Molecular Modes of Attosecond Charge Migration.

First-principles calculations are employed to elucidate the modes of attosecond charge migration (CM) in halogenated hydrocarbon chains. We use constrained density functional theory (DFT) to emulate the creation of a localized hole on the halogen and follow the subsequent dynamics via time-dependent DFT. We find low-frequency CM modes (∼1 eV) that propagate across the molecule and study their dependence on length, bond order, and halogenation. We observe that the CM speed (∼4 Å/fs) is largely independent of molecule length, but is lower for triple-bonded versus double-bonded molecules. Additionally, as the halogen mass increases, the hole travels in a more particlelike manner as it moves across the molecule. These heuristics will be useful in identifying molecules and optimal CM detection methods for future experiments, especially for halogenated hydrocarbons which are promising targets for ionization-triggered CM.

Simulated field-modulated x-ray absorption in titania.

In this paper, we present a method to compute the x-ray absorption near-edge structure (XANES) spectra of solid-state transition metal oxides using real-time time-dependent density functional theory, including spin-orbit coupling effects. This was performed on bulk-mimicking anatase titania (TiO2) clusters, which allows for the use of hybrid functionals and atom-centered all electron basis sets. Furthermore, this method was employed to calculate the shifts in the XANES spectra of the Ti L-edge in the presence of applied electric fields to understand how external fields can modify the electronic structure, and how this can be probed using x-ray absorption spectroscopy. Specifically, the onset of t2g peaks in the Ti L-edge was observed to red shift and the eg peaks were observed to blue shift with increasing fields, attributed to changes in the hybridization of the conduction band (3d) orbitals.

Resonant X-ray Sum-Frequency-Generation Spectroscopy of K-Edges in Acetyl Fluoride.

Resonant X-ray sum-frequency generation is calculated for excitations of the fluorine and the oxygen core K-edge in acetyl fluoride using real-time time-dependent density functional theory. The signal is generated by an extreme-ultraviolet pulse followed by an X-ray pulse with variable delay T. The X-ray pulse is tuned to different element-specific core excitations and used to probe the dynamics of a valence electronic wave packet. A two-dimensional signal is recorded depending on the dispersed X-ray pulse frequency and the frequency conjugated to T, revealing the couplings between core and valence excited states. Molecular orbital decomposition of the signal reveals which regions of the molecule contribute to the X-ray excitation.

X-ray linear and non-linear spectroscopy of the ESCA molecule.

Linear and nonlinear X-ray spectroscopy hold the promise to provide a complementary tool to the available ample body of terahertz to UV spectroscopic techniques, disclosing information about the electronic structure and the dynamics of a large variety of systems, spanning from transition metals to organic molecules. While experimental free electron laser facilities continue to develop, theory may take the lead in modeling and inspiring new cutting edge experiments, paving the way to their future use. As an example, the not-yet-available two-dimensional coherent X-ray spectroscopy (2DCXS), conceptually similar to 2D-NMR, is expected to provide a wealth of information about molecular structure and dynamics with an unprecedented level of detail. In the present contribution, we focus on the simulation of linear and non-linear (2DCXS) spectra of the ESCA molecule. The molecule has four inequivalent carbon K-edges and has been widely used as a benchmark for photoelectron spectroscopy. Two theoretical approaches for the computation of the system manifold of states, namely, TDDFT and RASSCF/RASPT2, are compared, and the possible signals that may appear in a 2DCXS experiment and their origin are surveyed.

Structure Based Modulation of Electron Dynamics in meso-(4-Pyridyl)-BODIPYs: A Computational and Synthetic Approach.

The effects of structural modification on the electronic structure and electron dynamics of cationic meso-(4-pyridyl)-BODIPYs were investigated. A library of 2,6-difunctionalized meso-(4-pyridyl)-BODIPYs bearing various electron-withdrawing substituents was designed, and DFT calculations were used to model the redox properties, while TDDFT was used to determine the effects of functionalization on the excited states. Structural modification was able to restructure the low-lying molecular orbitals to effectively inhibit d-PeT. A new meso-(4-pyridyl)-BODIPY bearing 2,6-dichloro groups was synthesized and shown to exhibit enhanced charge recombination fluorescence. The fluorescence enhancement was determined to be the result of functionalization modulating the kinetics of the excited state dynamics.

Attosecond Charge Migration with TDDFT: Accurate Dynamics from a Well-Defined Initial State.

We 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 superposition of Lewis dot resonance structures.

Accelerated Broadband Spectra Using Transition Dipole Decomposition and Padé Approximants.

We present a method for accelerating the computation of UV-visible and X-ray absorption spectra in large molecular systems using real-time time-dependent density functional theory (TDDFT). This approach is based on deconvolution of the dipole into molecular orbital dipole pairs developed by Repisky, et al. [Repisky et al., J. Chem. Theory Comput. 2015, 11, 980-911] followed by Padé approximants to their Fourier transforms. By combining these two techniques, the required simulation time is reduced by a factor of 5 or more, and moreover, the transition dipoles yield the molecular orbital contributions to each transition, akin to the coefficients in linear-response TDDFT. We validate this method on valence and core-level spectra of gas-phase water and nickel porphyrin, where the results are essentially equivalent to conventional linear response. This approach makes real-time TDDFT competitive against linear response for large molecular and material systems with a high density of states.