Experimental Photoelectron Spectra Research Articles

Highly-excited state properties of cumulenone chlorides in the vacuum-ultraviolet.

Recent observations of chloromethane in interstellar environments suggest that other organohalogens, which are known to be critically important in Earth's atmosphere, may also be of significance beyond our own terrestrial veil. This raises the question of how such molecules behave under extreme conditions such as when exposed to vacuum ultraviolet (VUV) radiation. VUV photons promote molecules to highly excited states that fragment in non-statistical patterns controlled by the initial femtosecond dynamics. A detailed understanding of VUV-driven photochemistry in complex organic molecules that consist of more than one functional group is a particularly challenging task. This quantum chemical analysis reports the electronic states and ionization potentials up to the VUV range (6-11 eV) of the chlorine-substituted cumulenone series molecules. The valence and Rydberg properties of lone-pair terminated, π-conjugated systems are explored for their potential resonance with lone pairs from elsewhere in the system. The carbon chain elongation within the family ClHCnO, where n = 1-4, influences the electronic excitations, associated wavefunctions, and ionization potentials of the molecules. The predicted geometries and ionization potentials are in good agreement with the available experimental photoelectron spectra for formyl chloride and chloroketene, n = 1-2. Furthermore, comparison between the regular cumulenone species and the corresponding chlorinated derivatives exhibit similar behaviors especially for n = 3, where the allene backbone in propadienone chloride is severely bent. Most notably for the excited states is that the Rydberg character becomes more dominant as the energy increases, with some retaining valence characters.

The ground-state structure, optical-absorption and photoelectron spectrum of silver clusters

Abstract The most stable structure, optical-absorption and photoelectron spectrum of neutral and anionic silver clusters up to 16 atoms have been studied by using particle-swarm optimization technique and density functional theory (DFT). A comprehensive structural search was carried out and many isomers were obtained. The ground state structures of neutral silver clusters have been determined by optical absorption spectrum and vertical ionization potential (VIP). Starting from n = 11, the growth of Ag n clusters follows a regular pattern. The infrared and Raman spectra of neutral silver clusters are predicted and can be used to identify these structures from experiments. The experimental photoelectron spectra (PES) of silver cluster anion are interpreted reasonably for the first time. Meantime, we find that the peak height of the measured PES in the range of binding energy close to the photon energy decreases significantly relative to the theoretical height. The reduction may be due to the small kinetic energy of photoelectron.

The ionic states of cyclooctatetraene: Analysis of a new experimental photoelectron spectrum by ab initio and density functional methods.

A synchrotron-based study of the photoelectron spectrum (PES) for cyclooctatetraene (COT) is reported, and this has been subjected to theoretical analysis in unprecedented detail. Weak vibrational structure was observed on the lowest ionization energy (IE1), but the peaks generally show very broad features. Multiconfiguration self-consistent field study confirms that the adiabatic IE (AIE) sequence is 12A1 < 12B1 < 22A1 < 12A2 < 22B1 < 32A1. The Tamm-Dancoff approximation gives an acceptable interpretation of the PES below 20 eV. Vibrational analysis of the PES bands by Franck-Condon methods predicts well-defined vibrational structure for these ionic states. The principal contributors to the PES envelopes are the a1 modes, and only a few are responsible for the overall shape of most bands. The high density of vibrational states, together with the known D2d ⇆ D2d interconversion process, where the C=C and C-C bonds interchange, is attributed to the lack of the observed structure. The transition state (TS) structures for the interconversion above of several ionic states of COT have been elucidated. The intrinsic reaction coordinate procedure gave a planar TS for the X1A1 (D2d) ⇆ 1A1g (D4h) ⇆ X1A1 (D2d) process; this shows alternating C-C (1.4791 Å), C=C (1.3261 Å), and C-H (1.0780 Å) bond lengths. The planar TS is a very shallow maximum, with energy varying with the method used, Hartree-Fock 0.575 eV and second order Møller-Plesset 0.653 eV. A polynomial containing quadratic, quartic, and sextic terms gives an effectively exact fit to the surface. The lowest ionic state of COT (X2A1) shows a similar TS for the process X2A1 (D2d) ⇆ 2B2u (D4h). This structure has alternating C-C (1.4366 Å), C=C (1.3572 Å), and C-H (1.0756 Å) bonds.

Local structures of nitrogen-doped graphdiynes determined by computational X-ray spectroscopy

Nitrogen doping is an important method to modulate electronic structure of two-dimensional carbon materials. The properties of the doped systems are heavily dependent on the local structure of nitrogen dopants involved, which are often determined by experimental X-ray photoelectron spectra (XPS) and near-edge X-ray absorption fine-structure (NEXAFS) at the nitrogen K-edge. In the present work, the N1s XPS and NEXAFS spectra of nitrogen-doped graphdiynes have been accurately calculated at the density functional theory level. Five representative nitrogen-dopants in graphdiynes, namely [pyridinic, amino, graphitic, and two sp-hybridized N (sp-N-1 and sp-N-2) local structures], are fully examined, from which all experimental features could be correctly assigned. The calculated results can be used to determine the ratio of different nitrogen dopants in graphdiyne at different elevated temperatures reported in previous experiments. Our findings provide the basic references for structure determination of nitrogen doped graphdiyne and new understanding of the underlying structure-property relationships.

B3Si12: A hexagonal prism with three short B[sbnd]B single bonds at the center coordinated by a Si12 cage and reinforced by aromaticity

The structures and properties of B3Si12ˉ/0/+ clusters were investigated by density functional theory calculations combined with mass-selected anion photoelectron spectroscopy. The experimental photoelectron spectrum of cluster anion is reasonably reproduced by theoretical simulations. The global minimum of B3Si12ˉ anion has a B3Si2 trigonal bipyramidal structure with ten capping Si atoms, whereas those of B3Si12 neutral and B3Si12+ cation adopt hexagonal prismatic structures with the B3 unit at the center. In particular, B3Si12ˉ anion is an outstanding closed-shell superatom cluster according to the jellium model (1S21P61D102S21F142P61G18). Furthermore, the NICS values of B3Si12ˉ, B3Si12, and B3Si12+ calculated at points 1 Å above the cage centers are to be −16.8, −56.8, and −51.1 ppm, respectively, suggesting that they are all aromatic.

Dynamical fluxionality, multiplicity of structural forms, and electronic properties of the B3Si11 cluster: anion photoelectron spectroscopy and theoretical calculations.

The geometrical structures and electronic properties of anionic, neutral, and cationic B3Si11 clusters were investigated by performing ab initio calculations combined with size-selected anion photoelectron spectroscopy. The experimental photoelectron spectrum of the B3Si11- anion is reasonably reproduced by theoretical simulations of two competing isomers. The global minimum of the B3Si11- anion is formed by the fusion of a B3Si7 bicapped tetragonal antiprism to a B3Si4 pentagonal bipyramid by sharing a B3 triangle, while that of neutral B3Si11 has a B3-endohedral sandwich structure composed of a Si5 five-membered ring and a Si6 six-membered ring, and that of the B3Si11+ cation adopts a Si11 tricapped tetragonal antiprism with three face-capping B atoms. It is interesting that a Si5 five-membered ring and a Si6 six-membered ring are stabilized by three B atoms in B3Si11. The three B atoms tend to bond with each other to form a B3 triangle with stronger B-B bonds than B-Si bonds. Moreover, neutral B3Si11 exhibits σ + π double delocalized bonding patterns. Anionic, neutral, and cationic B3Si11 clusters have multiplicity of structural forms and their low-lying isomers show dynamical fluxionality. The bond lengths, bond orders, MO, constant electronic charge density surfaces, and PDOS analyses showed that the three B atoms in B3Si11 have strong bonding interactions.

Experimental and theoretical threshold photoelectron spectra of methylene.

The threshold photoelectron spectrum of methylene (CH2), produced by consecutive H atom abstractions on methane, has been recorded using synchrotron radiation. The experimental spectrum spans the region of the X + 2Π u ← X 3 B 1 ionizing transition. It is modeled starting from ab initio bending potentials and using the bending approach introduced by Coudert et al. [J. Chem. Phys. 148, 054302 (2018)] accounting for the quasilinearity of CH2 and the strong Renner-Teller interaction in . This first calculation yields a theoretical threshold photoelectron spectrum which is in moderate agreement with the experimental one. A more accurate approach treating the three vibrational modes is developed for computing the threshold photoelectron spectrum of triatomic molecules. This new treatment is tested modeling the already measured threshold photoelectron spectrum of the X + 2Π u ← X 1 A 1 ionizing transition of the water molecule. The threshold photoelectron spectrum of CH2 computed with the new approach compares more favorably with the experimental spectrum and yields an adiabatic ionization potential of 10.386(6) eV.

Vibronic structure and photoelectron angular distribution in the photoelectron spectrum of ICN

The valence shell photoelectron spectrum of ICN has been simulated using the equation-of-motion coupled-cluster method to calculate the ionization energies and the norms of the Dyson orbitals to describe the intensity of the photoelectron bands. The simulated spectrum not only reproduces the position and intensity of the four main bands observed in the experimental photoelectron spectrum, but the vibronic structure present in the individual photoelectron bands is also reproduced to a reasonable extent. The agreement between the experimental and simulated vibronic structures at higher energies corresponding to the and states is particularly noteworthy. Additionally, the photoionization cross section and asymmetry parameter have been calculated for the ionization of four outermost valence molecular orbitals in the photon energy range of 0-50 eV. The computed asymmetry parameters are found to provide a qualitative description of the corresponding experimental measurements. The shape-resonance seen in the experimental asymmetry parameters and the trends seen in the calculated cross section for the four ionization channels have been explained by the partial wave analysis of the contribution of the individual angular momentum channels to the photoelectron.

Photoelectron spectrum of NO2 - : SAC-CI gradient study of vibrational-rotational structures.

Three-dimensional accurate potential energy surfaces around the local minima of NO2 - and NO2 were calculated with the SAC/SAC-CI analytical energy gradient method. Therefrom, the ionization photoelectron spectra of NO2 - , the equilibrium geometries and adiabatic electron affinity of NO2 , and the vibrational frequencies including harmonicity and anharmonicity of NO2 - and NO2 were obtained. The calculated electron affinity was in reasonable agreement with the experimental value. The SAC-CI photoelectron spectra of NO2 - at 350 K and 700 K including the rotational effects were calculated using the Franck-Condon approximation. The theoretical spectra reproduced well the fine experimental photoelectron spectra observed by Ervin et al. (J. Phys. Chem. 1988, 92, 5405). The results showed that the ionizations from many vibrational excited states as well as the vibrational ground state are included in the experimental photoelectron spectra especially at 700 K and that the rotational effects are important to reproduce the experimental photoelectron spectra of both temperatures. The SAC/SAC-CI theoretical results supported the analyses of the spectra by Ervin et al., except that we could show some small contributions from the asymmetric-stretching mode of NO2 - . © 2018 Wiley Periodicals, Inc.

Structural Stability and Evolution of Scandium-Doped Silicon Clusters: Evolution of Linked to Encapsulated Structures and Its Influence on the Prediction of Electron Affinities for ScSi n ( n = 4-16) Clusters.

Sc-doped semiconductor clusters are the simplest transition metal- and rare-earth metal-doped semiconductor clusters. In this work, the structural evolution behavior and electronic properties of Sc-doped neutral and anionic Si n ( n = 4-16) clusters were studied using the ABCluster global search technique coupled with a hybrid density functional method. The results revealed that although neutral and anionic configurations are different for ScSi n ( n = 6-14) clusters, the evolution pattern of the ground-state structures is consistent (evolution of linked to encapsulated structures starting from n = 14). The good agreement between the theoretical and experimental photoelectron spectra demonstrated that the obtained anionic global minimum structures are reasonable. The excellent agreement between the adiabatic electron affinities corrected by considering the structural correction factor and the experimental data indicated that the structural correction factor is important for reproducing the experimental data and that the obtained ground-state structures for the neutral ScSi n clusters reported herein are reliable. The relative stability and chemical bonding analysis showed that the fully encapsulated ScSi16- cluster is a magic cluster with good thermodynamic and chemical stability.

Electronic structure and photoelectron spectroscopy of manganese dihalides from quantum chemical methods and Dyson orbitals

Accurate treatment of electron correlation and spin-orbit coupling is essential for understanding the electronic structure and spectroscopy of high-spin, open-shell transition metal dihalides. The experimental photoelectron spectra of MnX2, X = Cl, Br, I, have been interpreted on the basis of multi-configurational ab initio electronic structure calculations. The intensity of the photoelectron bands have been calculated from the norms of the Dyson orbitals, which represent the transition probability to a particular ionic state from a given starting state. The present study provides a detailed account of the contribution of the one- and two-electron transitions in the experimental photoelectron spectra of the title molecules and outlines the nature of the photoelectron bands in terms of the closely spaced quintet and septet states of the cationic species.

Green's Function Coupled-Cluster Approach: Simulating Photoelectron Spectra for Realistic Molecular Systems.

In this paper, we present an efficient implementation for the analytical energy-dependent Green's function coupled-cluster with singles and doubles (GFCCSD) approach with our first practice being computing spectral functions of realistic molecular systems. Because of its algebraic structure, the presented method is highly scalable and is capable of computing spectral function for a given molecular system in any energy region. Several typical examples have been given to demonstrate its capability of computing spectral functions not only in the valence band but also in the core-level energy region. Satellite peaks have been observed in the inner valence band and core-level energy region where a many-body effect becomes significant and the single particle picture of ionization often breaks down. The accuracy test has been carried out by extensively comparing the computed spectral functions by our GFCCSD method with experimental photoelectron spectra as well as the theoretical ionization potentials obtained from other methods. It turns out the GFCCSD method is able to provide a qualitative or semiquantitative level of description of ionization processes in both the core and valence regimes. To significantly improve the GFCCSD results for the main ionic states, a larger basis set can usually be employed, whereas the improvement of the GFCCSD results for the satellite states needs higher-order many-body terms to be included in the GFCC implementation.

High-order above-threshold photoemission from nanotips controlled with two-color laser fields

We investigate the process of phase-controlled high-order above-threshold photoemission from metallic nanotips under bichromatic laser fields. Experimental photoelectron spectra resulting from two-color excitation with a moderately intense near-infrared fundamental field (1560 nm) and its weak second harmonic show a strong sensitivity on the relative phase and clear indications for a plateau-like structure that is attributed to elastic backscattering. To explore the relevant control mechanisms, characteristic features, and particular signatures from the near-field inhomogeneity, we performed systematic quantum simulations employing a one-dimensional nanotip model. Besides rich phase-dependent structures in the simulated above-threshold ionization photoelectron spectra we find ponderomotive shifts as well as substantial modifications of the rescattering cutoff as function of the decay length of the near-field. To explore the quantum or classical nature of the observed features and to discriminate the two-color effects stemming from electron propagation and from the ionization rate we compare the quantum results to classical trajectory simulations. We show that signatures from direct electrons as well as the modulations in the plateau region mainly stem from control of the ionization probability, while the modulation in the cutoff region can only be explained by the impact of the two-color field on the electron trajectory. Despite the complexity of the phase-dependent features that render two-color strong-field photoemission from nanotips intriguing for sub-cycle strong-field control, our findings support that the recollision features in the cutoff region provide a robust and reliable method to calibrate the relative two-color phase.

Probing the Structural and Electronic Properties of Dirhenium Halide Clusters: A Density Functional Theory Study

Dirhenium halide dianions received considerable attention in past decades due to the unusual metal–metal quadruple bond. The systematic structural evolution of dirhenium halide clusters has not been sufficiently studied and hence is not well-understood. In this work, we report an in-depth investigation on the structures and electronic properties of doubly charged dirhenium halide clusters Re2X82− (X = F, Cl, Br, I). Our computational efforts rely on the well-tested unbiased CALYPSO (Crystal structure AnaLYsis by Particle Swarm Optimization) method combined with density functional theory calculations. We find that all ground-state Re2X82− clusters have cube-like structures of D4h symmetry with two Re atoms encapsulated in halogen framework. The reasonable agreement between the simulated and experimental photoelectron spectrum of the Re2Cl82− cluster supports strongly the reliability of our computational strategy. The chemical bonding analysis reveals that the δ bond is the pivotal factor for the ground-state Re2X82− (X = F, Cl, Br, I) clusters to maintain D4h symmetric cube-like structures, and the enhanced stability of Re2Cl82− is mainly attributed to the chemical bonding of 5d orbital of Re atoms and 3p orbital of Cl atoms.

Photoelectron Energy Loss in Al(002) Revisited: Retrieval of the Single Plasmon Loss Energy Distribution by a Fourier Transform Method

A Fourier transform (FT) algorithm is proposed to retrieve the energy loss function (ELF) of solid surfaces from experimental X-ray photoelectron spectra. The intensity measured over a broad energy range towards lower kinetic energies results from convolution of four spectral distributions: photoemission line shape, multiple plasmon loss probability, X-ray source line structure and Gaussian broadening of the photoelectron analyzer. The FT of the measured XPS spectrum, including the zero-loss peak and all inelastic scattering mechanisms, being a mathematical function of the respective FT of X-ray source, photoemission line shape, multiple plasmon loss function, and Gaussian broadening of the photoelectron analyzer, the proposed algorithm gives straightforward access to the bulk ELF and effective dielectric function of the solid, assuming identical ELF for intrinsic and extrinsic plasmon excitations. This method is applied to aluminum single crystal Al(002) where the photoemission line shape has been computed accurately beyond the Doniach–Sunjic approximation using the Mahan–Wertheim–Citrin approach which takes into account the density of states near the Fermi level; the only adjustable parameters are the singularity index and the broadening energy Г (inverse hole lifetime). After correction for surface plasmon excitations, the q-averaged bulk loss function, q , of Al(002) differs from the optical value Im[− 1 / e(E, q = 0)] and is well described by the Lindhard–Mermin dispersion relation. A quality criterion of the inversion algorithm is given by the capability of observing weak interband transitions close to the zero-loss peak, namely at 0.65 and 1.65 eV in e(E, q) as found in optical spectra and ab initio calculations of aluminum.

КРЕМНИЙ-НИОБИЕВЫЕ БЛОКИ ДЛЯ КОНСТРУИРОВАНИЯ НАНОСТРУКТУРИРОВАННЫХ МАТЕРИАЛОВ

Представлены результаты компьютерного моделирования пространственной структуры анионных кремний-ниобиевых кластеров из 10, 12, 14 и 16 атомов кремния. Для каждого кластера представлен ряд стабильных изомеров с качественно различной структурой, которые могут служить элементарными блоками для построения более крупных наноструктурированных объектов – нанопроволок, нанопроводов, нанокристаллов.&#x0D; Для представленных расчетов были использованы вычислительные ресурсы Суперкомпьютерного центра Воронежского государственного университета.

Electronic structure of stoichiometric and oxygen-deficient ferroelectric Hf0.5Zr0.5O2

The electronic structure of oxygen-deficient Hf0.5Zr0.5O2 in the non-centrosymmetric orthorhombic (ferroelectric) phase was investigated by means of x-ray photoelectron spectroscopy and first-principle density functional theory calculations. It was established that a peak in the photoelectron spectra observed at an energy above the valence band top of ferroelectric Hf0.5Zr0.5O2 in ion-etched samples was due to oxygen vacancies. A method for evaluating the oxygen vacancies concentration in the material from the comparison of experimental and theoretical photoelectron spectra of the valence band is proposed. It is found that oxygen polyvacancies are not formed in ferroelectric Hf0.5Zr0.5O2: an energy-favorable spatial arrangement of several oxygen vacancies in the crystal corresponds to the configuration formed by noninteracting vacancies distant from each other. The oxygen vacancies in five charged states were simulated. The electron levels in the bandgap caused by charged oxygen vacancies indicate that any type of oxygen vacancies in ferroelectric Hf0.5Zr0.5O2 can capture both electrons and holes, i.e. can act as an amphoteric localization center for charge carriers.

Calculation of density of states of transition metals: From bulk sample to nanocluster.

A technique is presented of restoring the electronic density of states of the valence band from data of X-ray photoelectron spectroscopy (XPS). The originality of the technique consists in using a stochastic procedure to solve an integral equation relating the density of states and the experimental X-ray photoelectron spectra via the broadening function. To obtain the broadening function, only the XPS spectra of the core levels are needed. The results are presented for bulk sample of gold and tungsten and nanoclusters of tantalum; the possibility of using the results to determine the density of states of low-dimensional structures, including ensembles of metal nanoclusters, is demonstrated.

Electronic structure of lanthanide scandates

X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy and density functional theory calculations were used to study the electronic structure of three lanthanide scandates: GdScO3, TbScO3, and DyScO3. X-ray photoelectron spectra simulated from first principles calculations using a combination of on-site hybrid and GGA+U methods were found to be in good agreement with experimental x-ray photoelectron spectra. The hybrid method was used to model the ground state electronic structure and the GGA+U method accounted for the shift of valence state energies due to photoelectron emission via a Slater-Janak transition state approach. From these results, the lanthanide scandate valence bands were determined to be composed of Ln4f, O2p, and Sc3d states, in agreement with previous work. However, contrary to previous work the minority Ln4f states were found to be located closer to, and in some cases at, the valence band maximum. This suggests that minority Ln4f electrons may play a larger role in lanthanide scandate properties than previously thought.

Refined analysis of the X̃ 2A2←X̃ 1A1 photoelectron spectrum of furan.

The X̃ 2A2←X̃ 1A1 photoelectron spectrum of furan has been studied by a time-independent eigenstate-free Raman wave function approach based on multi-dimensional potential energy surfaces obtained from explicitly correlated distinguishable clusters calculations. Individual vibronic transitions with the most significant Franck-Condon factors were determined by our recently developed residual-based algorithm for the calculation of eigenpairs in conjunction with the formalism of contracted invariant Krylov subspaces. The account of anharmonic and temperature effects allowed us to explain most bands in an experimental high-resolution zero kinetic energy photoelectron spectrum. This led to the reassignment of many spectral features, as well as a refined interpretation of the intensity mechanism for the corresponding transitions.