We present an approach for generating local numerical basis sets of improving accuracy for first-principles nanoplasmonics simulations within time-dependent density functional theory. The method is demonstrated for copper, silver, and gold nanoparticles that are of experimental interest but computationally demanding due to the semi-core d-electrons that affect their plasmonic response. The basis sets are constructed by augmenting numerical atomic orbital basis sets by truncated Gaussian-type orbitals generated by the completeness-optimization scheme, which is applied to the photoabsorption spectra of homoatomic metal atom dimers. We obtain basis sets of improving accuracy up to the complete basis set limit and demonstrate that the performance of the basis sets transfers to simulations of larger nanoparticles and nanoalloys as well as to calculations with various exchange-correlation functionals. This work promotes the use of the local basis set approach of controllable accuracy in first-principles nanoplasmonics simulations and beyond.

We observe using ab initio methods that localized surface plasmon resonances in icosahedral silver nanoparticles enter the asymptotic region already between diameters of 1 and 2 nm, converging close to the classical quasistatic limit around 3.4 eV. We base the observation on time-dependent density-functional theory simulations of the icosahedral silver clusters Ag-55 (1.06 nm), Ag-147 (1.60 nm), Ag-309 (2.14 nm), and Ag-561 (2.68 nm). The simulation method combines the adiabatic GLLB-SC exchange-correlation functional with real time propagation in an atomic orbital basis set using the projector-augmented wave method. The method has been implemented for the electron structure code GPAW within the scope of this work. We obtain good agreement with experimental data and modeled results, including photoemission and plasmon resonance. Moreover, we can extrapolate the ab initio results to the classical quasistatically modeled icosahedral clusters.

We report a study on the temperature dependence of the valence electron excitation spectrum of CO2 performed using nonresonant inelastic X-ray scattering spectroscopy. The excitation spectra were measured at the temperatures of 300 and 850 K with momentum-transfer values of 0.4-4.8 Å(-1), i.e., from the dipole limit to the higher-multipole regime, and were simulated using high-level coupled cluster calculations on the dipole and quadrupole level. The results demonstrate the emergence of dipole-forbidden excitations owing to temperature-induced bending mode activation and finite momentum transfer.

The presence of plasmonic material influences the optical properties of nearby molecules in untrivial ways due to the dynamical plasmon-molecule coupling. We combine quantum and classical calculation schemes to study this phenomenon in a hybrid system that consists of a Na(2) molecule located in the gap between two Au/Ag nanoparticles. The molecule is treated quantum-mechanically with time-dependent density-functional theory, and the nanoparticles with quasistatic classical electrodynamics. The nanoparticle dimer has a plasmon resonance in the visible part of the electromagnetic spectrum, and the Na(2) molecule has an electron-hole excitation in the same energy range. Due to the dynamical interaction of the two subsystems the plasmon and the molecular excitations couple, creating a hybridized molecular-plasmon excited state. This state has unique properties that yield e.g. enhanced photoabsorption compared to the freestanding Na(2) molecule. The computational approach used enables decoupling of the mutual plasmon-molecule interaction, and our analysis verifies that it is not legitimate to neglect the back coupling effect when describing the dynamical interaction between plasmonic material and nearby molecules. Time-resolved analysis shows nearly instantaneous formation of the coupled state, and provides an intuitive picture of the underlying physics.

Recent experimental and theoretical works have demonstrated that quantum mechanical effects play an important role in materials design of some novel nano-plasmonic materials. In this work, electronic structure calculations are used to study these effects for the optical properties of metal nanostructures and small flakes of graphene. Their optical response is shown to depend on their exact atomic composition, and their similarities (size-dependent resonance frequency) and differences (metallic vs. semiconducting material) are discussed. The open-source computer code GPAW is used for the simulations, which can be done for systems of thousands of valence electrons. The calculations automatically include quantum effects such as tunneling, nonlocal response, and molecular orbital hybridization.

We report a study on the temperature dependence of the core-electron excitation spectra of CO2 and N2, performed using non-resonant inelastic X-ray scattering spectroscopy. The spectra were measured at two temperatures (300 K and 850 K) and at high pressure (40 bar). For CO2 a clear temperature dependence was observed at the C and O near-edge regions. The spectra of CO2 were simulated by density functional theory calculations, and the temperature was accounted for by sampling the initial state molecular geometries using the Metropolis algorithm. This model is able to account for the experimentally observed temperature dependence of the spectrum. The experiment fortifies the status of the non-resonant inelastic X-ray scattering spectroscopy as a valuable technique for physics and chemistry for in situ studies under extreme sample conditions. Especially in the case of gas phase the sample conditions of considerably elevated temperature and pressure are unfeasible for many other spectroscopic techniques.

The symmetry and energy distribution of unoccupied molecular orbitals is addressed in this work by means of NEXAFS and density functional theory calculations for planar, fluorinated organic semiconductors (perfluorinated copper phthalocyanines and perfluoropentacene). We demonstrate how molecular orbitals with significant density of states on the fluorine atoms show different symmetry from those mainly located on C and N atoms. As a result, the angle-dependent linear dichroism in NEXAFS F K-edge spectra is inversed with respect to that in the C and N K-edges. In addition, the significant overlap in energy of pi* and sigma* orbitals throughout the F K-edge spectrum hampers its use for analysis of molecular orientations from angle-dependent NEXAFS measurements.

ERKALE is a novel software program for computing X‐ray properties, such as ground‐state electron momentum densities, Compton profiles, and core and valence electron excitation spectra of atoms and molecules. The program operates at Hartree–Fock or density‐functional level of theory and supports Gaussian basis sets of arbitrary angular momentum and a wide variety of exchange‐correlation functionals. ERKALE includes modern convergence accelerators such as Broyden and ADIIS and it is suitable for general use, as calculations with thousands of basis functions can routinely be performed on desktop computers. Furthermore, ERKALE is written in an object oriented manner, making the code easy to understand and to extend to new properties while being ideal also for teaching purposes. © 2012 Wiley Periodicals, Inc.

Motivated by fundamental molecular physics and by atmospheric and planetary sciences, the valence excitations of N-2 gas have seen several decades of intensive study, especially by electron-energy-loss spectroscopy (EELS). It was consequently surprising when a comparison of nonresonant inelastic x-ray scattering (NIXS) and nonresonant EELS found strong evidence for violations of the first Born approximation for EELS when leaving the dipole scattering limit. Here we reassess the relative strengths of the constituent resonances of the lowest-energy excitations of N-2, encompassed by the so-called Lyman-Birge-Hopfield (LBH) band, expanding on the prior, qualitative interpretation of the NIXS results for N-2 by both quantifying the generalized oscillator strength of the lowest-energy excitations and also presenting a time-dependent density functional theory (TDDFT) calculation of the q dependence of the entire low-energy electronic excitation spectrum. At high q, we find that the LBH band has an unexpectedly large contribution from the octupolar w(1)Delta(u) resonance exactly in the regime where theory and EELS experiment for the presumed-dominant a(1)Pi(g) resonance have previously had substantial disagreement, and also where the EELS results must now be expected to show violations of the Born approximation. After correcting for this contamination, the a(1)Pi(g) generalized oscillator strength from the NIXS results is in good agreement with prior theory. The NIXS spectra, over their entire q range, also find satisfactory agreement with the TDDFT calculations for both bound and continuum excitations.

The interpretation of the oxygen near-edge spectrum of water has been debated intensively. We present new measurements of the temperature dependence of the spectrum and perform a van't Hoff analysis for the pre-edge intensity. Many microscopical and thermodynamic properties of liquid water have been described in the literature in terms of mixture models, which presume the existence of two distinct species with different local structures. Assuming such a two-component model here leads to a van't Hoff enthalpy change ΔH = 0.9 ± 0.2 kcal/mol for the conversion between the two presumed components contributing to the pre-edge intensity. The small value of ΔH compared to the average bond energy implies that the components are nearly equally bonded, suggesting that the pre-edge is sensitive to structural changes that leave hydrogen bonds intact. We further show that the pre-edge intensity in the vapor, liquid, and ice Ih spectra can be correlated with enthalpy changes. While the pre-edge intensity in water has often been interpreted to imply a large fraction of broken hydrogen bonds in the liquid, we propose that the current results indicate that those bonds would not be considered broken by energetical criteria.

We present two complementary inelastic X-ray scattering studies on the structure of tetrahydrofuran (THF) clathrate hydrate and the supercooled stoichiometric liquid mixture of water and THF. Compton scattering experiments of the liquid mixture show that formation of hydrate precursors is unlikely. By comparing experimental spectra of THF hydrate and water/THF mixtures at temperatures above 250 K with density functional theory calculations, structural changes that manifest in OH bond length changes are observed. X-ray Raman scattering measurements of the oxygen K-edge in the same temperature range corroborate these results. The experimental results of THF hydrate at temperatures between 20 and 244 K can be modeled best by assuming thermal expansion only. Therefore, dependency on the system's temperature different structural behavior of THF hydrate is reported.

We report non-resonant inelastic X-ray scattering experiments of several gaseous samples in the inner-shell excitation energy range. The experimental near-edge spectra from all the K-edges of N(2), N(2)O, and CO(2) including the momentum transfer dependence are presented. The results are analyzed using density functional theory calculations that accurately reproduce the experimental spectral features. We observe vibrational effects in the measured spectrum and in the calculations the atomic motion is modeled using the Franck-Condon approximation and the linear coupling model. Our findings show that vibrational effects cannot be neglected in the analysis of high resolution inelastic X-ray scattering spectroscopy. The results also support the validity of the transition potential approximation for calculating core excited state potential energy surfaces.

The microscopic structure of disproportionating amorphous silicon monoxide is studied by inelastic x-ray scattering at the silicon ${L}_{\text{II},\text{III}}$ edge. This material arranges into nanocrystalline regions of Si embedded in amorphous ${\text{SiO}}_{2}$ at proper annealing temperatures and in this work we demonstrate how the contribution of the suboxide interfaces between these regions can be extracted from the experimental data. The resulting near-edge spectra are analyzed in detail using a computational framework that combines molecular-dynamics simulations and density-functional theory calculations. The results indicate that the amount of silicon atoms with oxidation states between $+1$ and $+3$ is significant and depends strongly on the annealing temperature. Furthermore, the presented $s$, $p$, and $d$-type local densities of states $(\ensuremath{\ell}\text{DOS})$ demonstrate that the most significant differences are found in the $p$-type $\ensuremath{\ell}\text{DOS}$.

We present a study of the local electronic structure surrounding the OH group in a series of alcohols by X-ray Raman scattering at the oxygen K edge. The samples include the linear alcohols from methanol to butanol as well as the isomers isopropanol, isobutanol, and 2-butanol. For interpretation and computational benchmarks, we combine classical molecular dynamics (MD) simulations and density functional theory (DFT) spectrum calculations. The results indicate that intramolecular structure influences the spectra considerably. Nevertheless, hydrogen bonding produces a clear spectral signature that is nearly identical in all of the alcohols. The spectral calculations using MD structures closely reproduce the experimental results and support the picture provided by the MD simulations.

We report the oxygen K-edge spectra of ices Ih, VI, VII, and VIII measured with X-ray Raman scattering. The pre-edge and main-edge contributions increase strongly with density, even though the hydrogen bond arrangements are very similar in these phases. While the near-edge spectral features in water and ice have often been linked to hydrogen bonding, we show that the spectral changes in the phases studied here can be quantitatively related to structural changes in the second coordination shell. Density-functional theory calculations reproduce the experimental results and support the conclusion. Our results suggest that non-hydrogen-bonded neighbors can have a significant effect also in the liquid water spectrum. We discuss the implications of the results for the actively debated interpretation of the liquid water spectrum in terms of local structure.

We present an x-ray scattering study of the temperature-induced phase separation and Si nanocrystal formation in bulk amorphous SiOx with x≈1. X-ray Raman scattering at the Si LII,III-edge reveals a significant contribution of suboxides present in native amorphous SiO. The suboxide contribution decreases with increasing annealing temperature between 800–1200 °C pointing toward a phase separation of SiO into Si and SiO2 domains. In combination with x-ray diffraction and small angle x-ray scattering the SiO microstructure is found to be dominated by internal suboxide interfaces in the native state. For higher annealing temperatures above 900 °C growth of Si nanocrystals with rough surfaces embedded in a silicon oxide matrix can be observed.

Changes in the ground-state electron momentum density of tetrahydrofuran clathrate hydrate are studied in a temperature range between 93 and 275 K by means of X-ray Compton scattering. At temperatures above 253 K, large rather unexpected differences from the Compton profiles of ice are observed. Configurational enthalpies are extracted and exhibit a rapid rise above 253 K, whereas a constant configurational heat capacity of 0.23 ± 0.07 J g−1 K−1 is found below 253 K. Density functional theory calculations suggest that this anomalous behavior originates from the structural change of the hydrate, however, no indication was found for the formation of hydrogen bonds between tetrahydrofuran and water molecules.

We apply time-dependent density functional theory to study the valence electron excitations of molecules and generalize the typically used time-propagation scheme and Casida's method to calculate the full wavevector dependent response function. This allows the computational study of dipole-forbidden valence electron transitions and the dispersion of spectral weight as a function of the wavevector. The method provides a novel analysis tool for spectroscopic methods such as inelastic x-ray scattering and electron energy loss spectroscopy. We present results for benzene and CF(3)Cl and make a comparison with experimental results.