Core-level Absorption Spectra Research Articles

Intruder Peak-Free Transient Inner-Shell Spectra Using Real-Time Simulations.

Real-time methods are convenient for simulating core-level absorption spectra but suffer from nonphysical intruder peaks when using atom-centered basis sets. In transient absorption spectra, these peaks exhibit highly nonphysical time-dependent modulations in their energies and oscillator strengths. In this paper, we address the origins of these intruder peaks and propose a straightforward and effective solution based on a filtered dipole operator. In combination with real-time time-dependent density functional theory (RT-TDDFT), we demonstrate how to compute intruder-free attosecond transient X-ray absorption spectra for the aminophenol (C6H7NO) oxygen and nitrogen K-edges and the α-quartz (SiO2) silicon L-edge. Without filtering, the computed spectra are qualitatively wrong. This procedure is suitable for both static and transient inner-shell spectroscopy studies and can easily be implemented in a range of real-time methodologies.

NaI revisited: Theoretical investigation of predissociation via ultrafast XUV transient absorption spectroscopy.

Avoided crossings can trigger abrupt changes of electronic character and redirect the outcomes of photochemical reactions. Here, we report a theoretical investigation into core-level spectroscopic probing of predissociation dynamics of sodium iodide (NaI), a prototype system for studies of avoided-crossing dynamics. The elegant femtochemistry work of Zewail and co-workers pioneered the real-time dynamics of NaI, detecting the Na atoms bursting forth from the avoided crossing and the residual NaI molecules oscillating inside the quasibound potential. The simulated results show that core-level spectroscopy not only observes these integrated outcomes but also provides a direct measure of the abrupt switching of electronic character at the avoided crossing. The valence and core-excited electronic structures of NaI are computed by spin-orbit general multiconfigurational quasidegenerate perturbation theory, from which core-level absorption spectra of the predissociation dynamics are constructed. The wave-packet motion on the covalent potential is continuously mapped as shifts in the absorption energies, and the switching between the covalent and ionic character at the avoided crossing is characterized as the sharp rise and fall of the Na+ signal. The Na+ signal is found to be insensitive to the wave-packet motion in the asymptotic part of the ionic potential, which, in turn, enables a direct measure of the nonadiabatic crossing probability excluding the effect of wave-packet broadening.

Conformational Adaptation and Electronic Structure of 2H-Tetraphenylporphyrin on Ag(111) during Fe Metalation

In-situ metalation of porphyrin molecules in ultrahigh vacuum (UHV) is of great interest for the characterization of pure species in a controlled environment. Here, we report the characterization of the electronic states and the moleculesʼ geometrical adaptation during the formation of pure 2H-5,10,15,20-tetraphenylporphyrin (2H-TPP) and Fe- tetraphenylporphyrin (Fe-TPP) layers on Ag(111) single crystal. Core level absorption spectra indicate the flat conformation of the monolayer suggesting an adatom hopping instead of a surface mediated dopant diffusion for the metalation process. Photoemission points out that the interaction between Fe dz-states and Ag bands increases the monolayer metallic character already induced by the charge transfer from the substrate.

Electronic properties of potassium-doped FePc

The evolution of electronic structure of the organic semiconductor iron-phthalocyanine with potassium doping has been studied by means of photoemission spectroscopy, near-edge X-ray absorption fine structure and density functional theory (DFT) calculations. The DFT study and detailed analysis of the core-level spectra permit us to suggest possible lattice sites for the potassium ions. The data disclosed filling of the lowest unoccupied molecular orbital upon doping and associated changes of the core level absorption spectra. None of the films prepared in our studies showed a finite electronic density of states at the Fermi level.

The unoccupied electronic structure of potassium doped copper phthalocyanine studied by near edge absorption fine structure

The unoccupied electronic structure of potassium doped copper-phthalocyanine thin films has been studied using x-ray absorption spectroscopy. The data reveal filling of the lowest unoccupied molecular orbital upon doping and related changes of the core level absorption spectra. The spectral changes can be rationalized taking into account the core level binding energies which also depend on doping.

Identification of polymorphs of sp 3 bonded carbon and boron nitride using core-level absorption spectroscopy

The 1s core-level absorption spectra of the sp 3 bonded carbon (cubic and hexagonal polymorphs) and boron nitrides (zincblende and wurtzite polymorphs) were calculated using an ab initio pseudopotential plane wave method including the core-hole effects. A comprehensive comparison with the available experimental data shows that there exist characteristic features that can be used to spectroscopically identify the different polymorphs. The anisotropic characters of K-edge absorption spectra in hexagonal diamond and wurtzite boron nitride can provide additional phase specific signatures. The application of the spectroscopic fingerprinting method in the experimental identification of these metastable sp 3 bonded phases is discussed.

MAGNETIC CIRCULAR DICHROISM IN THE SOFT X-RAY ABSORPTION SPECTRA OF Co-BASED HEUSLER ALLOYS

Magnetic circular dichroism (MCD) of core-level absorption spectra (XAS) in the soft X-ray region has been measured for the ferromagnetic Heusler alloys Co 2 TiSn , Co 2 ZrSn , and Co 2 NbSn . The Co 2p → 3d XAS–MCD spectra show an obvious contribution of the orbital angular momentum component to the magnetic moment in these alloys. The ratio between the orbital and the spin magnetic moments derived from the MCD spectra varies by more than a factor of 2 within these alloys. The mechanism of this variation is discussed in comparison with the band structure calculation.

Synchrotron x-ray photoemission and reflectance study of the dipole forbidden diamond core exciton.

The diamond 1s core level absorption spectra measured with x-ray reflectance and partial photoelectron yield are compared. A steep rise and a feature around 290 eV in the partial yield are absent in the reflectance. This anomaly in the partial yield is due to the predominance of the Auger yield over the core exciton decay. A binding energy of 1.25 eV for a dipole forbidden core exciton is determined from these spectra. The spectral assignments of absorption features are revised, and discrepancies in diamond core exciton models are resolved.

Si2p Core Level Absorption and Photoemission Spectra of Porous Si

ABSTRACTSi2p core level absorption and photoemission spectra are taken for different porous Si layers using synchrotron radiation, toknow the electronic structures of porous Si. The core level absorption spectra show the high energy shift of the conduction band which correlates with the photo-luminescence blue shift. The oxidation states of porous Si are clarified from the photoemission spectra.

The d and f resonances in the core level absorption spectra of TeF 6

The post-edge resonant features observed in the Te p and d core level absorption spectra of TeF 6 have been characterized with stabilized bound state and continuum multiple scattering Xα calculations. The nature of the resonances above the Te 4p edge is similar to that observed in SF 6 and can be attributed to excitations into quasibound channels with substantial kd character. The localization of the continuum channel is due to strong interactions between the Te d and F p and d orbitals. The origin of the resonances above the Te 4d edge is different. It is better described as a manifestation of the atomic-like d→ kf delayed onset as a consequence of the Te centrifugal potential.

Electronic and atomic structure of amorphous carbon.

The electronic structure of amorphous carbon and hydrogenated amorphous carbon (a-C:H) has been investigated through calculations on a number of model structures containing different configurations of ${\mathrm{sp}}^{2}$ and ${\mathrm{sp}}^{3}$ sites. We find that the most stable arrangement of ${\mathrm{sp}}^{2}$ sites is in compact clusters of fused sixfold rings, i.e., graphitic layers. The width of the optical gap is found to vary inversely with the ${\mathrm{sp}}^{2}$ cluster size, and the \ensuremath{\sim}0.5-eV optical gap of evaporated amorphous carbon is found to be consistent with a model of disordered graphitic layers of about 15 A\r{} in diameter, bounded by ${\mathrm{sp}}^{3}$ sites. It is argued that a-C forms such finite clusters in order to relieve strain. It is then shown that the 1.5--2.5-eV optical gap of a-C:H is unusually small and requires that both its valence and conduction band consist of \ensuremath{\pi} states on ${\mathrm{sp}}^{2}$ sites and that these sites must also be significantly clustered, such as in graphitic clusters containing four or more rings. In other words, the optical gap of both a-C and a-C:H depends on their degree of medium-range order, rather than just on their short-range order as is the case in most amorphous semiconductors. We have also studied the nature of states away from the gap in order to interpret the photoemission data and the carbon 1s core-level absorption spectra. The nature of defects and midgap states is discussed, and it is predicted that the defect density decreases with increasing band gap. Finally it is argued that the doping of a-C:H by group-III and -V elements proceeds via a substitution mechanism, as in a-Si:H, in spite of the coordination disorder present in a-C:H. Doping is also expected to be accompanied by an increase in gap states, as in a-Si:H.

Comments on the energy-band structures of the solid-rare-gas mixtures

In this paper, we further discuss the optical properties of solid-rare-gas mixtures in the soft-x-ray region. We consider the systems Ne-Ar, Ar-Kr, and Ne-Kr. We consider only the lowest conduction band whose state density is computed in a virtual-crystal approximation including off-diagonal randomness and the lowest $n=1$ exciton position. Selected compositions are, also as a check, computed using a self-consistent single-site model including off-diagonal randomness. From this we consider the core-level absorption spectra for these crystals. The paper differs from a paper by Kunz and Mickish in that we actually calculate the alloy state density rather than infer it from the pure materials, and in that we actually calculate the exact position for the core-state band edges (this is the first such calculation for a solid rare gas) and actually calculate the position of the $n=1$ exciton line.