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Modeling core-level excitations with variationally optimized reduced-density matrices and the extended random phase approximation
摘要： The information contained within ground-state one- and two-electron reduced-density matrices (RDMs) can be used to compute wave functions and energies for electronically excited states through the extended random phase approximation (ERPA). The ERPA is an appealing framework for describing excitations out of states obtained via the variational optimization of the two-electron RDM (2-RDM), as the variational 2-RDM (v2RDM) approach itself can only be used to describe the lowest-energy state of a given spin symmetry. The utility of the ERPA for predicting near-edge features relevant to x-ray absorption spectroscopy is assessed for the case that the 2-RDM is obtained from a ground-state v2RDM-driven complete active space self-consistent field (CASSCF) computation. A class of killer conditions for the CASSCF-speci?c ERPA excitation operator is derived, and it is demonstrated that a reliable description of core-level excitations requires an excitation operator that ful?lls these conditions; the core-valence separation (CVS) scheme yields such an operator. Absolute excitation energies evaluated within the CASSCF/CVS-ERPA framework are slightly more accurate than those obtained from the usual random phase approximation (RPA), but the CVS-ERPA is not more accurate than RPA for predicting the relative positions of near-edge features. Nonetheless, CVS-ERPA is established as a reasonable starting point for the treatment of core-level excitations using variationally optimized 2-RDMs.
关键词： reduced-density matrices,extended random phase approximation,v2RDM,CASSCF,core-level excitations,x-ray absorption spectroscopy
更新于2025-09-04 15:30:14
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Applicability of the thin-film approximation in terahertz photoconductivity measurements
摘要： Thin mesoporous photoconductive layers are critically important for efficient water-spitting solar cells. A detailed understanding of photoconductivity in these materials can be achieved via terahertz transient absorption measurements. Such measurements are commonly interpreted using the thin-film approximation. We compare this approximation with a numerical solution of the transfer function without approximations using experimental results for thin-film mesoporous tin oxide (SnO2) samples which range in thickness from 3.3 to 12.6 mm. These samples were sensitized with either a ruthenium polypyridyl complex or a porphyrin dye. The two sensitizers have markedly different absorption coefficients, resulting in penetration depths of 15 mm and 1 mm, respectively. The thin-film approximation results are in good agreement with the numerical work-up for the short penetration length dye. For the longer penetration length samples, the thin-film formula fails even for thicknesses of only 3 mm (cid:2) k/100. The imaginary part of the conductivity calculated using the thin-film formula was significantly larger in magnitude than the value without approximations. This discrepancy between the commonly used thin-film approximation and the numerical solution demonstrates the need for a careful analysis of the thin-film formula.
关键词： thin-film approximation,terahertz photoconductivity,mesoporous SnO2,ruthenium polypyridyl complex,porphyrin dye
更新于2025-09-04 15:30:14
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Ionization of Impurities by Constant and Alternating Electric Fields in a Gap Graphene Modification
摘要： The probability of the ionization of impurities in graphene with a gap in its energy spectrum in co-directional constant and alternating electric fields is examined in the quasiclassical approximation. Analytical expressions for the probability of ionization in a constant electric field and a strong alternating electric field acting individually are obtained.
关键词： quasiclassical approximation,ionization,impurities,electric fields,graphene
更新于2025-09-04 15:30:14
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Resonance Energy Transfer in Arbitrary Media: Beyond the Point Dipole Approximation
摘要： In this work, we present a comprehensive theoretical and computational study of donor/acceptor resonance energy transfer (RET) beyond the dipole approximation, in arbitrary inhomogeneous and dispersive media. The theoretical method extends Fo?rster theory for RET between particles (molecules or nanoparticles) to the case where higher multipole transitions in the donor and/or acceptor play a significant role in the energy transfer process. In our new formulation, the energy transfer matrix element is determined by a fully quantum electrodynamic expression, but its evaluation requires only classical electrodynamics calculations. By means of a time domain electrodynamical approach (TED), the matrix element evaluation involves the electric and magnetic fields generated by the donor and evaluated at the position of the acceptor, including fields associated with transition electric dipoles, electric quadrupoles, and magnetic dipoles in the donor, and the acceptor response to the electric and magnetic fields and to the electric field gradient. As an illustration of the benefits of the new formalism, we tested our method with a 512 atom lead sulfide (PbS) quantum dot as the donor/acceptor in vacuum, and with spherical nanoparticles (toy model) possessing designed transition multipoles. This includes an analysis of the effects of interferences between multipoles in the energy transfer rate. The results show important deviations from the conventional Fo?rster dipole theory that are important even in vacuum but that can be amplified by interaction with a plasmonic nanoparticle.
关键词： multipole transitions,resonance energy transfer,plasmonic nanoparticle,quantum electrodynamics,dipole approximation,RET
更新于2025-09-04 15:30:14
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centers in amorphous
摘要： We investigate the charge-trapping behavior in nitrogen-deficient amorphous silicon nitride (a-Si3N4?x) using first-principles calculations. The amorphous ensembles with one nitrogen atom missing are generated through melt-quench procedures. The nitrogen deficiency mainly produces one Si—Si bond and one K center (Si dangling bond). The energy level of defect states indicates that the K centers act as possible trap sites. The transition levels of K centers are estimated, and it is found that the Hubbard U energy ranges from ?1.14 to 1.11 eV. Even though most K centers show positive U, the charge states of most centers in the ensemble are either positive or negative under the charge-neutrality condition, resulting in 'seemingly negative-U' behavior. This is consistent with the diamagnetic signal in experiments. The charge-injection energy of K centers is evaluated on the basis of the Franck-Condon approximation, and the average trap depths for electrons (1.33 eV below the conduction edge) and holes (1.54 eV above the valence edge) are in good agreement with experimental data.
关键词： K centers,Franck-Condon approximation,first-principles calculations,Hubbard U energy,amorphous silicon nitride,charge-trapping behavior
更新于2025-09-04 15:30:14