Core Ionization Thresholds Research Articles

Molecular inner-shell photoabsorption/photoionization cross sections at core-valence-separated coupled cluster level: Theory and examples.

Oxygen, nitrogen, and carbon K-shell photoabsorption and photoionization cross sections have been calculated within core-valence-separated coupled cluster (CC) linear response theory for a number of molecular systems, namely, water, ammonia, ethylene, carbon dioxide, acetaldehyde, furan, and pyrrole. The cross sections below and above the K-edge core ionization thresholds were obtained, on the same footing, from L2 basis set calculations of the discrete electronic pseudospectrum yielded by an asymmetric-Lanczos-based formulation of CC linear response theory at the CC singles and doubles (CCSD) and CC singles and approximate doubles (CC2) levels. An analytic continuation procedure for both discrete and continuum cross sections as well as a Stieltjes imaging procedure for the photoionization cross section were applied and the results critically compared.

Neon in ultrashort and intense x-rays from free electron lasers

We theoretically examine neon atoms in ultrashort and intense x-rays from free electron lasers and compare our results with data from experiments conducted at the Linac Coherent Light Source. For this purpose, we treat in detail the electronic structure in all possible nonrelativistic cationic configurations using a relativistic multiconfiguration approach. The interaction with the x-rays is described in rate-equation approximation. To understand the mechanisms of the interaction, a path analysis is devised which allows us to investigate what sequences of photoionization and decay processes lead to a specific configuration and with what probability. Thereby, we uncover a connection to the mathematics of graph theory and formal languages. In detail, we study the ion yields and find that plain rate equations do not provide a satisfactory description. We need to extend the rate equations for neon to incorporate double Auger decay of a K-shell vacancy and photoionization shake off for neutral neon. Shake off is included for valence and core ionization; the former has hitherto been overlooked but has important consequences for the ion yields from an x-ray energy below the core ionization threshold. Furthermore, we predict the photon yields from xuv and x-ray fluorescence; these allow one insights into the configurations populated by the interaction with the x-rays. Finally, we discover that inaccuracies in those Auger decay widths employed in previous studies have only a minor influence on ion and photon yields.

Photoionization and ionic dissociation of the C3 H3 NS molecule induced by soft X-ray near the C1s edge.

Time of flight mass spectrometry, electron-ion coincidence, and ion yield spectroscopy were employed to investigate for the first time the thiazole (C3 H3 NS) molecule in the gas phase excited by synchrotron radiation in the soft X-ray domain. Total ion yield (TIY) and photoelectron-photoion coincidence (PEPICO) spectra were recorded as a function of the photon energy in the vicinity of the carbon K edge (C1s). The C1s resonant transitions as well as the core ionization thresholds have been determined from the profile of TIY spectrum, and the features were discussed. The corresponding partial ion yields were determined from the PEPICO spectra for the cation species produced upon the molecular photodissociation. Additional ab initio calculations have also been performed from where relevant structural and electronic configuration parameters were obtained for this molecule.

Yields and Time-of-Flight Spectra of Neutral High-Rydberg Fragments at the K Edges of the CO2 Molecule.

We have studied the production of neutral fragments in high-Rydberg (HR) states at the C 1s and O 1s edges of the CO2 molecule by performing two kinds of experiments. First, the yields of neutral HR fragments were measured indirectly by ionizing such fragments in a static electric field and by collecting resulting singly charged positive ions as a function of the photon energy. Such measurements reveal not only excitations below the core ionization thresholds but also thresholds for single core-hole and shakeup photoionization. Second, we obtained the mass spectra of neutral HR fragments at selected photon energies by exploiting pulsed field ionization; they show atomic fragments C(HR) and O(HR). We discuss dissociation pathways leading to the production of neutral HR fragments in core excitation and ionization of CO2.

An experimental NEXAFS and computational TDDFT and ΔDFT study of the gas-phase core excitation spectra of nitroxide free radical TEMPO and its analogues

Core-hole spectroscopy adds to the fundamental understanding of the electronic structure of stable nitroxide free radicals thus paving way for a sensible design of new analogues with desired functionalities. We study the gas-phase C 1s, N 1s and O 1s excitation spectra of three nitroxide free radicals - TEMPO and two of its amide-substituted analogues - using the experimental NEXAFS technique and the theoretical TDDFT and ΔDFT methods in the unrestricted setting. The short-range corrected SRC1-BLYP and SRC2-BLYP exchange-correlation functionals are used with TDDFT, and the standard B3LYP functional with ΔDFT. The TDDFT-based detailed spectral assignment includes the valence, mixed valence-Rydberg and Rydberg portions of the spectra from the onset of absorptions to the vicinity of the core-ionization thresholds. The relative overlaps between the experimental and TDDFT-modelled spectra are reasonably good, in the range of 0.7-0.8, 0.6-0.8, and 0.7-0.8 for the C 1s, N 1s, and O 1s spectra, respectively. The extent of spin contamination within the unrestricted framework and its effect on the accuracy of the calculated excitation energies and dipole intensities are discussed in detail. It is concluded that, despite the sizeable spin contamination, the presently used methods are capable of predicting the core-excitation spectra of comparatively large free radical species fairly reliably over a wide spectral range.

Rydberg-resolved resonant inelastic soft x-ray scattering: dynamics at core ionization thresholds.

Resonant inelastic x-ray scattering spectra excited in the immediate vicinity of the core-level ionization thresholds of N2 have been recorded. Final states of well-resolved symmetry-selected Rydberg series converging to valence-level ionization thresholds with vibrational excitations are observed. The results are well described by a quasi-two-step model which assumes that the excited electron is unaffected by the radiative decay. This threshold dynamics simplifies the interpretation of resonant inelastic x-ray scattering spectra considerably and facilitates characterization of low-energy excited final states in molecular systems.

Spectral distributions of vibrational excitations accompanying 1s shell photoionization in small molecules

Distribution of vibrational excitations accompanying inner-shell ionization in small molecules is investigated. Within the vibrationally dependent-fixed-nuclei and the harmonic oscillator models the distribution is presented as a product of the Franck–Condon distribution and the interferential factor. It is shown that this factor varies strongly in the shape resonance vicinity and oscillates far above the core-ionization threshold. Analytical relationships between the spectral distributions and the potential functions of initial and final molecular states as well as intramolecular interference of scattered photoelectron waves are obtained and applied for the description of C 1s ionization in CO. The extended oscillations of the intensity ratio of vibrational components v′ = 0 and v′ = 1 of C 1s photoelectron spectrum of CO far above the threshold are discussed in more detail. The non-Franck–Condon, chemical and multi-electron effects on the distributions are also examined.

Multi-charged rare gas ions measured with a coincidence technique

Multi-charged ions of rare gas toms have been measured in coincidence with a photoelectron from a core orbital using a time-of-flight mass spectrometer combined with an electron energy analyzer of the spherical mirror type. Probability ratios obtained for double Auger processes of Ar 2p, Kr 3d and Xe 4d hole states have confirmed that those estimated from previously observed ion yield spectra around core-ionization thresholds are correct. These ratios for spin split states in Ar 2p and Kr 3d holes were found to be almost the same, although those in Xe 4d hole states are different.

Ionic fragmentation processes following silicon:2p core level photoexcitation and photoionization of 1,1,1-trimethyltrichlorodisilane

Ionic fragmentation process following Si:2p core-level photoexcitation and photoionization of 1,1,1-trimethyltrichlorodisilane (Me3SiSiC13) have been studied in the range 100-110 eV by means of photoelectron-photoion and photoion-photoion coincidence techniques. The total photoionization efficiency curve of Me 3 SiSiCl 3 has two peaks near the 2p core-ionization threshold of the silicon atom bonded to three methyl groups (Si[Me]). These two peaks are assigned to the excitation of the Si[Me]:2p electron into unoccupied molecular orbitals below the threshold

Dissociative double ionization following valence and Al : 2p core level photoexcitation of Al(CH3)3

Dissociation processes following double photoionization of Al(CH3)3 have been studied in the range of valence and Al : 2p core-level ionization by means of the photoelectron-photoion and photoion-photoion coincidence methods. The double-ionization threshold and the Al : 2p core-ionization threshold are estimated to be about 30 and 80eV, respectively. The variation of the fragmentation pattern with photon energy is discussed in conjunction with the relevant electronic states. The relative yields of the H+ -Al+ and H+-CHm+ (m' = 0-3) ion pairs are enhanced around the Al : 2p core-ionization threshold.

Strong 5p hole-5d electron interaction in the electron emission processes for Eu and Yb metals

Abstract We report the observation of intense constant kinetic energy emission structures, in the photoemission spectra of the divalent rare earth elements, Eu and Yb, in the solid state, when excited by photon energies larger than the 5p core ionization thresholds; and we propose a possible explanation of this phenomenon.

Strong 5p hole - 5d electron interaction in the electron emission processes for Eu and Yb metals

We report the observation of intense constant kinetic energy emission structures, in the photoemission spectra of the divalent rare earth elements, Eu and Yb, in the solid state, when excited by photon energies larger than the 5p core ionization thresholds; and we propose a possible explanation of this phenomenon.

Resonance-Enhanced Shakeup in Near-Threshold Core Excitation of CO andN2

Auger and x-ray photoelectron spectroscopies indicate that the probabilities for $1\ensuremath{\pi}\ensuremath{\rightarrow}2\ensuremath{\pi}$ shakeup in the C $1s$ ionization of CO and for the $1{\ensuremath{\pi}}_{\mathrm{u}}\ensuremath{\rightarrow}1{\ensuremath{\pi}}_{\mathrm{g}}$ shakeup in ${\mathrm{N}}_{2}$ are about an order of magnitude greater near the threshold than at energies well above it (the sudden limit). This enhancement is attributed to a conjugate process that is strengthened by the intense core-to-$\ensuremath{\pi}$ resonance just below the core-ionization threshold.

Resonant photoemission as a tool to studyfanddelectrons in actinide compounds

Using synchrotron radiation in the energy range $60\ensuremath{\le}h\ensuremath{\nu}\ensuremath{\le}130$ eV, we have measured $h\ensuremath{\nu}$-dependent photoemission energy distribution curves and constant-initial-state spectra from single crystals of ${\mathrm{U}}_{x}{\mathrm{Th}}_{1\ensuremath{-}x}\mathrm{Sb}$, UTe, U${\mathrm{O}}_{2}$, and U${\mathrm{Pd}}_{3}$. The U $5f$ and U and Th $6d$ conduction bands, as well as the Sb and Te $5p$ valence-band emission, all exhibit Fano-type resonance behavior at the $5d$ core ionization thresholds. The $5f$ emission shows the strongest intensity variation, which can be explained by the super-Coster-Kronig character of the decay of the autoionizing core excited state. Thus $f$ and $d$ emission features in the photoemission spectra are unambiguously identified by their different resonance behavior. It is concluded that the spectra are dominated by localized $5f$ states in all of these compounds.

Evidence for multi-electron excitations in photon stimulated ion desorption: CO and NO on Ni(100)

The photon stimulated ion desorption (PSID) from saturation layers ( T ≲ 150 K) of CO and NO on Ni(100) has been monitored through the C(1s), N(1s), O(1s) and Ni(2p) core ionization thresholds. O + ions are found to be the major desorption product of both molecular adsorption systems. Whereas the O + yield from CO exhibits an absorption edge jump at the C(1s) threshold, no jump is found at the N(1s) threshold in the case of NO. Above the O(1s) threshold, the O + yield is found to deviate significantly from the O(KVV) Auger yield for both molecules. It exhibits an onset delayed by more than 20 eV and a broad maximum more than 100 eV above threshold. This indicates that, above the O(1s) threshold, the O + yield is dominated by desorption processes which are assisted by multi-electron excitations. Fine structure in the O + yield from both molecules around 50 eV above threshold is assigned to a 1 σ, 3 σ two-electron excitation where one electron is excited into the σ shape resonance of the molecules.

Electron stimulated desorption of O + ions: Energy dependence of the desorption efficiency from adsorbed oxygen on metal surfaces and its bearing on the mechanism

Measurements of the structure in the curve of desorption yield versus electron energy for ESD of O + from β 1-oxygen on polycrystalline W and W(100), and from oxidized V are reported. They show definite structure around the core ionization thresholds of the metal atoms. This may be taken as evidence for the existence of Auger mechanisms of ESD, e.g. that proposed recently by Knotek and Feibelman for oxidic oxygen.