251 research outputs found
IL14. Core-Hole Initiated Charge Migration with TDDFT
No full textAttosecond electron dynamics in molecules underpins a range of important processes such as light harvesting, photochemistry, and ultrafast spectroscopy. Modeling these dynamics from first principles is important for predicting and interpreting ultrafast experiments. In the case of large molecules, however, correlated techniques can be prohibitively expensive. Here, time-dependent density functional theory (TDDFT) o↵ers a promising alternative, but limitations in the exchange-correlation functional, especially the adiabatic (local-in-time) approximation limit the accuracy of the results.
In this talk, I will present a study demonstrating the validity of TDDFT for core-hole triggered charge migration in nitrosobenzene. Specifically, by initializing the system with an unambiguous initial state (a nitrogen K-edge core-hole), real-time TDDFT with hybrid functionals captures hole migration across the molecule with accuracy comparable to ADC(4) [1]. These results suggest that given an initial state that is a good reflection of a molecule after interaction with a exciting or ionizing pump field, adiabatic TDDFT adequately capures the dynamics. Adam Bruner, Louisiana State University Samuel Hernandez, Louisiana State University Francois Mauger, Louisiana State University Mette Gaarde, Louisiana State University Kenneth Schafer, Louisiana State University Kenneth Lopata, Louisiana State Universit
Nonlinear nanopolaritonics: Finite-difference time-domain Maxwell–Schrödinger simulation of molecule-assisted plasmon transfer
No full textThe effect of nonlinear excitations of a nearby two-state dipolar molecule on plasmon transfer across a pair of spherical gold nanoparticles is studied numerically using a split field finite-difference time-domain Maxwell-Schrödinger approach [K. Lopata and D. Neuhauser, J. Chem. Phys. 130, 104707 (2009)]. It is observed in the linear response regime that the molecule has a drastic effect on plasmon transfer; specifically, there is a Fano-type resonance that serves to scatter localized plasmons from x -polarization to y -polarization. With increasing nonlinearity of the molecular excitation, the scattering effect saturates due to the limited capacity of the molecule to absorb and radiate energy once the excited and ground states are equally populated. © 2009 American Institute of Physics
Attosecond Charge Migration with TDDFT: Accurate Dynamics from a Well Defined Initial State
No full textWe investigate the ability of time-dependent density functional theory (TDDFT) to capture attosecond valence electron dynamics resulting from sudden X-ray ionization of a core electron. In this special case the initial state can be constructed unambiguously, allowing for a simple test of the accuracy of the dynamics. The response following nitrogen K-edge ionization in nitrosobenzene shows excellent agreement with fourth order algebraic diagrammatic construction (ADC(4)) results, suggesting that a properly chosen initial state allows TDDFT to adequately capture attosecond charge migration. Visualizing hole motion using an electron localization picture (ELF), we provide an intuitive chemical interpretation of the charge migration as a time-dependent superposition of Lewis-dot resonance structures. Coupled with the initial state solution to obtain such dynamics with TDDFT, this chemical picture facilitates interpretation of electron .Non UBCUnreviewedAuthor affiliation: Louisiana State UniversityFacult
High-harmonic spectroscopy of transient two-center interference calculated with time-dependent density-functional theory
No full textWe demonstrate high-harmonic spectroscopy in many-electron molecules using time-dependent density-functional theory. We show that a weak attosecond-pulse-train ionization seed that is properly synchronized with the strong driving mid-infrared laser field can produce experimentally relevant high-harmonic generation (HHG) signals, from which we extract both the spectral amplitude and the target-specific phase (group delay). We also show that further processing of the HHG signal can be used to achieve molecular-frame resolution, i.e., to resolve the contributions from rescattering on different sides of an oriented molecule. In this framework, we investigate transient two-center interference in CO2 and OCS, and how subcycle polarization effects shape the oriented/aligned angle-resolved spectra. (C) 2019 Author(s)
Molecular nanopolaritonics: Cross manipulation of near-field plasmons and molecules. I. Theory and application to junction control
No full textNear-field interactions between plasmons and molecules are treated in a simple unified approach. The density matrix of a molecule is treated with linear-response random phase approximation and the plasmons are treated classically. The equations of motion for the combined system are linear, governed by a simple Liouvillian operator for the polariton (plasmon+molecule excitation) dynamics. The dynamics can be followed in time or directly in frequency space where a trace formula for the transmission is presented. A model system is studied, metal dots in a forklike arrangement, coupled to a two level system with a large transition-dipole moment. A Fano-type resonance [Phys. Rev. 103, 1202 (1956)] develops when the molecular response is narrower than the width of the absorption spectrum for the plasmons. We show that the direction of the dipole of the molecule determines the direction the polariton chooses. Further, the precise position of the molecule has a significant effect on the transfer. © 2007 American Institute of Physics
Quantum Drude friction for time-dependent density functional theory
No full textFriction is a desired property in quantum dynamics as it allows for localization, prevents backscattering, and is essential in the description of multistage transfer. Practical approaches for friction generally involve memory functionals or interactions with system baths. Here, we start by requiring that a friction term will always reduce the energy of the system; we show that this is automatically true once the Hamiltonian is augmented by a term of the form ∫a(q;nn0)×[∂j(q,t)∂t] ·J (q) dq, which includes the current operator times the derivative of its expectation value with respect to time, times a local coefficient; the local coefficient will be fitted to experiment, to more sophisticated theories of electron-electron interaction and interaction with nuclear vibrations and the nuclear background, or alternately, will be artificially constructed to prevent backscattering of energy. We relate this term to previous results and to optimal control studies, and generalize it to further operators, i.e., any operator of the form ∫ a(q;n0)[∂c (q,t)/∂t] ·C(q)dq (or a discrete sum) will yield friction. Simulations of a small jellium cluster, both in the linear and highly nonlinear excitation regime, demonstrate that the friction always reduces energy. The energy damping is essentially double exponential; the long-time decay is almost an order of magnitude slower than the rapid short-time decay. The friction term stabilizes the propagation (split-operator propagator here), therefore increasing the time-step needed for convergence, i.e., reducing the overall computational cost. The local friction also allows the simulation of a metal cluster in a uniform jellium as the energy loss in the excitation due to the underlying corrugation is accounted for by the friction. We also relate the friction to models of coupling to damped harmonic oscillators, which can be used for a more sophisticated description of the coupling, and to memory functionals. Our results open the way to very simple finite grid description of scattering and multistage conductance using time-dependent density functional theory away from the linear regime, just as absorbing potentials and self-energies are useful for noninteracting systems and leads. © 2008 American Institute of Physics
Multiscale Maxwell–Schrödinger modeling: A split field finite-difference time-domain approach to molecular nanopolaritonics
No full textWe present a combined finite-difference time-domain/linear response approach for modeling plasmon/molecule systems. The self-interaction of the molecule is avoided by splitting the fields and currents into two parts: those due to the molecule and those from everything else. This approach is suitable for describing surface plasmons on metal nanostructures interacting in the near field with nearby dipolar molecules or semiconductor nanostructures. The approach is applied to three collinear 5 nm diameter gold nanoparticles; the results demonstrate that a nearby molecule strongly affects surface plasmon transfer along the array. Specifically, an xy oriented molecule situated midway between the second and third nanoparticles exhibits a symmetric Fano-type inference effect. Transmission of incident x -polarized energy from the second nanoparticle to the third is enhanced over a frequency range below the molecular resonance, and partially scattered into y -polarized currents for frequencies above. At the molecule\u27s resonance frequency, the magnitude of the resulting y -current is approximately 20% of the x -current. © 2009 American Institute of Physics
Modeling Fast Electron Dynamics with Real-Time Time-Dependent Density Functional Theory: Application to Small Molecules and Chromophores
No full textThe response of matter to external fields forms the basis for a vast wealth of fundamental physical processes ranging from light harvesting to nanoscale electron transport. Accurately modeling ultrafast electron dynamics in excited systems thus offers unparalleled insight but requires an inherently nonlinear time-resolved approach. To this end, an efficient and massively parallel real-time real-space time-dependent density functional theory (RT-TDDFT) implementation in NWChem is presented. The implementation is first validated against linear-response TDDFT and experimental results for a series of molecules subjected to small electric field perturbations. Second, nonlinear excitation of green fluorescent protein is studied, which shows a blue-shift in the spectrum with increasing perturbation, as well as a saturation in absorption. Next, the charge dynamics of optically excited zinc porphyrin is presented in real time and real space, with relevance to charge injection in photovoltaic devices. Finally, intermolecular excitation in an adenine-thymine base pair is studied using the BNL range separated functional [Baer, R.; Neuhauser, D.Phys. Rev. Lett. 2005, 94, 043002], demonstrating the utility of a real-time approach in capturing charge transfer processes. © 2011 American Chemical Society
Near and Above Ionization Electronic Excitations with Non-Hermitian Real-Time Time-Dependent Density Functional Theory
No full textWe present a real-time time-dependent density functional theory (RT-TDDFT) prescription for capturing near and post-ionization excitations based on non-Hermitian von Neumann density matrix propagation with atom-centered basis sets, tuned range-separated DFT, and a phenomenological imaginary molecular orbital-based absorbing potential to mimic coupling to the continuum. The computed extreme ultraviolet absorption spectra for acetylene (C 2H2), water (H2O), and Freon 12 (CF 2Cl2) agree well with electron energy loss spectroscopy (EELS) data over the range of 0-50 eV. The absorbing potential removes spurious high-energy finite basis artifacts, yielding correct bound-to-bound transitions, metastable (autoionizing) resonance states, and consistent overall absorption shapes. © 2013 American Chemical Society
First-Principles Simulations of X-ray Transient Absorption for Probing Attosecond Electron Dynamics
No full textCopyright © 2020 American Chemical Society. X-ray transient absorption spectroscopy (XTAS) is a promising technique for measuring electron dynamics in molecules and solids with attosecond time resolutions. In XTAS, the elemental specificity and spatial locality of core-to-valence X-ray absorption is exploited to relate modulations in the time-resolved absorption spectra to local electron density variations around particular atoms. However, interpreting these absorption modulations and frequency shifts as a function of the time delay in terms of dynamics can be challenging. In this paper, we present a first-principles study of attosecond XTAS in a selection of simple molecules based on real-time time-dependent density functional theory (RT-TDDFT) with constrained DFT to emulate the state of the system following the interaction with a ultraviolet pump laser. In general, there is a decrease in the optical density and a blue shift in the frequency with increasing electron density around the absorbing atom. In carbon monoxide (CO), modulations in the O K-edge occur at the frequency of the valence electron dynamics, while for dioxygen (O2) they occur at twice the frequency, due to the indistinguishability of the oxygen atoms. In 4-aminophenol (H2NC6H4OH), likewise, there is a decrease in the optical density and a blue shift in the frequency for the oxygen and nitrogen K-edges with increasing charge density on the O and N, respectively. Similar effects are observed in the nitrogen K-edge for a long-range charge-transfer excitation in a benzene (C6H6)-tetracyanoethylene (C6N4; TCNE) dimer but with weaker modulations due to the delocalization of the charge across the entire TCNE molecule. Additionally, in all cases, there are pre-edge features corresponding to core transitions to depopulated orbitals. These potentially offer a background-free signal that only appears in pumped molecules
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