules, it is up to 0.4-0.5 eV away from coupled-cluster single double triple (CCSD(T)), but for tetracene and hexacene, it matches the ionization energies from both CCSD(T) and evGW 0 to better than 0.05 eV. For solids, as exemplified here by periodic supercells of semiconductors and insulators with 6192 valence electrons, the method matches evGW 0 quite well and both methods are in good agreement with the experiment.
关键词： quasiparticle energies,large systems,self-consistency,stochastic approach,GW approximation
更新于2025-09-10 09:29:36
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Resolution-of-identity accelerated relativistic two- and four-component electron dynamics approach to chiroptical spectroscopies
摘要： We present an implementation and application of electron dynamics based on real-time time-dependent density functional theory (RT-TDDFT) and relativistic 2-component X2C and 4-component Dirac–Coulomb (4c) Hamiltonians to the calculation of electron circular dichroism and optical rotatory dispersion spectra. In addition, the resolution-of-identity approximation for the Coulomb term (RI-J) is introduced into RT-TDDFT and formulated entirely in terms of complex quaternion algebra. The proposed methodology was assessed on the dimethylchalcogenirane series, C4H8X (X = O, S, Se, Te, Po, Lv), and the spectra obtained by non-relativistic and relativistic methods start to disagree for Se and Te, while dramatic differences are observed for Po and Lv. The X2C approach, even in its simplest one-particle form, reproduces the reference 4c results surprisingly well across the entire series while offering an 8-fold speed-up of the simulations. An overall acceleration of RT-TDDFT by means of X2C and RI-J increases with system size and approaches a factor of almost 25 when compared to the full 4c treatment, without compromising the accuracy of the final spectra. These results suggest that one-particle X2C electron dynamics with RI-J acceleration is an attractive method for the calculation of chiroptical spectra in the valence region.
关键词： resolution-of-identity approximation,complex quaternion algebra,chiroptical spectroscopies,real-time time-dependent density functional theory,relativistic Hamiltonians,electron dynamics
更新于2025-09-10 09:29:36
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Gauge invariance and origin independence of electronic charge density and current density induced by optical fields
摘要： Expressions for the first-order polarization charge density ρ(1) and current density J(1) induced in a molecule by a monochromatic plane wave, obtained by time-dependent quantum mechanical perturbation theory, have been investigated to assess their gauge invariance and independence of the coordinate system in passive and active translations. The conditions arrived at show that, within the (long wavelengths) dipole approximation, only the electric contributions to these densities are needed to rationalize the phenomenology. To the next higher quadrupole approximation, assuming that the magnetic field and the electric field gradient are uniform over the molecular dimensions, corresponding contributions to ρ(1) and J(1) are considered. It has been found that total densities are independent of the origin, whereas the contributions from electric and magnetic fields are not separately invariant. A magnetic contribution to J(1), which is by itself origin independent, can be defined by means of an approach based on continuous translation of the origin of the coordinate system.
关键词： time-dependent perturbation theory,origin independence,quadrupole approximation,current density,optical fields,gauge invariance,dipole approximation,electronic charge density
更新于2025-09-09 09:28:46