Experimental Photoelectron Spectra Research Articles

Decoding the structural and electronic variations in M2Bn- (M = Sc, Y, La; n = 6–9) clusters: Insights for nanomaterial design

A series of low-lying energy isomers of M2Bn- (M = Sc, Y, La; n = 6–9) clusters were investigated using density functional theory. By conducting functional tests and comparing with reported experimental photoelectron spectra, we confirmed that when n = 6 − 8, their ground-state structures are nearly identical, differing only in the distance between the M atom and the boron skeleton. Moreover, for n = 7–8, all ground states exhibit consistent inverse-sandwich structures. However, for n = 9, three systems exhibit different types of ground-state structures: The Sc-doped system has a quasi-inverse-sandwich structure, the Y-doped system has two possible coexisting structures, and the La-doped system has an inverse-sandwich structure. Subsequent average binding energy analyses, chemical bonding analyses and stability analyses further elucidated the reasons. Our research provides theoretical insights for a deeper understanding of the various properties of clusters doped with different elements and for exploring potential building blocks for nanomaterials.

Photoionization of aziridine: Nonadiabatic dynamics of the first six low-lying electronic states of the aziridine radical cation.

In this article, the theoretical photoionization spectroscopy of the aziridine (C2H5N) molecule is investigated. To start with, we have optimized the geometry of this molecule at the neutral electronic ground state at the density functional theory/augmented correlation-consistent polarized valence triple zeta level of theory using the G09 program. The electronic structure calculations were restricted to the first six low-lying electronic states in order to account for the experimental photoelectron spectrum of the C2H5N molecule. The first six low-lying electronic states (X̃2A', Ã2A', B̃2A″, C̃2A″, D̃2A', and Ẽ2A') of the potential energy surfaces (PESs) are calculated by both equation of motion-ionization potential-coupled cluster singles and doubles and multi-configuration quasi-degenerate perturbation theory abinitio quantum chemistry methods along the dimensionless normal displacement coordinates in which multiple conical intersections were established among the considered electronic states. A (6 × 6) model vibronic Hamiltonian is constructed on a diabatic electronic basis, using the symmetry selection rules and Taylor series expansion. The Cs symmetry point group of the aziridine molecule leads to electronic states symmetry of either A' or A″, and these states are close in energy, due to which the same symmetry electronic states avoid each other. To get a smooth diabatic PES, a fourfold diabatization scheme is used, which is implemented in the General Atomic and Molecular Electronic Structure Systems suite of programs. All the parameters used in the diabatic vibronic coupling model Hamiltonian are calculated in terms of the normal modes of vibrational coordinates. Finally, the vibronic model Hamiltonian constructed for the coupled six electronic states is used to solve both time-independent and time-dependent Schrödinger equations using the multi-configuration time-dependent Hartree program module to obtain the dynamical observables. The theoretical vibronic band structure is found to be in good accord with the available experimental results.

PGA: A new particle swarm optimization algorithm based on genetic operators for the global optimization of clusters.

We have developed a global optimization program named PGA based on particle swarm optimization algorithm coupled with genetic operators for the structures of atomic clusters. The effectiveness and efficiency of the PGA program can be demonstrated by efficiently obtaining the tetrahedral Au20 and double-ring tubular B20, and identifying the ground state clusters through the comparison between the simulated and the experimental photoelectron spectra (PESs). Then, the PGA was applied to search for the global minimum structures of (n = 3-30) clusters, new structures have been found for sizes n = 6, 7, 12, 14, and medium-sized 21-30 were first determined. The high consistency between the simulated spectra and the experimental ones once again demonstrates the efficiency of the PGA program. Based on the ground-state structures of these (n = 3-30) clusters, their structural evolution and electronic properties were subsequently explored. The performance on Au20, B20, , and (n = 3-30) clusters indicates the promising potential of the PGA program for exploring the global minima of other clusters. The code is available for free upon request.

Non-negligible Outer-Shell Reorganization Energy for Charge Transfer in Nonpolar Systems.

Many charge-transporting molecular systems function as ordered or disordered arrays of solid state materials composed of nonpolar (or weakly polar) molecules. Due to low dielectric constants for nonpolar systems, it is common to ignore the effects of outer-shell reorganization energy (λout). However, ignoring λout has not been properly supported and it can severely impact predictions and insights derived. Here, we estimate λout by two means: from experimental ultraviolet photoelectron spectra, in which vibronic progression in these spectra can be fitted with the widths of peaks determining the low-frequency component in reorganization energy, regarded to be closely associated with λout, and from molecular dynamic (MD) simulation of nonpolar molecules, in which disorder or fluctuation statistics for energies of charged molecules are calculated. An upper bound for λout was obtained as 505 and 549 meV for crystalline anthracene (140 K) and pentacene (50 K), respectively, by fitting of experimental data, and 212 and 170 meV, respectively, from MD simulations. These values are comparable to the inner-sphere reorganization energy (λin) arising from intramolecular vibration. With corresponding spectral density functions calculated, we found that λout is influenced both by low- and high-frequency dynamics, in which the former arises from constrained translational and rotational motions of surrounding molecules. In an amorphous state, about half of the λout's obtained are from high-frequency components, which is quite different from the conventional polar solvation. Moreover, crystalline systems exhibit super-Ohmic spectral density, whereas amorphous systems are sub-Ohmic.

B-O-O-B Impurity Induces Ultralong Room-Temperature Phosphorescence of Boric Acid.

Organic materials that can emit ultralong room-temperature phosphorescence (RTP) have attracted a great deal of interest. Whether the pure boric acid (BA) solid can emit RTP and the origin of the RTP in BA caused a debate recently. Herein, our first-principles calculations and experimental measurements suggest that RTP of BA originates from the B-O-O-B group in a (H2BO3)2 species, which can be formed by polymerization of two dehydrogenated BA molecules under light irradiation. The calculated absorption, fluorescence, and phosphorescence spectra of B-O-O-B match well with the experiments. Experimental X-ray photoelectron and X-ray absorption spectra evidence the existence of B-O-O-B in BA. The O-O bond in B-O-O-B can break upon optical excitation, creating two B-O radicals. Radiative transition from localized dangling orbitals of the B-O radicals to the delocalized orbitals of the crystal bulk leads to the observed RTP. Our calculated phosphorescence lifetime is ∼1 s, which agrees well with the experiment.

The ground-state structures and spectra of neutral, anionic and cationic copper clusters

The structures and spectra of Cunb (n = 3–16; b = 0, ±1) clusters have been researched by means of density functional theory (DFT) coupled with CALYPSO structure prediction method. It is found that the growth of Cun (n = 9–16) clusters shows a clear regularity. Based on the global minimum structure of the Cun cluster, the UV spectra have been predicted and are in good agreement with the available experimental results. The most stable structures of anionic copper clusters have been identified by comparing experimental and theoretical photoelectron spectra (PES). The infrared (IR) and Raman spectra of cationic copper clusters have been simulated and the calculated IR spectra of Cun+ -Arm (n = 3–10) complexes are in accord with experimental spectra. The Ar atoms adsorbed to small Cun+ cluster have partly influence on the IR spectrum of the host cluster. The Cu6, Cu8 and Cu3+ clusters can be regard as three magic clusters.

Ab initio spectroscopy and thermochemistry of the platinum hydride ions, PtH+ and PtH.

Rovibrational levels of low-lying electronic states of the gas-phase, diatomic molecules, PtH+ and PtH-, are computed on potential-energy functions obtained by using a hybrid spin-orbit configuration-interaction procedure. PtH- has a well-separated Σ0++1 ground state, while the first two electronic states of PtH+ (Σ0++1 and 3Δ3) are nearly degenerate. Combining the experimental photoelectron (PE) spectra of PtH- with theoretical photodetachment spectroscopy leads to an improved value for the electron affinity of PtH, EA(PtH) = (1.617 ± 0.015) eV. When PtH- is a product of photodissociation of PtHCO2-, its PE spectrum is broad because of rotational excitation. Temperature-dependent thermodynamic functions and thermochemistry of dissociation are computed from the theoretical energy levels. Previously published energetic quantities for PtH+ and PtH- are revised. The ground 1Σ+ term of PtH+ is not well described using single-reference theory.

Determination of Ground State Structures of Snx - (x=21-35) Clusters.

In this work, an unbiased global search with a homemade genetic algorithm was performed to investigate the structural evolution and electronic properties of Snx - (x=21-35) clusters with density functional theory (DFT) calculations. All the ground-state structures for all these Snx - (x=21-35) clusters have been confirmed by the comparison of the experimental and simulated photoelectron spectra (PESs). It has been revealed that all Snx - (x=21-35) clusters are tricapped trigonal prism (TTP)-based structures consisting of two (for sizes x=21-28) or three (for x=29-35) TTP units, with the remaining atoms adsorbed on the surface or inserted between TTP units. The gradually decreasing HOMO-LUMO gaps indicate that these clusters are undergoing semiconductor-to-metal transformation. The average binding energies show that the structural stabilities of Snx - clusters are not as good as that of silicon and germanium clusters. It found that sizes x=23, 25, 29, 33 show high relative stability.

Resolving the Experimental Photoelectron Spectra of CAl3Si.

The experimental photoelectron spectra concerning the six electronic states of CAl3Si- are resolved through electronic structure calculations and quantum nuclear dynamics in this study. It incorporates a model diabatic Hamiltonian to evaluate the coupling parameters and fit the potential energy curves (PECs). The analysis of these PECs showed us that there are sufficient nonadiabatic effects in the photoelectron spectra through the presence of various conical intersections. Poisson intensity distributions (PIDs) and the wave packet density plots are utilized for assigning the fundamental and first overtone excitations. The nuclear dynamics study is accomplished by employing time-dependent (TD) and time-independent (TI) quantum chemistry methods. Ultimately, our theoretical results concurred well with the experimental findings exhibiting vibronic coupling amidst the nearly positioned electronic states.

Exploring the stability and aromaticity of rare earth doped tin cluster MSn16- (M = Sc, Y, La).

Rare earth elements have high chemical reactivity, and doping them into semiconductor clusters can induce novel physicochemical properties. The study of the physicochemical mechanisms of interactions between rare earth and tin atoms will enhance our understanding of rare earth functional materials from a microscopic perspective. Hence, the structure, electronic characteristics, stability, and aromaticity of endohedral cages MSn16- (M = Sc, Y, La) have been investigated using a combination of the hybrid PBE0 functional, stochastic kicking, and artificial bee colony global search technology. By comparing the simulated results with experimental photoelectron spectra, it is determined that the most stable structure of these clusters is the Frank-Kasper polyhedron. The doping of atoms has a minimal influence on density of states of the pure tin system, except for causing a widening of the energy gap. Various methods such as ab initio molecular dynamics simulations, the spherical jellium model, adaptive natural density partitioning, localized orbital locator, and electron density difference are employed to analyze the stability of these clusters. The aromaticity of the clusters is examined using iso-chemical shielding surfaces and the gauge-including magnetically induced currents. This study demonstrates that the stability and aromaticity of a tin cage can be systematically adjusted through doping.

The low-lying electronic states of ScO2 and ScO2 −: a computational study

The low-lying electronic states of ScO2 and ScO2 – were studied using the SAC-CI method. The lowest six doublet states (X2B2, 12A1/22A′, 22A1, 12A2, 12B1/12A″, and 22B2) of ScO2 and the valence-bound singlet/triplet states (X1A1, 21A1/13A1, 11A2/13A2, 11B1/13B1, 11B2/13B2, and 21B2/23B2) of ScO2 – were optimised, and the relative energies among these electronic states were obtained. The electron detachment energies from the X1A1 state of ScO2 – to the lowest six doublet states of ScO2 and from the 13B2 state of ScO2 – to the X2B2 state of ScO2 can be compared with the previous experimental photoelectron spectra of ScO2 –. The vertical excitation spectra of ScO2 and ScO2 – were computed. The singlet/triplet dipole-bound states (1B2(DB)/3B2(DB)) of ScO2 – were found.

Memory properties and short-range order in silicon oxynitride-based memristors

Silicon oxynitride films of various compositions were grown by the plasma-enhanced chemical deposition method using a setup with remote plasma and an inductive excitation from a gas mixture of 10% monosilane (diluted with argon) and nitrogen in the presence of residual oxygen in working gas mixtures. The high-frequency generator power (13.56 MHz) was varied in the range of 50–150 W. The samples composition was studied using x-ray photoelectron spectroscopy. A comparison of the experimental Si 2p photoelectron spectrum with the calculation showed that, at a low generator power (50 W), the short-range order in silicon oxynitride films is better described by the “random mixture” model. As the generator power is increased (100 W and higher), the excess silicon content in a silicon oxynitride film is decreased, and the short-range order is better described by the “random bonding” model. On metal–insulator–semiconductor-structures based on the silicon oxynitride films obtained, measurements of current–voltage characteristics were carried out, and the resistive switching of the obtained structures is studied in the present contribution. It is found that, in films in which the short-range order is described by the random bonding model, a stable switching in the bipolar regime is observed.

Electronic and optical properties of thiogermanate AgGaGeS4: theory and experiment.

The electronic and optical properties of an AgGaGeS4 crystal were studied by first-principles calculations, where the full-potential augmented plane-wave plus local orbital (APW+lo) method was used together with exchange-correlation pseudopotential described by PBE, PBE+U, and TB-mBJ+U approaches. To verify the correctness of the present theoretical calculations, we have measured for the AgGaGeS4 crystal the XPS valence-band spectrum and the X-ray emission bands representing the energy distribution of the electronic states with the biggest contributions in the valence-band region and compared them on a general energy scale with the theoretical results. Such a comparison indicates that, the calculations within the TB-mBJ+U approach reproduce the electron-band structure peculiarities (density of states - DOS) of the AgGaGeS4 crystal which are in fairly good agreement with the experimental data based on measurements of XPS and appropriate X-ray emission spectra. In particular, the DOS of the AgGaGeS4 crystal is characterized by the existence of well-separated peaks/features in the vicinity of -18.6 eV (Ga-d states) and around -12.5 eV and -7.5 eV, which are mainly composed by hybridized Ge(Ga)-s/p and S-p state. We gained good agreement between the experimental and theoretical data with respect to the main peculiarities of the energy distribution of the electronic S 3p, Ag 4d, Ga 4p and Ge 4p states, the main contributors to the valence band of AgGaGeS4. The bottom of the conduction band is mostly donated by unoccupied Ge-s states, with smaller contributions of unoccupied Ga-s, Ag-s and S-p states, too. The AgGaGeS4 crystal is almost transparent for visible light, but it strongly absorbs ultra-violet light where the significant polarization also occurs.

Structural Evolution and Electronic Properties of Selenium-Doped Boron Clusters SeBn0/- (n = 3-16).

A theoretical research of structural evolution, electronic properties, and photoelectron spectra of selenium-doped boron clusters SeBn0/- (n = 3-16) is performed using particle swarm optimization (CALYPSO) software in combination with density functional theory calculations. The lowest energy structures of SeBn0/- (n = 3-16) clusters tend to form quasi-planar or planar structures. Some selenium-doped boron clusters keep a skeleton of the corresponding pure boron clusters; however, the addition of a Se atom modified and improved some of the pure boron cluster structures. In particular, the Se atoms of SeB7-, SeB8-, SeB10-, and SeB12- are connected to the pure quasi-planar B7-, B8-, B10-, and B12- clusters, which leads to planar SeB7-, SeB8-, SeB10-, and SeB12-, respectively. Interestingly, the lowest energy structure of SeB9- is a three-dimensional mushroom-shaped structure, and the SeB9- cluster displays the largest HOMO-LUMO gap of 5.08 eV, which shows the superior chemical stability. Adaptive natural density partitioning (AdNDP) bonding analysis reveals that SeB8 is doubly aromatic, with 6 delocalized π electrons and 6 delocalized σ electrons, whereas SeB9- is doubly antiaromatic, with 4 delocalized π electrons and 12 delocalized σ electrons. Similarly, quasi-planar SeB12 is doubly aromatic, with 6 delocalized π electrons and 14 delocalized σ electrons. The electron localization function (ELF) analysis shows that SeBn0/- (n = 3-16) clusters have different local electron delocalization and whole electron delocalization effects. The simulated photoelectron spectra of SeBn- (n = 3-16) have different characteristic bands that can identify and confirm SeBn- (n = 3-16) combined with future experimental photoelectron spectra. Our research enriches the geometrical structures of small doped boron clusters and can offer insight for boron-based nanomaterials.

Theoretical and Experimental Study of the Spectroscopy and Thermochemistry of UC+/0/.

A combination of high-level ab initio calculations and anion photoelectron detachment (PD) measurements is reported for the UC, UC-, and UC+ molecules. To better compare the theoretical values with the experimental photoelectron spectrum (PES), a value of 1.493 eV for the adiabatic electron affinity (AEA) of UC was calculated at the Feller-Peterson-Dixon (FPD) level. The lowest vertical detachment energy (VDE) is predicted to be 1.500 eV compared to the experimental value of 1.487 ± 0.035 eV. A shoulder to lower energy in the experimental PD spectrum with the 355 nm laser can be assigned to a combination of low-lying excited states of UC- and excited vibrational states. The VDEs calculated for the low-lying excited electronic states of UC at the SO-CASPT2 level are consistent with the observed additional electron binding energies at 1.990, 2.112, 2.316, and 3.760 eV. Potential energy curves for the Ω states and the associated spectroscopic properties are also reported. Compared to UN and UN+, the bond dissociation energy (BDE) of UC (411.3 kJ/mol) is predicted to be considerably lower. The natural bond orbitals (NBO) calculations show that the UC0/+/- molecules have a bond order of 2.5 with their ground-state configuration arising from changes in the oxidation state of the U atom in terms of the 7s orbital occupation: UC (5f27s1), UC- (5f27s2), and UC+ (5f27s0). The behavior of the UN and UC sequence of molecules and anions differs from the corresponding sequences for UO and UF.

The low-lying electronic states of VO4 and VO4 –: a computational study

Vanadium tetraoxide (VO4) has several isomers, and VO2(η2-O2) is the most stable one. The low-lying doublet states of VO2(η2-O2) and low-lying singlet states of VO2(η2-O2)− were studied using the SAC-CI theory. The electron detachment energies from the ground electronic state of VO2(η2-O2)− to the low-lying doublet states of VO2(η2-O2) were obtained. These electron detachment energies can be compared with the previous experimental photoelectron spectrum of VO4 −, and possible candidate states for the unassigned bands were found. The ground electronic states of VO2(η2-O2)/VO2(η2-O2)− and seven doublet excited states of VO2(η2-O2) were optimised. The vertical excitation spectra of VO2(η2-O2) and VO2(η2-O2)− were calculated. The lowest doublet/quartet states of VO4 and lowest singlet/triplet states of VO4 – were also computed at the BP86/aug-cc-pVTZ level. Highlights VO2(η2-O2) is the most stable isomer of VO4. The low-lying electronic states of VO2(η2-O2) and VO2(η2-O2)− were calculated using the SAC-CI theory. The unassigned bands in previous photoelectron spectrum of VO4 – were assigned, and the main structures of this spectrum can be described by the electron detachments from VO2(η2-O2)− to the low-lying doublet states of VO2(η2-O2). The computational results presented in this study were useful for the spectroscopic studies of VO4/VO4 –. Knowledge obtained in this study would be also helpful in understanding the electronic structures of MO4/MO4 – (M = Metal).

Structural evolution, electronic properties and spectra of titanium clusters

The ground-state structures, electronic properties and spectra of Ti n c ( n = 3–16 and c = 0, ±1) clusters have been investigated based on density functional theory (DFT) and CALYPSO structure prediction. Geometry optimizations show that the neutral, cationic and anionic titanium clusters have the same growth pattern. The pentagonal bipyramid plays an important role in the growth process. Analysis of electronic properties shows that the thermal stability of cationic and anionic clusters is greater than that of neutral cluster. The pentagonal bipyramid and icosahedron clusters have relatively high stability. Chemical activity of titanium clusters decreases with the cluster size. The preferred dissociation pathway of Ti n + clusters is the loss of a single Ti atom to form Ti n − 1 + . Optical absorption of Ti n clusters and infrared and Raman spectra of Ti n + clusters have been simulated and can be used for their structural identification. The ground states of Ti n − clusters have been determined by comparing experimental and theoretical photoelectron spectra. • The pentagonal bipyramid plays an important role in the growth process of titanium clusters • The titanium cluster with pentagonal bipyramid or icosahedron geometry is more table than other clusters. • The preferred dissociation pathway of Ti n + clusters is the loss of a single Ti atom to form Ti n − 1 + . • The ground state of anionic Ti n − clusters are identified by photoelectron spectra.s

Halides and the carbon-carbon double bond: Interactions of ethylene with bromide and iodide

A combined experimental and theoretical approach has been used to study the bromide-ethylene and iodide-ethylene gas-phase complexes. Experimental anion photoelectron spectra show the electron stabilisation of the complexes to be 0.19 eV and 0.14 eV associated with the bromide and iodide complexes respectively. High-level CCSD(T) calculations predict two minima with respect to each halide; the halide either appends linearly to a single hydrogen of the ethylene molecule or appends orthogonally to the carbon–carbon double bond. The two structural motifs are found to be close in energy, with the difference in dissociation energy associated with both halides determined to be approximately 1.3 kJ mol−1.

Electronic structure of the solvated benzene radical anion

The benzene radical anion is a molecular ion pertinent to several organic reactions, including the Birch reduction of benzene in liquid ammonia. The species exhibits a dynamic Jahn-Teller effect due to its open-shell nature and undergoes pseudorotation of its geometry. Here, we characterize the complex electronic structure of this condensed-phase system based on ab initio molecular dynamics simulations and GW calculations of the benzene radical anion solvated in liquid ammonia. Using detailed analysis of the molecular and electronic structure, we find that the spatial character of the excess electron of the solvated radical anion follows the underlying Jahn-Teller distortions of the molecular geometry. We decompose the electronic density of states to isolate the contribution of the solute and to examine the response of the solvent to its presence. Our findings show the correspondence between instantaneous molecular structure and spin density; provide important insights into the electronic stability of the species, revealing that it is, indeed, a bound state in the condensed phase; and offer electronic densities of states that aid in the interpretation of experimental photoelectron spectra.

The atomic and electronic structure of Hf0.5Zr0.5O2 and Hf0.5Zr0.5O2:La films

HfxZr1-xO2 and lanthanum-doped HfxZr1-xO2:La thin films are candidates for applications in ferroelectric random-access memory. Here, we explore the atomic and electronic structure of Hf0.5Zr0.5O2 and Hf0.5Zr0.5O2:La thin films grown by atomic layer deposition. Using X-ray photoelectron spectroscopy, it was found that the oxides under study have an almost identical electronic structure and a bandgap of about 5.4 eV. The Hf0.5Zr0.5O2:La film was shown to consist of the mixture of Hf0.5Zr0.5O2 and La2O3 phases. The bombardment with argon ions of the studied films leads to oxygen vacancy generation in the near-surface layer. The oxygen vacancy concentrations in the bombarded films were evaluated from the comparison of experimental valence band photoelectron spectra with the theoretical ones calculated using the density functional theory.