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The interaction of formic acid (HCOOH) with solid water, deposited on tungsten at 80 K, was investigated. We have prepared and annealed formic acid (FA)/water interfaces (FA layers on thin films of solid water and H2O adlayers on thin FA films). Metastable impact electron spectroscopy and ultraviolet photoemission spectroscopy (He I and II) were utilized to study the electron emission from the 10a′ to 6a′ molecular orbitals (MOs) of FA, and the 1b1, 3a1, and 1b2 MOs of H2O. These spectra were compared with results of density-functional theory calculations on FA-H2O complexes reported in Ref. 14 [A. Allouche, J. Chem. Phys. 122, 234703(2005), (preceding paper)]. Temperature programmed desorption was applied for information on the desorption kinetics. Initially, FA is adsorbed on top of the water film. The FA spectra are distorted with respect to those from FA monomers; it is concluded that a strong interaction exists between the adsorbates. Even though partial solvation of FA species takes place during annealing, FA remains in the top layer up to the desorption of the water film. When H2O molecules are offered to FA films at 80 K, no water network is formed during the initial stage of water exposure; H2O molecules interact individually via H bonds with the formic acid network. Experiment and theory agree that no water-induced deprotonation of the formic acid molecules takes place. [ABSTRACT FROM AUTHOR]

We investigate the electron emission from 3D chiral silver alloy nanohelices initiated by femtosecond laser pulses with a central photon energy of hν = 1.65 eV, well below the work function of the material. We find hot but thermally distributed electron spectra and a strong anisotropy in the electron yield with left- and right-circularly polarized light excitations, which invert in sign between left- and right-handed helices. We analyze the kinetic energy distribution and discuss the role of effective temperatures. Measurements of the reflectance and simulations of the absorbance of the helices based on retarded field calculations are compared to the anisotropy in photoemission. We find a significant enhancement of the anisotropy in the electron emission in comparison to the optical absorption. Neither simple thermionic nor a multiphoton photoemission can explain the experimentally observed asymmetries. Single photon deep-UV photoemission from these helices together with a change of the work function suggests a contribution of the chirally induced spin selectivity effect to the observed asymmetries. [ABSTRACT FROM AUTHOR]

We present a time-dependent theory for non-resonant x-ray emission spectrum (XES) and normal Auger spectrum (NAS) calculation, based on a fully quantum description of nuclear dynamics using the vibrational wave packet concept. We compare two formulations of the time-dependent theory, either employing a two-time propagation scheme or using spectral integration over the electron energy continuum. We find that the latter formulation is more efficient for numerical simulations, providing a reasonable accuracy when the integration step is shorter than the lifetime broadening of the core-ionized state. We demonstrate our approach using the example of non-resonant x-ray emission from a water molecule, considering the lowest core-ionized K−1 and first core-ionized shake-up K−1V−1V1 intermediate states. These channels exemplify the developed theory on bound–bound, bound–continuum, continuum–bound, and continuum–continuum transitions. Our results suggest that the time-dependent approach is efficient for simulating XES involving dissociative states, whereas the time-independent approach, based on Franck–Condon factors, is more efficient for bound–bound transitions expressed as discrete frequency dependence in the energy domain. The methods and discussion have general applicability, including both NAS and more complex systems, such as liquid water. [ABSTRACT FROM AUTHOR]

In this paper we explore the use of the novel relativistic intermediate Hamiltonian Fock-space coupled-cluster method in the calculation of the electronic spectrum for small actinyl ions (NpO2+, NpO22+, and PuO22+). It is established that the method, in combination with uncontracted double-zeta quality basis sets, yields excitation energies in good agreement with experimental values, and better than those obtained previously with other theoretical methods. We propose the reassignment of some of the peaks that were observed experimentally, and confirm other assignments. [ABSTRACT FROM AUTHOR]

In this paper we explore the use of the novel relativistic intermediate Hamiltonian Fock-space coupled-cluster method in the calculation of the electronic spectrum for small actinyl ions (NpO2+, NpO22+, and PuO22+). It is established that the method, in combination with uncontracted double-zeta quality basis sets, yields excitation energies in good agreement with experimental values, and better than those obtained previously with other theoretical methods. We propose the reassignment of some of the peaks that were observed experimentally, and confirm other assignments. [ABSTRACT FROM AUTHOR]

The emission of an Auger electron is the predominant relaxation mechanism of core-vacant states in molecules composed of light nuclei. In this non-radiative decay process, one valence electron fills the core vacancy, while a second valence electron is emitted into the ionization continuum. Because of this coupling to the continuum, core-vacant states represent electronic resonances that can be tackled with standard quantum-chemical methods only if they are approximated as bound states, meaning that Auger decay is neglected. Here, we present an approach to compute Auger decay rates of core-vacant states from coupled-cluster and equation-of-motion coupled-cluster wave functions combined with complex scaling of the Hamiltonian or, alternatively, complex-scaled basis functions. Through energy decomposition analysis, we illustrate how complex-scaled methods are capable of describing the coupling to the ionization continuum without the need to model the wave function of the Auger electron explicitly. In addition, we introduce in this work several approaches for the determination of partial decay widths and Auger branching ratios from complex-scaled coupled-cluster wave functions. We demonstrate the capabilities of our new approach by computations on core-ionized states of neon, water, dinitrogen, and benzene. Coupled-cluster and equation-of-motion coupled-cluster theory in the singles and doubles approximation both deliver excellent results for total decay widths, whereas we find partial widths more straightforward to evaluate with the former method. [ABSTRACT FROM AUTHOR]

The electronic emission spectrum of YH has been investigated in the 690 nm to 3 μm spectral region using a Fourier transform spectrometer. The YH bands were excited in an yttrium hollow cathode lamp operated with neon gas and a trace of hydrogen. The observed bands have been classified into three different electronic transitions: C 1Σ+–X 1Σ+, d0+(3Σ-)–X 1Σ+, and e 3[uppercase_phi_synonym]–a 3Δ. The rotational analysis of the 0–0, 1–1, 2–2, 3–3, 0–1, 1–2, and 2–3 bands of the C 1Σ+–X 1Σ+ system provided the following set of equilibrium molecular constants for the ground state: ωe=1530.456(15) cm-1, ωexe=19.4369(72) cm-1, ωeye=0.0361(9) cm-1, Be =4.575 667(38) cm-1, αe=0.091 449(23) cm-1, and re=1.922 765(8) Å. Two weaker bands with origins at 14 264.7256(49) and 12 811.5753(49) cm-1 have been assigned as the 0–1 and 0–2 bands of a new d0+(3Σ-)–X 1Σ+ electronic transition with T0=15 756.4251(49) cm-1. Three bands observed in the red region with origins at 11 377.9784(39), 11 499.3408(26), and 11 583.8294(23) cm-1 have been assigned as the 3[uppercase_phi_synonym]2–3Δ1, 3[uppercase_phi_synonym]3–3Δ2, and 3[uppercase_phi_synonym]4–3Δ3 subbands of a new e 3[uppercase_phi_synonym]–a 3Δ electronic transition. © 1994 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Double-differential cross sections for the emission of secondary electrons, as a function of emission angle and energy, from a water molecule, a cluster of water molecules, and liquid water due to proton impact have been calculated using the dielectric response function of the target material and a method which uses an integral formulation of the density-functional theory. From these double-differential cross sections, single-differential and total cross sections, and the energy loss per unit path length, have been calculated by successive integration. The results have been compared to available experimental results and, in the case of single-differential cross sections, also to empirical models. A comparison has been made between the results for the molecule and the central molecule of the cluster in order to obtain insight into how the cross sections might change in the condensed phase, namely in liquid water. [ABSTRACT FROM AUTHOR]

Recent observations of considerable spin polarization in photoemission from metal surfaces through monolayers of chiral molecules were followed by several efforts to rationalize the results as the effect of spin-orbit interaction that accompanies electronic motion on helical, or more generally strongly curved, potential surfaces. In this paper we (a) argue, using simple models, that motion in curved force-fields with the typical energies used and the characteristic geometry of DNA cannot account for such observations; (b) introduce the concept of induced spin filtering, whereupon selectivity in the transmission of the electron orbital angular momentum can induce spin selectivity in the transmission process provided there is strong spin-orbit coupling in the substrate; and (c) show that the spin polarization in the tunneling current as well as the photoemission current from gold covered by helical adsorbates can be of the observed order of magnitude. Our results can account for most of the published observations that involved gold and silver substrates; however, recent results obtained with an aluminum substrate can be rationalized within the present model only if strong spin-orbit coupling is caused by the built-in electric field at the molecule-meta l interface. [ABSTRACT FROM AUTHOR]

New photoresists are needed to advance extreme ultraviolet (EUV) lithography. The tailored design of efficient photoresists is enabled by a fundamental understanding of EUV induced chemistry. Processes that occur in the resist film after absorption of an EUV photon are discussed, and a new approach to study these processes on a fundamental level is described. The processes of photoabsorption, electron emission, and molecular fragmentation were studied experimentally in the gas-phase on analogs of the monomer units employed in chemically amplified EUV resists. To demonstrate the dependence of the EUV absorption cross section on selective light harvesting substituents, halogenated methylphenols were characterized employing the following techniques. Photoelectron spectroscopy was utilized to investigate kinetic energies and yield of electrons emitted by a molecule. The emission of Auger electrons was detected following photoionization in the case of iodo-methylphenol. Mass-spectrometry was used to deduce the molecular fragmentation pathways following electron emission and atomic relaxation. To gain insight on the interaction of emitted electrons with neutral molecules in a condensed film, the fragmentation pattern of neutral gas-phase molecules, interacting with an electron beam, was studied and observed to be similar to EUV photon fragmentation. Below the ionization threshold, electrons were confirmed to dissociate iodo-methylphenol by resonant electron attachment. [ABSTRACT FROM AUTHOR]

We report low-temperature photoelectron spectra of isolated gas-phase complexes of the hexachloroplatinate dianion bound to the nucleobases uracil, thymine, cytosine, and adenine. The spectra display well-resolved, distinct peaks that are consistent with complexes where the hexachloroplatinate dianion is largely intact. Adiabatic electron detachment energies for the hexachloroplatinate-nucleobase complexes are measured as 2.26-2.36 eV. The magnitudes of the repulsive Coulomb barriers (RCBs) of the complexes are all ∼1.7 eV, values that are lower than the RCB of the uncomplexed PtCl62- dianion as a result of charge solvation by the nucleobases. In addition to the resolved spectral features, broad featureless bands indicative of delayed electron detachment are observed in the 193 nm photoelectron spectra of the four clusters. The 266 nm spectra of the PtCl62-·thymine and PtCl62-·adenine complexes also display very prominent delayed electron emission bands. These results mirror recent results on the related Pt(CN)42-·nucleobase complexes [A. Sen et al., J. Phys. Chem. B 119, 11626 (2015)]. The observation of delayed electron emission bands in the PtCl62-·nucleobase spectra obtained in this work, as for the previously studied Pt(CN)42-·nucleobase complexes, is attributed to one-photon excitation of nucleobase-centred excited states that can effectively couple to the electron detachment continuum, producing strong electron detachment. Moreover, the selective, strong excitation of the delayed emission bands in the 266 nm spectra is linked to fundamental differences in the individual nucleobase photophysics at this excitation energy. This strongly supports our previous suggestion that the dianion within these clusters can be viewed as a "dynamic tag" which has the propensity to emit electrons when the attached nucleobase decays over a time scale long enough to allow autodetachment. [ABSTRACT FROM AUTHOR]

Electrospray ionization (ESI) has become an essential tool in chemical physics and physical chemistry for the production of novel molecular ions from solution samples for a variety of spectroscopic experiments. ESI was used to produce free multiply-charged anions (MCAs) for photoelectron spectroscopy (PES) in the late 1990 s, allowing many interesting properties of this class of exotic species to be investigated. Free MCAs are characterized by strong intramolecular Coulomb repulsions, which create a repulsive Coulomb barrier (RCB) for electron emission. The RCB endows many fascinating properties to MCAs, giving rise to meta-stable anions with negative electron binding energies. Recent development in the PES of MCAs includes photoelectron imaging to examine the influence of the RCB on the electron emission dynamics, pump-probe experiments to examine electron tunneling through the RCB, and isomer-specific experiments by coupling PES with ion mobility for biological MCAs. The development of a cryogenically cooled Paul trap has led to much better resolved PE spectra for MCAs by creating vibrationally cold anions from the room temperature ESI source. Recent advances in coupling the cryogenic Paul trap with PE imaging have allowed high-resolution PE spectra to be obtained for singly charged anions produced by ESI. In particular, the observation of dipole-bound excited states has made it possible to conduct vibrational autodetachment spectroscopy and resonant PES, which yield much richer vibrational spectroscopic information for dipolar free radicals than traditional PES. [ABSTRACT FROM AUTHOR]

Using a combination of photoelectron spectroscopy measurements and quantum chemistry calculations, we have identified competing electron emission processes that contribute to the 350-315 nm photoelectron spectra of the deprotonated green fluorescent protein chromophore anion, phydroxybenzylidene- 2,3-dimethylimidazolinone. As well as direct electron detachment from S0, we observe resonant excitation of the 21ππ* state of the anion followed by autodetachment. The experimental photoelectron spectra are found to be significantly broader than photoelectron spectrum calculated using the Franck-Condon method and we attribute this to rapid (∼10 fs) vibrational decoherence, or intramolecular vibrational energy redistribution, within the neutral radical. [ABSTRACT FROM AUTHOR]

Biomolecular polyanions mainly relax by electron emission after UV excitation. Here, we study photodetachment of protein polyanions in the 6-16 eV VUV range by coupling a linear quadrupole ion trap with a synchrotron beamline. Gas-phase VUV action spectra of electrospray-produced multiply deprotonated insulin (5.6 kDa) and myoglobin (16.7 kDa) proteins are reported, which significantly increases the amount of data available on the optical response of proteins in the VUV. The influence of the protein charge and oxidation state upon the electron detachment efficiency is discussed. For small protein such as insulin, it appears that higher charge states produce higher detachment yields. Investigations on oxidized species show that the nature of the groups bearing the negative charges has an influence on the yields. For larger proteins, comparison of two forms of myoglobin clearly indicate that the three-dimensional structure does not impact much on the shape and the magnitude of the photodetachment spectra, in spite of a slight shift for the first electronic excited states. [ABSTRACT FROM AUTHOR]

We describe an approach for constructing analytical models for the energy-dependence of photoelectron angular distributions in the one-electron, non-relativistic approximation. We construct such a model for electron emission from an orbital described as a superposition of s- and p-type functions, using linearly polarized light. In the limits of pure s or pure p electron photodetachment or photoionization, the model correctly reproduces the familiar Cooper-Zare formula. The model predictions are compared to experimental results for strongly solvated H- and NH2-, corresponding to predominantly s and predominantly p character parent states, respectively. [ABSTRACT FROM AUTHOR]

Excited states of atoms and molecules lying above the ionization threshold can decay by electron emission in a process commonly known as autoionization. The autoionization widths can be calculated conveniently using Fano formalism and discretized atomic and molecular spectra by a standard procedure referred to as Stieltjes imaging. The Stieltjes imaging procedure requires the use of the full discretized spectrum of the final states of the autoionization, making its use for poly-atomic systems described by high-quality basis sets impractical. Following our previous work on photoionization cross-sections, here we show that also in the case of autoionization widths, the full diagonalization bottleneck can be overcome by the use of Lanczos pseudospectra. We test the proposed method by calculating the well-documented autoionization widths of inner-valence-excited neon and apply the new technique to autoionizing states of hydrofluoric acid and benzene. [ABSTRACT FROM AUTHOR]

The decay processes of core-valence doubly excited states near the N K edge of NO have been studied using electron spectroscopy. Electron yields measured as a function of photon energy and kinetic energy enable the clear identification of atomic Auger lines associated with the dissociation of doubly excited states. The atomic Auger lines exhibit Doppler profiles, allowing the entire reaction scheme of such dissociation processes to be determined. [ABSTRACT FROM AUTHOR]

With the nonstochastic quantum mechanical study of a quantum dot light emitter, we find that fluorescence intermittency statistics are universal and insensitive to the microscopic nature of the tunneling fluctuation between quantum dot and trapping state. We also investigate the power-law exponent θ and the crossover time τC of the on-time (τon) probability P(τon)∝τon-θ (for τon<=τC) and ∝e-Γτon (for τon>=τC) under an optical field of given energy and strength. For easy off-resonance excitation, it is found in both numerical and analytic ways that τC-1 is proportional to the intensity of the optical field (i.e., the square of the field strength) independent of the internal parameters of a quantum dot. Furthermore, it is also found that θ=2 in the limit of vanishing field strength is the upper bound of the exponent and θ becomes less than 2 as the field strength increases. [ABSTRACT FROM AUTHOR]

We explore the potential of double core hole electron spectroscopy for chemical analysis in terms of x-ray two-photon photoelectron spectroscopy. The creation of deep single and double core vacancies induces significant reorganization of valence electrons. The corresponding relaxation energies and the interatomic relaxation energies are evaluated by complete active space self-consistent field (CASSCF) calculations. We propose a method on how to experimentally extract these quantities by the measurement of single ionization potentials (IPs) and double core hole ionization potentials (DIPs). The influence of the chemical environment on these DIPs is also discussed for states with two holes at the same atomic site and states with two holes at two different atomic sites. Electron density difference between the ground and double core hole states clearly shows the relaxations accompanying the double core hole ionization. The effect is also compared to the sensitivity of single core hole IPs arising in single core hole electron spectroscopy. We have demonstrated the method for a representative set of small molecules LiF, BeO, BF, CO, N2, C2H2, C2H4, C2H6, CO2, and N2O. The scalar relativistic effect on IPs and on DIPs are briefly addressed. [ABSTRACT FROM AUTHOR]

A single layer of bismuth deposited on the Cu(100) surface forms long range ordered structural phases at various Bi density. A highly ordered c(2×2) reconstruction is accomplished at 0.5 ML, further Bi deposition induces a c(92×2)R45° structure and a subsequent p(10×10) phase related to the formation of regular dislocations arrays. The transition from a c(2×2) superstructure to the c(92×2)R45° phase is accompanied by a sudden decrease in the work function. Photoemission measurements reveal that the Bi induced states close to the Fermi level, associated to the c(2×2) phase, are strongly quenched when the arrays of dislocations are formed, while at higher binding energies, they undergo an energy shift probably due to a confinement effect. The low-energy single particle excitations and the electron dispersion of the Bi induced states of the c(2×2) phase are compared to the electronic states deduced by theoretical band structure obtained by ab initio calculation performed within the embedding method applied to a realistic semi-infinite system. [ABSTRACT FROM AUTHOR]

We present for static density functional theory and time-dependent density functional theory calculations an all-electron method which employs high-order hierarchical finite-element bases. Our mesh generation scheme, in which structured atomic meshes are merged to an unstructured molecular mesh, allows a highly nonuniform discretization of the space. Thus it is possible to represent the core and valence states using the same discretization scheme, i.e., no pseudopotentials or similar treatments are required. The nonuniform discretization also allows the use of large simulation cells, and therefore avoids any boundary effects. [ABSTRACT FROM AUTHOR]

The π-conjugated organic molecules 3,4,9,10-perylene-tetracarboxylic dianhydride, 1,4,5,8-naphthalene-tetracarboxylic dianhydride, and 1,8-naphthalene-dicarboxylic anhydride were investigated via gas phase and bulk ultraviolet photoemission spectroscopy and compared to density functional theory calculations. Values for final state effects such as intermolecular polarization were determined and the differing features in the spectra interpreted as a consequence of interactions in the thin films. Additionally, the highest occupied molecular orbitals of the molecules clearly show distinctive peaks originating from vibrational excitations, leading to results for Franck–Condon factors. [ABSTRACT FROM AUTHOR]

The quantum dynamics of the photodetached electron of H- in electric field near a surface are studied in the time domain. The evolution of wave packet for different manifold eigenstates with limited lifetimes is obtain analytically. It is found that the quantum coherence and temporal evolution of surface electronic wave packet can be controlled by the laser central energy and electric field. The correspondence between classical and quantum mechanics is shown explicitly in the system. Numerical simulation shows that the temporal evolution of photodetached electronic wave packet on elastic surface exhibits some similar properties of time-resolved two-photon photoemission signal of surface electron. [ABSTRACT FROM AUTHOR]

We present results on the fragmentation of adenine dication as a function of the excitation energy. The adenine molecule is charged and excited in a single collision with Cl+ ion at 3 keV and the excitation energy distribution is obtained for each fragmentation channel by measuring the kinetic energy loss of the projectile. This method named collision induced dissociation under energy control is based on the formation of a negative scattered projectile as a result of double electron capture from the target molecule. Comparison between the main dissociation channels of singly and doubly charged adenine shows that fragmentation patterns are very similar consisting mainly of the successive emission of neutral HCN or H2CN+. The energy distributions of the parent adenine dication and the kinetic energy release of the fragments measured for the most abundant fragmentation channels confirms the assumption of successive emission dynamics. A specific fragmentation pathway of the adenine requiring less energy than the usual successive emission of neutral HCN could be identified. It consists of the emission of a charged H2CN+ following on by the emission of a dimer of HCN (precisely HC2N2). This new channel, measured for a mean excitation energy of 8.4 eV for the adenine dication is very closed to the emission of HCN monomer measured at 7.9 eV. The implications of these results concern the formation of adenine in the sealed-tube reaction of HCN with liquid ammonia as well as the possible formation of the adenine molecule in the interstellar medium. This last point is briefly discussed in relation to astrobiology and exobiology interests. [ABSTRACT FROM AUTHOR]

Molecular dynamics (MD) simulations generate a canonical ensemble only when integration of the equations of motion is coupled to a thermostat. Three extended phase space thermostats, one version of Nosé–Hoover and two versions of Nosé–Poincaré, are compared with each other and with the Berendsen thermostat and Langevin stochastic dynamics. Implementation of extended phase space thermostats was first tested on a model Lennard-Jones fluid system; subsequently, they were implemented with our physics-based protein united-residue (UNRES) force field MD. The thermostats were also implemented and tested for the multiple-time-step reversible reference system propagator (RESPA). The velocity and temperature distributions were analyzed to confirm that the proper canonical distribution is generated by each simulation. The value of the artificial mass constant, Q, of the thermostat has a large influence on the distribution of the temperatures sampled during UNRES simulations (the velocity distributions were affected only slightly). The numerical stabilities of all three algorithms were compared with each other and with that of microcanonical MD. Both Nosé–Poincaré thermostats, which are symplectic, were not very stable for both the Lennard-Jones fluid and UNRES MD simulations started from nonequilibrated structures which implies major changes of the potential energy throughout a trajectory. Even though the Nosé–Hoover thermostat does not have a canonical symplectic structure, it is the most stable algorithm for UNRES MD simulations. For UNRES with RESPA, the “extended system inside-reference system propagator algorithm” of the RESPA implementation of the Nosé–Hoover thermostat was the only stable algorithm, and enabled us to increase the integration time step. [ABSTRACT FROM AUTHOR]

In this work a new method is proposed for the choice of basis functions in diffusion theory (DT) calculations. This method, named hybrid basis approach (HBA), combines the two previously adopted long time sorting procedure (LTSP) and maximum correlation approximation (MCA) techniques; the first emphasizing contributions from the long time dynamics, the latter being based on the local correlations along the chain. In order to fulfill this task, the HBA procedure employs a first order basis set corresponding to a high order MCA one and generates upper order approximations according to LTSP. A test of the method is made first on a melt of cis-1,4-polyisoprene decamers where HBA and LTSP are compared in terms of efficiency. Both convergence properties and numerical stability are improved by the use of the HBA basis set whose performance is evaluated on local dynamics, by computing the correlation times of selected bond vectors along the chain, and on global ones, through the eigenvalues of the diffusion operator L. Further use of the DT with a HBA basis set has been made on a 71-mer of syndiotactic trans-1,2-polypentadiene in toluene solution, whose dynamical properties have been computed with a high order calculation and compared to the “numerical experiment” provided by the molecular dynamics (MD) simulation in explicit solvent. The necessary equilibrium averages have been obtained by a vacuum trajectory of the chain where solvent effects on conformational properties have been reproduced with a proper screening of the nonbonded interactions, corresponding to a definite value of the mean radius of gyration of the polymer in vacuum. Results show a very good agreement between DT calculations and the MD numerical experiment. This suggests a further use of DT methods with the necessary input quantities obtained by the only knowledge of some experimental values, i.e., the mean radius of gyration of the chain and the viscosity of the solution, and by a suitable vacuum trajectory, with great savings in computational time required. This offers a theoretical bridge between the experimental static and dynamical properties of polymers. [ABSTRACT FROM AUTHOR]

The valence electronic states of three kinds of fluorinated derivatives of perylene diimides, D2MFPP, D3MFPP, and D4MFPP, on Cu(110) and SiO2/Si surface were studied by photoemission and density functional calculations. When these organic molecules were deposited on the Cu(110) and thermally oxidized SiO2 surfaces, five well-resolved photoemission features originating from the molecules were observed. On Cu(110) surface, two emission features with π-like character increased their binding energy with increasing the coverage of organic molecule, indicating a strong interaction between the organic molecules and Cu substrate. The density functional calculations suggest flat-lying adsorption geometry for D3MFPP and D4MFPP on Cu(110) surface. [ABSTRACT FROM AUTHOR]

The linear combination of atomic orbital approach to the calculation of the molecular continuum spectrum with B-spline basis functions has been extended to calculate first-order nondipolar corrections to the photoelectron angular distributions. The authors give an account of the new implementation and provide an application to the calculation of dipolar and nondipolar contributions to the photoelectron angular distributions from the SF6 molecule. The proposed approach is a promising tool for the investigation of such effects in the photoemission of large molecules and clusters. [ABSTRACT FROM AUTHOR]

We examine the propagation of shaped (amplitude- and frequency-modulated) ultrafast laser pulses through optically dense rubidium vapor. Pulse reshaping, stimulated emission dynamics, and residual electronic excitation all strongly depend on the laser pulse shape. For example, frequency swept pulses, which produce adiabatic passage in the optically thin limit (independent of the sign of the frequency sweep), behave unexpectedly in optically dense samples. Paraxial Maxwell optical Bloch equations can model our ultrafast pulse propagation results well and provide insight. [ABSTRACT FROM AUTHOR]

The (1×1)→quasihexagonal (HEX) phase transition on a clean Pt(100) surface was investigated by monitoring the time evolution of the Pt4f7/2 core level photoemission spectra. The spectral component originating from the atoms forming the (1×1) metastable unreconstructed surface was found at -570±20 meV with respect to the bulk peak. Ab initio calculations based on density functional theory confirmed the experimental assignment. At temperatures above 370 K, the (1×1) phase irreversibly reverts to the more stable HEX phase, characterized by a surface core level shifted component at -185±40 meV. By analyzing the intensity evolution of the core level components, measured at different temperatures in the range of 393–475 K, we determined the activation energy of the phase transformation, E=0.76±0.04 eV. This value is considerably lower than the one previously determined by means of low energy electron diffraction. Possible reasons for this discrepancy are discussed. [ABSTRACT FROM AUTHOR]

Interatomic Coulombic decay (ICD) is a radiationless decay mechanism occurring via electron emission in an inner-valence ionized weakly bound cluster. The ICD has been studied for the neon clusters Nen (n=2,...,5). The decay widths of the neon clusters are calculated using ab initio Green’s function method. The non-Dyson version of Green’s function is employed. This propagator is analytically continued into the complex energy plane with the aid of a complex absorbing potential, and the decaying states are found as resonance states in this plane. [ABSTRACT FROM AUTHOR]

The fluorobenzene-ammonia van der Waals complex has been studied using a combination of two-color resonance enhanced multiphoton ionization (REMPI) spectroscopy, counterpoise corrected RICC2 ab initio molecular orbital calculations, and multidimensional Franck-Condon analysis. The experimental REMPI spectrum is characterized by a dominant, blueshifted band origin, and weak activity in intermolecular vibrational modes. RICC2 geometry optimizations and numerical vibrational frequency calculations of the neutral ground and first excited states have been performed on a number of different structural isomers of the complex using basis sets ranging from augmented double-zeta to quadruple-zeta level. Ground state basis set superposition error corrected zero-point binding energies show the in-plane σ complex, forming a pseudo-six-membered ring connecting the fluorine atom and ortho-hydrogen, to be consistently the most stable of all six conformations considered, at all levels of theory. Comparison of computed zero-point excitation energies for the most stable π and σ conformers with fluorobenzene show that the σ complex is the only conformer predicted to exhibit a spectral blueshift upon electronic excitation. The computed neutral ground and first excited state geometries and frequencies were used to perform multidimensional Franck-Condon simulations of the S1-S0 vibronic spectrum for each of the most stable conformers. These simulations yielded null spectra for transitions involving the most stable of the π complexes, πbridge; a spectrum rich in strong intermolecular vibrational structure for the second of the π complexes, in complete contrast to the experimental spectrum; and for the σ complex, a spectrum exhibiting weak intermolecular activity in line with that observed experimentally. This last simulation allowed an almost complete vibrational assignment of the intermolecular structure in the REMPI spectrum. The agreement between computational results and experiment overwhelmingly favors assignment of the spectrum to the in-plane σ complex. [ABSTRACT FROM AUTHOR]

The electron trapping or attachment cross section of carbon dioxide (CO2) condensed as thin films on a spacer of Ar is obtained using a simple model for electron trapping in a molecular film and then charge releasing from the same film by photon absorption. The measurements are presented for different electron exposures and impact energies, film thicknesses, and probing photon energies. The cross section for trapping an electron of incident energy between 0 and 5 eV reveals three different attachment processes characterized by a maximum at about 0.75 eV, a structured feature around 2.25 eV, and a shoulder around 3.75 eV. From the measurement of their dependence with the probing photon energy, the two lowest processes produce traps having a vertical electron binding energy of ∼3.5 eV, whereas the highest one yields a slightly higher value of ∼3.7 eV. The 0.75 eV maximum corresponds to the formation of vibrational Feshbach resonances in (CO2)n- anion clusters. The 2.25 eV feature is attributed to the formation of a vibrationally excited 2Πu anion in (CO2)n- clusters, followed by fast decay into its vibrational ground state without undergoing autodetachment. Finally, 3.75 eV shoulder is assigned to the well-known dissociative electron attachment process from 2Πu anion state producing the O- anion in the gas phase and the (CO2)nO- anions in clusters. [ABSTRACT FROM AUTHOR]

Since the classic work of Roothaan [Rev. Mod. Phys. 32, 179 (1960)], the one-electron energies of a ROHF method are known as ambiguous quantities having no physical meaning. Together with this, it is often assumed in present-day computational studies that Koopmans’ theorem is valid in a ROHF method. In this work we analyze the specific dependence of orbital energies on the choice of the basic equations in a ROHF method which are the Euler equations and different forms of the generalized Hartree-Fock equation. We first prove that the one-electron open-shell energies [variant_greek_epsilon]m derived by the Euler equations can be related to the respective ionization potentials Im via the modified Koopmans’ formula Im=-[variant_greek_epsilon]m/fm where fm is an occupation number. As compared to this, neither the closed-shell orbital energies nor the virtual ones derived by the Euler equations can be related to the respective ionization potentials and electron affinities via Koopmans’ theorem. Based on this analysis, we derive the new (canonical) form for the Hamiltonian of the Hartree-Fock equation, the eigenvalues of which obey Koopmans’ theorem for the whole energy spectrum. A discussion of new orbital energies is presented on the examples of a free N atom and an endohedral N@C60 (Ih). The vertical ionization potentials and electron affinities estimated via Koopmans’ theorem are compared with the respective observed data and, for completeness, with the respective estimates derived via a ΔSCF method. The agreement between observed data and their estimates via Koopmans’ theorem is qualitative and, in general, appears to possess the same accuracy level as in the closed-shell SCF. [ABSTRACT FROM AUTHOR]

The inner-shell C 1s photoionization of randomly oriented molecules of the chiral compound carvone has been investigated using circularly polarized synchrotron radiation up to 30 eV above threshold. Binding energies of the C_==O and C_H2== carbon 1s orbitals were determined to be 292.8±0.2 and 289.8±0.2 eV, respectively. The remaining C_–H C 1s levels substantially overlap under an intense central peak centered at 290.5±0.2 eV. The angle-resolved photoemission from the carbonyl carbon C_==O core orbital in pure carvone enantiomers shows a pronounced circular dichroism of ∼6% at the magic angle of 54.7° to the light beam propagation direction. This corresponds to an expected 0°–180° forward-backward electron emission asymmetry of ∼10%. On changing between the R and S enantiomers of carvone the sense or sign of the asymmetry and associated dichroism effectively reverses. The observed circular dichroism, and its energy dependence, is well accounted for by calculations performed in the pure electric dipole approximation. [ABSTRACT FROM AUTHOR]

Model catalysts were prepared by deposition of size-selected Aun (n=1–7) on rutile TiO2(110), and characterized by a combination of electron spectroscopy, ion scattering, temperature-programmed desorption, and pulse-dosing mass spectrometry. CO oxidation activity was found to vary strongly with deposited cluster size, with significant activity appearing at Au3. Activity is not obviously correlated with affinity for CO, or with cluster morphology, but is strongly correlated with the clusters’ ability to bind oxygen (during O2 exposure) on top of the gold. The temporal dependence of CO2 evolution in reaction of O2 pre-exposed samples with CO pulses shows an interesting cluster size dependence. For Au5 and Au6, the peak CO2 production is coincident with the peak CO flux, but for Au3, Au4, and Au7, there are significant induction periods for CO2 evolution. In addition, it is observed that some of the most active cluster sizes have the slowest CO2 evolution rates. Several mechanistic scenarios capable of accounting for the observations are laid out. [ABSTRACT FROM AUTHOR]

InP nanocrystals with narrow size distribution and mean particle diameter tunable from ∼2 up to ∼7 nm were synthesized via the dehalosilylation reaction between InCl3 and tris(trimethylsilyl)phosphine. Specific capping of the nanocrystal surface with a shell of organic ligands protects the nanocrystals from oxidation and provides solubility of the particles in various organic solvents. InP nanocrystals with enhanced photoluminescence (PL) efficiency were obtained from the initial nanocrystals by photoassisted etching of the nanocrystal surface with HF. The resulting PL quantum efficiency of InP nanocrystals dispersed in n-butanol is about three orders of magnitude higher when compared to the nonetched InP samples and approaches ∼40% at room temperature. High-resolution photoelectron spectroscopy with the use of synchrotron radiation was applied to reveal the changes of the nanocrystal surface responsible for the dramatic improvement of the PL efficiency. The analysis of high-resolution P 2p core-level spectra confirmed significant changes of the nanocrystal surface structure induced by the postpreparative treatments and allowed us to propose the description of the etching mechanism. In the nonetched InP nanocrystals, some surface P atoms generate energy states located inside the band gap which provide nonradiative recombination pathways. Photoassisted treatment of InP nanocrystals with HF results in selective removal of these phosphorous atoms from the nanocrystal surface. The reconstructed surface of the etched InP nanocrystals is terminated mainly with In atoms and is efficiently passivated with tri-n-octylphosphine oxide ligands. [ABSTRACT FROM AUTHOR]

Recent experiments using time- and angle-resolved two-photon photoemission (2PPE) spectroscopy at metal/polar adsorbate interfaces succeeded in time-dependent analysis of the process of electron solvation. A fully quantum mechanical, two-dimensional simulation of this process, which explicitly includes laser excitation, is presented here, confirming the origin of characteristic features, such as the experimental observation of an apparently negative dispersion. The inference of the spatial extent of the localized electron states from the angular dependence of the 2PPE spectra has been found to be non-trivial and system-dependent. [ABSTRACT FROM AUTHOR]

The C K-edge photoabsorption and 1s core-level photoemission of pentacene (C22H14) free molecules are experimentally measured, and calculated by self-consistent-field and static-exchange approximation ab initio methods. Six nonequivalent C atoms present in the molecule contribute to the C 1s photoemission spectrum. The complex near-edge structures of the carbon K-edge absorption spectrum present two main groups of discrete transitions between 283 and 288 eV photon energy, due to absorption to π* virtual orbitals, and broader structures at higher energy, involving σ* virtual orbitals. The sharp absorption structures to the π* empty orbitals lay well below the thresholds for the C 1s ionizations, caused by strong excitonic and localization effects. We can definitely explain the C K-edge absorption spectrum as due to both final (virtual) and initial (core) orbital effects, mainly involving excitations to the two lowest-unoccupied molecular orbitals of π* symmetry, from the six chemically shifted C 1s core orbitals. [ABSTRACT FROM AUTHOR]

Core level and valence band photoemission measurements combined with near edge x-ray absorption fine structure measurements were performed on a single C70 layer adsorbed on polycrystalline Al (1 ML-C70/Al) (ML—monolayer), pure and doped with sodium atoms. The data obtained from the pure ML chemisorbed on Al surface show a semiconducting behavior of the system, which is characterized by a covalent bond between the adsorbate and the substrate. The same data show also that the C70 molecules tend to orient themselves with the C5v axis perpendicular to the surface in analogy to what observed for 1 ML-C70/Cu(111). By doping the sample with sodium atoms a charge transfer from the alkali atoms to the lowest unoccupied molecular orbital (LUMO) of the C70 molecules takes place, as underlined by the gradual increasing intensity of the C70 LUMO peak as a function of doping. Nevertheless, no metallic phases are observed for any doping step. © 2005 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The single electron emission behaviors and characteristics from the well-defined quantized energy levels, corresponding to localized electronic states at the dome-structure tips, in single-walled carbon nanotubes (SWNTs) are investigated and illuminated by use of the energy level emission model in combination with the first-principles calculations on the electronic structures. Under the external electric field, the confined electrons are emitted simultaneously from each quantized energy level by virtue of the resonant tunneling effects. With increasing applied voltage, the emission current increases monotonically and exponentially up to the first peak value, and then steps into the increasing and decreasing “sawtoothlike” variations in sequence. The negative differential resistance or conductivity and the maximum current for SWNTs are simulated. The influences of localized electronic states and curvatures of the different closed tips on the single electron emission behaviors of SWNTs are evaluated and discussed. Also a few issues and applications relevant to electron emission of carbon nanotubes are addressed. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We present a high-resolution C1s and O1s x-ray photoemission (XPS) study for condensed films of π-conjugated organic molecules, namely, of the anhydrides 3,4,9,10-perylene-tetracarboxylic acid dianhydride, 1,4,5,8-naphthalene-tetracarboxylic acid dianhydride, 1,8-naphthalene dicarboxylic acid anhydride, and benzoperylene-(1,8)-dicarboxylic acid anhydride as well as the quinoic acenaphthenequinone. Although the functional groups are identical for the anhydrides, the molecules show very different photoemission fine structure thus providing a detailed fingerprint. A simultaneous peak fit analysis of the XPS spectra of all molecules allows to consistently determine the ionization potentials of all chemically different carbon and oxygen atoms. Additional structures in the C1s and O1s spectra are interpreted as shakeup satellites and assigned with the help of singles and doubles configuration interaction calculations. These satellites provide further information on multielectron excitations and must be taken into account for quantitative investigations. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The interatomic Coulombic decay (ICD) in the Ne dimer is discussed in view of the recent experimental results. The ICD electron spectrum and the kinetic energy release of the Ne+ fragments resulting after Coulomb explosion of Ne22+ are computed and compared to the measured ones. A very good agreement is found, confirming the dynamics predicted for this decay mechanism. The effect of the temperature on the electron spectrum is briefly investigated.© 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Adiabatic ionization energy (AIE) and two-color threshold ion vibrational spectra of p-fluorostyrene have been measured by mass analyzed threshold ionization (MATI) method via three different intermediate levels in the first excited state, vibrationless S1 origin, 421411, and 231 vibronic levels. Features of the ion vibrational spectra indicates that the geometry of the molecular ion including the conformation of the vinyl chain in the ionic ground state (D0) is almost identical to that of its neutral ground state (S0), and ionization has very little effect on the vibrational potentials of the aromatic ring modes. Comparison of the AIE with the reported value of styrene shows that fluorination at the para position of the aromatic ring has little effect on energy of the electron ejected in ionization process from the styrene chromophore. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We compare theoretical results on statistical electron emission from electronically hot but vibrationally cold C[SUB60] with recent experimental results involving excitation with ultrashort laser pulses. Both photoelectron spectra and ion yields of C[SUB60] as well as fragment ions are compared with the predictions of the statistical electron emission model. Quantitative agreement is obtained between the calculations and the experimentally measured photoelectron spectra, total ion yields and singly-doubly charged branching ratio. The electron-phonon coupling time is fitted to a few hundred femtoseconds, consistent with independent measurements. The data allow a determination of the thermal properties of the electron system, which are consistent with the theoretical input. The data also allow a fit of the averaged photon absorption cross section. [ABSTRACT FROM AUTHOR]

Selective ion photodesorption from NO adsorbed on Si(111)7 × 7 has been investigated by core electron excitation. In a first step, the adsorption of nitric oxide on the clean Si(111)7 × 7 surface has been studied over the temperature range from 15 K to room temperature using ultraviolet photoemission spectroscopy, x-ray photoemission spectroscopy, and photon stimulated desorption of ions. At room temperature, NO was found to react with the surface silicon adatoms to form silicon oxynitride sites (SiN[SUBx]O[SUBy]). At low temperature and low coverage, in addition to a permanent dissociative adsorption, a transient molecular adsorption has been observed. At low temperature and high coverage, NO was found to be condensed. In a second step, ion photodesorption induced by core electron excitation of both the adsorbate and the substrate has been studied. Quite interestingly, selective ion photodesorption was observed by core electron excitation of both dissociatively adsorbed NO at room temperature and condensed NO molecules at low temperature. An "ultrafast dissociation" is proposed to explain the results of condensed NO molecules at low temperature. [ABSTRACT FROM AUTHOR]

The electronic decay of the Ne[SUP-,SUB2] cation by electron emission is studied. This interatomic Coulombic decay (ICD) follows inner valence ionization of the neon dimer and the decay rate depends strongly on the internuclear distance. The time-dependent theory of wave packet propagation is applied allowing to follow the evolution of the decay process in time. The impact of nuclear dynamics on the decay spectrum is investigated. Among others, the spectrum corresponding to the decay of the 2 [SUP2]Σ[SUP+,SUB2] electronic state of the Ne[SUP-,SUB2] cation is calculated at different times. Its characteristics are found to be influenced considerably by the nuclear motion. A pronounced oscillatory structure appears: Its origin is explained and related to the interatomic nature of the ICD process. Particularly enlightening for the understanding of the ICD process is the analysis of the total energy distribution in the final system resulting after the fragmentation of the Ne[SUP2+,SUB2] dication, produced by the ICD of Ne[SUP+,SUB2]. [ABSTRACT FROM AUTHOR]

We introduce an analytical model designed to capture the most important features of the electronic matrix elements arising in non-Born-Oppenheimer couplings between a bound anion state and a neutral-molecule-plus-ejected-electron state. In this particle-in-a-radial-box model, vibrations are assumed to cause modulations in the depth (U[SUP0]) and length (L) parameters of the box. The most important elements of this model are that L is chosen to reproduce the proper dependence of the radial size of the anion's orbital on electron binding energy, and U[SUP0] is chosen to produce the correct electron affinity. Within this model, which is shown to be consistent with trends seen in ab initio calculations of associated electron ejection rates, the coupling matrix elements can be evaluated analytically to provide closed-form expressions for how the rates depend upon (1) the kinetic energy of the ejected electron, (2) the energy spacing between the anion and neutral energy surfaces as a function of geometry, (3) the difference in the slopes of the anion and neutral energy surfaces, and (4) overlaps of the neutral's vibration-rotation wave function with the spatial derivative of that of the anion. [ABSTRACT FROM AUTHOR]

The interaction of C+60 and C+70 ion beams with a surface of highly oriented pyrolitic graphite was investigated by probing the ionization and fragmentation rates of scattered species within a time window of 20 μs following impact. Neutralization/reionization and fragmentation behavior was observed and followed by a pulsed deflection field applied to the surface at variable delays after the collision event. An almost complete collisional neutralization of the incident projectile was found. For an impact energy of 140–180 eV, a significant part of the scattered species was found to reionize by delayed electron emission within the experimental time window. The associated reionization and fragmentation kinetics were modeled with a system of differential equations assuming a simple unimolecular reaction diagram. Rate constants for delayed ionization and fragmentation were calculated as functions of internal energy and respective activation energies with the ‘‘finite heat bath’’ model (Klots) and the Rice–Ramsperger–Kassel–Marcus expression, respectively. The calculated and measured (deflection field delay dependent) ion intensities were compared in a fit procedure. The best fit led to an activation energy for the fragmentation of C+60 (C+60→C+58+C2) of 6.6±0.5 eV. This translates to an activation energy of 7.1±0.5 eV for the fragmentation of neutral C60 (using the experimentally determined ionization potential of C58). For C+70 we obtained an identical (within error) activation energy for fragmentation (C+70→C+68+C2) of 6.6±0.5 eV. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We report on delayed electron emission from free tungsten clusters, excited by light from a Q-switched YAG laser. Using a novel ion extraction lens, electron emission can be analyzed over a time range of 50 ns–5 μs after the laser pulse without interference from prompt ions. All clusters of size 5≤n≤40 exhibit delayed emission on this time scale, while delayed emission from smaller clusters does not occur. We analyze the time dependence and size dependence of the emission rate for different wavelengths and fluences. The yield of delayed ions may exceed the yield of prompt ions for intermediate laser fluences. A statistical model is proposed which is based on the assumption that energy randomization in the electronically excited clusters proceeds much faster than in 50 ns, i.e., that the observed phenomenon is the (cluster) analog of thermionic emission. Good agreement with all our experimental findings is achieved, although the model invokes only one adjustable parameter. We argue that other delayed deexcitation channels, namely, emission of atoms or photons, are not significant under our experimental conditions. [ABSTRACT FROM AUTHOR]