Simulated Photoelectron Spectra Research Articles

Evolution of the Structural and Electronic Properties of Medium-Sized Sodium Clusters: A Honeycomb-Like Na20 Cluster

Sodium is one of the best examples of a free-electron-like metal and of a certain technological interest. However, an unambiguous determination of the structural evolution of sodium clusters is challenging. Here, we performed an unbiased structure search among neutral and anionic sodium clusters in the medium size range of 10-25 atoms, using the Crystal structure AnaLYsis by Particle Swarm Optimization (CALYPSO) method. Geometries are determined by CALYPSO structure searches, followed by reoptimization of a large number of candidate structures. For most cluster sizes the simulated photoelectron spectra of the lowest-energy structures are in excellent agreement with the experimental data, indicating that the current ground-state structures are the true minima. The equilibrium geometries show that, for both neutral and anionic species, the structural evolution from bilayer structures to layered outsides with interior atoms occurs at n = 16. A novel unprecedented honeycomb-like structure of Na20 cluster with C3 symmetry is uncovered, which is more stable than the prior suggested structure based on pentagonal structural motifs.

Retrieving plasmonic near-field information: A quantum-mechanical model for streaking photoelectron spectroscopy of gold nanospheres

Streaked photoemission from nanostructures is characterized by size- and material-dependent nanometer-scale variations of the induced nanoplasmonic response to the electronic field of the streaking pulse and thus holds promise of allowing photoelectron imaging with both subfemtosecond temporal and nanometer spatial resolution. In order to scrutinize the driven collective electronic dynamics in 10--200-nm-diameter gold nanospheres, we calculated the plasmonic field induced by streaking pulses in the infrared and visible spectral range and developed a quantum-mechanical model for streaked photoemission by extreme ultraviolet pulses. Our simulated photoelectron spectra reveal a significant amplitude enhancement and phase shift of the photoelectron streaking trace relative to calculations that exclude the induced plasmonic field. Both are most pronounced for streaking pulses tuned to the plasmon frequency and retrace the plasmonic electromagnetic field enhancement and phase shift near the nanosphere surface.

Study on the structures and properties of praseodymium-doped silicon clusters PrSi n (n = 3–9) and their anions with density functional schemes

The equilibrium geometries and properties such as adiabatic electron affinities (AEAs), simulated photoelectron spectra (PES), dissociation energies, relative stabilities, HOMO–LUMO gaps, charges transfer, and magnetic moments of PrSin (n = 3–9) and their anions have been made a detailed study by means of the ABCluster global search technique combined with density functional methods. The structure optimization is carried out with three exchange correlation functionals (B3LYP, PBE0, and mPW2PLYP). The ground state structures predicted by mPW2PLYP are thought to be trustworthy. The experimental PES of PrSi4− is reassigned in light of the theoretical results, and the experimental AEAs of 2.0 ± 0.1 eV are obtained. The mPW2PLYP AEAs of PrSin are in excellent agreement with the experimental values. The average absolute deviations from experiment are only 0.05 eV, and the maximum deviations are 0.10 eV. The accordance between the experimental PES and the theoretical simulations indicates that the ground state structures of PrSin− (n = 4–9) are trustworthy. Doping Pr atom to Sin (n = 3–9) clusters raises the photochemical sensitivity. A large proportion of the total magnetic moments for all of these species are contributed by Pr atom.

Hollow Gold Cages and Their Topological Relationship to Dual Fullerenes.

Golden fullerenes have recently been identified by photoelectron spectra by Bulusu et al. [S. Bulusu, X. Li, L.-S. Wang, X. C. Zeng, PNAS 2006, 103, 8326-8330]. These unique triangulations of a sphere are related to fullerene duals having exactly 12 vertices of degree five, and the icosahedral hollow gold cages previously postulated are related to the Goldberg-Coxeter transforms of C20 starting from a triangulated surface (hexagonal lattice, dual of a graphene sheet). This also relates topologically the (chiral) gold nanowires observed to the (chiral) carbon nanotubes. In fact, the Mackay icosahedra well known in gold cluster chemistry are related topologically to the dual halma transforms of the smallest possible fullerene C20 . The basic building block here is the (111) fcc sheet of bulk gold which is dual to graphene. Because of this interesting one-to-one relationship through Euler's polyhedral formula, there are as many golden fullerene isomers as there are fullerene isomers, with the number of isomers Niso increasing polynomially as O(Niso9 ). For the recently observed Au16- , Au17- , and Au18- we present simulated photoelectron spectra including all isomers. We also predict the photoelectron spectrum of Au32- . The stability of the golden fullerenes is discussed in relation with the more compact structures for the neutral and negatively charged Au12 to Au20 and Au32 clusters. As for the compact gold clusters we observe a clear trend in stability of the hollow gold cages towards the (111) fcc sheet. The high stability of the (111) fcc sheet of gold compared to the bulk 3D structure explains the unusual stability of these hollow gold cages.

Analysis of valence XPS and AES of (PP, P4VP, PVME, PPS, PTFE) polymers by DFT calculations using the model molecules

We simulated valence X-ray photoelectron spectra (VXPS) of five [(CH2CH(CH3))n {poly(propyrene) PP}, ((CH2CH(C5NH4))n {poly(4-vinyl-pyridine) P4VP}, (CH2CHO(CH3))n {poly(vinyl methyl ether) PVME}, (C6H4S)n {poly(phenylene) sulphide PPS}, (CF2CF2)n {poly(tetrafluoroethylene) PTFE}] polymers by density-functional theory (DFT) calculations using the model oligomers. The spectra reflect the differences in the chemical structures between each polymer, since the peak intensities of valence band spectra are seen to be due to photo-ionization cross-section of (C, N, O, S, F) atoms by considering the orbital energies and cross-section values of the polymer models, individually. In the Auger electron spectra (AES) simulations, theoretical kinetic energies of the AES are obtained with our modified calculation method. The modified kinetic energies correspond to two final-state holes at the ground state and at the transition-state in DFT calculations, respectively. Experimental peaks of (C, N, O)- KVV, and S L2,3VV AES for each polymer are discussed in detail by our modified calculation method.

Insights into the Photoelectron Spectroscopy of Chlorofluoroethenes Studied by Density-Functional and Coupled-Cluster Theories.

The first two ionic states of chlorofluoroethenes were studied by using both time-independent and time-dependent density-functional theories. We calculated the equilibrium geometries and harmonic vibrational frequencies of 1,1-, cis-, and trans-C2H2FCl and their cations by using the B3LYP and B3PW91 functionals together with the cc-pVTZ and aug-cc-pVTZ basis sets. Franck-Condon factors were computed by the method developed in our group, in which the Duschinsky effect was treated explicitly. A new technique, named alignment transformation, followed by Euler transformations was developed to achieve the Eckart conditions. The adiabatic ionization energies were calculated by the CCSD(T) method extrapolated to the complete basis set limit. Insights into the simulated photoelectron spectra of C2H2FCl indicate that the resolutions of recent threshold photoelectron experiments are not high enough to detect individual transitions. The high-resolution photoelectron spectra of C2H2FCl are predicted for future reference. The computed adiabatic ionization energies of the three isomers of C2H2FCl are in accord with the experiments with the absolute deviations ranging from 0.004 to 0.021 eV. We suggest that the agreement between experimental and theoretical spectra should be a key criterion to judge whether a spectral assignment is reasonable.

Periodicity, Electronic Structures, and Bonding of Gold Tetrahalides [AuX4]− (X = F, Cl, Br, I, At, Uus)

Systematic theoretical and experimental investigations have been performed to understand the periodicity, electronic structures, and bonding of gold halides using tetrahalide [AuX4](-) anions (X = F, Cl, Br, I, At, Uus). The [AuX4](-) (X = Cl, Br, I) anions were experimentally produced in the gas phase, and their negative-ion photoelectron spectra were obtained, exhibiting rich and well-resolved spectral peaks. As expected, Au-X bonds in such series contain generally increasing covalency when halogen ligands become heavier. We calculated the adiabatic electron detachment energies as well as vertical electron detachment energies using density functional theory methods with scalar relativistic and spin-orbit coupling effects. The computationally simulated photoelectron spectra are in good agreement with the experimental ones. Our results show that the trivalent Au(III) oxidation state becomes progressively less stable while Au(I) tends to be preferred when the halides become heavier along the Periodic Table. This series of molecules provides an example for manipulating the oxidation state of metals in complexes through ligand design.

Structures and Photoelectron Spectra of Helium Nano Clusters

The ionization energies and photoelectron spectra of van der Waals (vdW) helium nano clusters (Hen) were studied in this work. Clusters with different numbers of helium atoms, in the range from n = 2 to n = 100, and belonging to different symmetry point groups were selected. Density functional theory (DFT) employing the CAM-B3LYP functional and the 6-31+G(d) basis set was used for optimizing the structures. Symmetry adapted cluster-configuration interaction (SAC-CI) methodology and the 6-31+G(3df) basis set were also used to calculate the ionization energies and their intensities. The calculated ionization energies and their intensities were used to simulate the photoelectron spectrum of each structure. It was found that the clustering of He atoms decreases the first ionization energy of the cluster compared to the isolated helium atom in the gas phase. The variation of the first ionization energies of the nanostructures were plotted versus their sizes and fitted into a mathematical equation. The simulated photoelectron spectra of the nano clusters were compared with each other comprehensively to investigate the change in the shape of the spectrum with the size of the cluster. The calculated ionization bands of each cluster were assigned using natural bonding orbital (NBO) analysis, and their changes with the size were studied. Also, it was studied how much the ionization processes are governed by the electron correlation, and the effect of the size on the electron correlation was examined.

Isomerism and structural fluxionality in the Au26 and Au26(-) nanoclusters.

Using the minima hopping global optimization method at the density functional level, we found low-energy nanostructures for neutral Au26 and its anion. The local-density and a generalized gradient approximation of the exchange–correlation functional predict different nanoscale motifs. We found a vast number of isomers within a small energy range above the respective putative global minima with each method. Photoelectron spectroscopy of Au26(-) under different experimental conditions revealed definitive evidence of the presence of multiple isomers, consistent with the theoretical predictions. Comparison between the experimental and simulated photoelectron spectra suggests that the photoelectron spectra of Au26(-) contain a mixture of three isomers, all of which are low-symmetry core–shell-type nanoclusters with a single internal Au atom. We present a disconnectivity graph for Au26(-) that has been computed completely at the density functional level. The transition states used to build this disconnectivity graph are complete enough to predict Au26(-) to have a possible fluxional shell, which facilitates the understanding of its catalytic activity.

A theoretical study on the equilibrium structures, vibrational frequencies and photoelectron spectroscopy of thiocarbonyl fluoride by using density functional and coupled-cluster theories

The equilibrium geometries, vibrational frequencies and normal modes of F2CS and F2CS+X∼2B2, A∼2B1, and B∼2A1 states were obtained by utilizing both density functional and coupled-cluster (CC2) theories. Franck–Condon factors were calculated by using the harmonic-oscillator model taking into account the Duschinsky effect, based on which photoelectron spectra were simulated. The adiabatic ionization energies were computed by the CCSD(T) method extrapolated to the complete basis set limit. The computed equilibrium structures and vibrational frequencies are generally in agreement with the experiment, except in few cases. The B3LYP and CC2 approaches perform equally well in the computations of F2CS. The simulated photoelectron spectra of F2CS are also in accord with the experiment, indicating that the calculated structures are reliable. The computed adiabatic ionization energies are in agreement with the experiment within 0.01, 0.02, and 0.06eV for the three ionic states, respectively.

B30]−: A Quasiplanar Chiral Boron Cluster

Chirality is vital in chemistry. Its importance in atomic clusters has been recognized since the discovery of the first chiral fullerene, the D2 symmetric C76. A number of gold clusters have been found to be chiral, raising the possibility to use them as asymmetric catalysts. The discovery of clusters with enantiomeric structures is essential to design new chiral materials with tailored chemical and physical properties. Herein we report the first inherently chiral boron cluster of [B30](-) in a joint photoelectron spectroscopy and theoretical study. The most stable structure of [B30](-) is found to be quasiplanar with a hexagonal hole. Interestingly, a pair of enantiomers arising from different positions of the hexagonal hole are found to be degenerate in our global minimum searches and both should co-exist experimentally because they have identical electronic structures and give rise to identical simulated photoelectron spectra.

B30]−: A Quasiplanar Chiral Boron Cluster

AbstractChirality is vital in chemistry. Its importance in atomic clusters has been recognized since the discovery of the first chiral fullerene, the D2 symmetric C76.1 A number of gold clusters have been found to be chiral,2 raising the possibility to use them as asymmetric catalysts. The discovery of clusters with enantiomeric structures is essential to design new chiral materials with tailored chemical and physical properties.3 Herein we report the first inherently chiral boron cluster of [B30]− in a joint photoelectron spectroscopy and theoretical study. The most stable structure of [B30]− is found to be quasiplanar with a hexagonal hole. Interestingly, a pair of enantiomers arising from different positions of the hexagonal hole are found to be degenerate in our global minimum searches and both should co‐exist experimentally because they have identical electronic structures and give rise to identical simulated photoelectron spectra.

A combined stochastic search and density functional theory study on the neutral and charged silicon-based clusters MSi6 (M = La, Ce, Yb and Lu)

Structures and simulated photoelectron spectra of MSi6− (M = La, Ce, Yb and Lu).

An analytical approach for computing Franck‐Condon integrals of harmonic oscillators with arbitrary dimensions

We have developed an analytical approach for computing Franck-Condon integrals (FCIs) of harmonic oscillators (HOs) with arbitrary dimensions in which the mode-mixing Duschinsky effect is taken into account. A general formula of FCIs of HOs was obtained and was applied to study the photoelectron spectroscopy of vinyl alcohol and ovalene (C(32)H(14)). The equilibrium geometries, harmonic vibrational frequencies and normal modes of vinyl alcohol, ovalene, and their cations were computed at the B3LYP/aug-cc-pVTZ or the B3LYP/6-31G(d) level, from which Franck-Condon factors were calculated and photoelectron spectra were simulated. The adiabatic ionization energies of vinyl alcohol were also computed by extrapolating the CCSD(T) energies to the complete basis set limit with aug-cc-pVXZ (X = D, T, Q, 5). The simulated photoelectron spectra of both vinyl alcohol and ovalene are in agreement with the experiments. The computed adiabatic ionization energies of syn- and anti-vinyl alcohol are in consistent with the experiment within 0.008 eV and 0.014 eV, respectively. We show, for the first time, that the analytical approach of computing FCIs is also efficient and promising for the studies of vibronic spectra of macrosystems.

Simulations of light induced processes in water based on ab initio path integrals molecular dynamics. II. Photoionization

We have applied ab initio based reflection principle to simulate photoelectron spectra of small water clusters, ranging from monomer to octamer. The role of quantum and thermal effects on the structure of the water photoelectron spectra is discussed within the ab initio path integral molecular dynamics (PIMD) framework. We have used the PIMD method with up to 40 beads to sample the ground state quantum distribution at temperature T = 180 K. We have thoroughly tested the performance of various density functionals (B3LYP, BHandHLYP, M06HF, BNL, LC-ωPBE, and CAM-B3LYP) for the ionization process description. The benchmarking based on a comparison of simulated photoelectron spectra to experimental data and high level equation-of-motion ionization potential coupled clusters with singles and doubles calculations has singled out the BHandHLYP and LC-ωPBE functionals as the most reliable ones for simulations of light induced processes in water. The good performance of the density functional theory functionals to model the water photoelectron spectra also reflects their ability to reliably describe open shell excited states. The width of the photoelectron spectrum converges quickly with the cluster size as it is controlled by specific interactions of local character. The peak position is, on the other hand, defined by long-range non-specific solvent effects; it therefore only slowly converges to the corresponding bulk value. We are able to reproduce the experimental valence photoelectron spectrum of liquid water within the combined model of the water octamer embedded in a polarizable dielectric continuum. We demonstrate that including the long-range polarization and the state-specific treatment of the solvent response are needed for a reliable liquid water ionization description.

A combined ab initio and Franck–Condon simulation study of the photodetachment spectrum of the [formula omitted] anion

A general analytical expression for the calculation of the two-dimensional four-mode Franck–Condon overlap integrals was presented and applied to scrutinize the photoelectron spectroscopy of ClO 2 - . Geometry optimization and harmonic vibrational frequency calculations have been performed on the X ∼ 2B 1 state of ClO 2 and X ∼ 1A 1 state of ClO 2 - by using the density functional theory (B2PLYP functional) and the coupled cluster singles and doubles with perturbative triples [CCSD(T)] methods with various basis sets up to aug-cc-pVQZ. The adiabatic electron affinity of ClO 2 was computed by using the CCSD(T) method extrapolated to the complete basis set limit with aug-cc-pVXZ (X = D, T, Q). Franck–Condon analyses and spectral simulation were carried out on the ClO 2( X ∼ 2B 1)–( ClO 2 - X ∼ 1A 1) photodetachment process. The simulated photoelectron spectrum of ClO 2 - are in accord with the experiment. While the Duschinsky effect plays a minor role and can be neglected. In addition, the equilibrium geometry parameters, R(Cl–O) = 1.572 ± 0.005 Å and ∠ (O–Cl–O) = 112.5 ± 0.5°, of the X ∼ 1A 1 state of ClO 2 - , are obtained by employing an iterative Franck–Condon analysis procedure in the spectral simulations.

On the simulation of photoelectron spectra complicated by conical intersections: Higher-order effects and hot bands in the photoelectron spectrum of triazolide (CH)2N3−

We report simulated photoelectron spectra for 1,2,3-triazolide (CH)(2)N(3)(-), which reveal the vibronic energy levels of the neutral radical 1,2,3-triazolyl, (CH)(2)N(3). The spectral simulation using a quasidiabatic Hamiltonian H(d) comprised of polynomials through 4th order (thereby extending conventional quadratic expansions), is compared to both the experimental spectrum and a standard Franck-Condon (adiabatic) simulation. The quartic H(d) is far superior to the quadratic H(d), reproducing the main features of the experimental spectrum and allowing for their subsequent assignment. The contributions from excited anion states successfully reproduce the observed vibronic transitions to the red of the assigned band origin of the neutral species. The algorithmic extensions required for the determination of these hot band contributions to the total spectrum are discussed. Convergence of the spectral envelope with respect to the vibronic basis, including both the principal and hot bands, required more than 10(9) terms.

Photoelectron spectra of dihalomethyl anions: Testing the limits of normal mode analysis

We report the 364-nm negative ion photoelectron spectra of CHX(2)(-) and CDX(2)(-), where X = Cl, Br, and I. The pyramidal dihalomethyl anions undergo a large geometry change upon electron photodetachment to become nearly planar, resulting in multiple extended vibrational progressions in the photoelectron spectra. The normal mode analysis that successfully models photoelectron spectra when geometry changes are modest is unable to reproduce qualitatively the experimental data using physically reasonable parameters. Specifically, the harmonic normal mode analysis using Cartesian displacement coordinates results in much more C-H stretch excitation than is observed, leading to a simulated photoelectron spectrum that is much broader than that which is seen experimentally. A (2 + 1)-dimensional anharmonic coupled-mode analysis much better reproduces the observed vibrational structure. We obtain an estimate of the adiabatic electron affinity of each dihalomethyl radical studied. The electron affinity of CHCl(2) and CDCl(2) is 1.3(2) eV, of CHBr(2) and CDBr(2) is 1.9(2) eV, and of CHI(2) and CDI(2) is 1.9(2) eV. Analysis of the experimental spectra illustrates the limits of the conventional normal mode approach and shows the type of analysis required for substantial geometry changes when multiple modes are active upon photodetachment.

Simulated Photoelectron Spectra of the Cyanide-Water Anion via Quasiclassical Molecular Dynamics

We present the simulated photoelectron spectrum (PES) for cyanide-water CN(H(2)O)(-) based on quasiclassical trajectory molecular dynamics (QCT-MD). Using density functional theory to generate trajectories and to calculate vertical detachment energies, we obtain simulated spectra that are in qualitative agreement with experiment. We obtain a theoretical 12 → 300 K temperature red shift of 0.1 eV as compared to an experimental redshift of 0.25 eV. The calculated linewidths of 0.3 eV are in excellent agreement with experiment. Our trajectories show that the temperature red shift as being dominated by dynamics within the basin of the N-bound minimum, however, at 300 K we predict conversion into the basin of the C-bound minimum, equilibrating at a 80:20 ratio of N- vs C-bound mixture. We discuss the potential advantages of QCT-MD over anharmonic Franck-Condon analysis such as natural incorporation of anharmonicity (as necessary for weakly bound systems), and reduced computational scaling, but also drawbacks such as neglect of final-state (e.g., Duschinsky) effects.

Flexible H2O2 in Water: Electronic Structure from Photoelectron Spectroscopy and Ab Initio Calculations

The effect of hydration on the electronic structure of H(2)O(2) is investigated by liquid-jet photoelectron spectroscopy measurements and ab initio calculations. Experimental valence electron binding energies of the H(2)O(2) orbitals in water are, on average, 1.9 eV red-shifted with respect to the gas-phase molecule. A smaller width of the first peak was observed in the photoelectron spectrum from the solution. Our experiment is complemented by simulated photoelectron spectra, calculated at the ab initio level of theory (with EOM-IP-CCSD and DFT methods), and using path-integral sampling of the ground-state density. The observed shift in ionization energy upon solvation is attributed to a combination of nonspecific electrostatic effects (long-range polarization) and of the specific interactions between H(2)O(2) and H(2)O molecules in the first solvation shell. Changes in peak widths are found to result from merging of the two lowest ionized states of H(2)O(2) in water due to conformational changes upon solvation. Hydration effects on H(2)O(2) are stronger than on the H(2)O molecule. In addition to valence spectra, we report oxygen 1s core-level photoelectron spectra from H(2)O(2)(aq), and observed energies and spectral intensities are discussed qualitatively.