Valence Ionization Spectra Research Articles

Electronic Characterization of Glycolaldehyde: Experimental and Theoretical Insights from the Core- and Valence-Level Spectroscopy.

The core-level electron excitation and ionization spectra of glycolaldehyde have been investigated by photoabsorption and photoemission spectroscopy at both carbon and oxygen K-edges; the valence ionization spectra were also recorded by photoelectron spectroscopy in the UV-vis region. The spectra are interpreted by means of ab initio calculations based on the equation-of-motion coupled cluster singles and doubles (EOM-CCSD) and coupled cluster singles, doubles, and perturbative are in good agreement with the experimental results, and many of the observed features are assigned. The photoabsorption spectra are not only dominated by transitions from core-level orbitals to unoccupied π and σ orbitals but also show structures due to Rydberg transitions.

Investigation of the valence ionization spectrum of chromium carbonyl using an ab initio quantum dynamical approach.

The nuclear dynamics of the chromium carbonyl cation following an ionization process corresponding to the 2T2g ← 1A1g transition is studied theoretically, for the first time, using a fully quantal approach as well as high levels of the ab initio electronic structure and semiempirical density functional theory (DFT) methods. The photoelectron spectrum is calculated by the construction of a Hamiltonian model, in which the two totally symmetric modes ν19 (the Cr-C stretching mode) and ν39 (the C-O stretching mode) together with the spin-orbit (SO) coupling up to the zeroth-order SO splitting are treated. The potential energy curves along these two vibrational modes are computed by using the DFT. The simulated photoelectron spectrum is found to be in good agreement with the corresponding experimental one, leading to the conclusion that the potential energy surfaces and the diabatic population analysis are accurately determined. Our calculation confirms that the vibrational modes ν19 and ν39 are the vibrational progression of the valence ionization spectrum of the chromium carbonyl cation.

The cage fragmentation of doubly ionized norbornane: A Born-Oppenheimer molecular dynamics study

Results are reported of Born-Oppenheimer molecular dynamics calculations performed on the singlet dication of norbornane, starting from the neutral ground state geometry. Intramolecular rearrangements and charge dissociation processes, which probably take place in the innermost valence ionization spectrum, are discussed and an analysis by means of natural bond orders and Wiberg bond indices has been performed. The outcome of these simulations and the observed cage fragmentation might explain a tremendous rise of electron-impact (e, 2e) ionization cross sections of norbornane at electron binding energies around the double-ionization threshold.

Valence photoelectron spectra of alkali bromides calculated within the propagator theory

The valence ionization spectra covering the binding energy range 0-45 eV of alkali bromide XBr (X = Li, Na, K, Rb) vapors are studied within the framework of the propagator theory. Relativistic Algebraic Diagrammatic Construction calculations have been carried out in order to investigate photoionization processes and to describe molecular electronic structure. Theoretical results are compared with available experimental data.

Intramolecular interactions of L‐phenylalanine: Valence ionization spectra and orbital momentum distributions of its fragment molecules

Intramolecular interactions between fragments of L-phenylalanine, i.e., phenyl and alaninyl, have been investigated using dual space analysis (DSA) quantum mechanically. Valence space photoelectron spectra (PES), orbital energy topology and correlation diagram, as well as orbital momentum distributions (MDs) of L-phenylalanine, benzene and L-alanine are studied using density functional theory methods. While fully resolved experimental PES of L-phenylalanine is not yet available, our simulated PES reproduces major features of the experimental measurement. For benzene, the simulated orbital MDs for 1e(1g) and 1a(2u) orbitals also agree well with those measured using electron momentum spectra. Our theoretical models are then applied to reveal intramolecular interactions of the species on an orbital base, using DSA. Valence orbitals of L-phenylalanine can be essentially deduced into contributions from its fragments such as phenyl and alaninyl as well as their interactions. The fragment orbitals inherit properties of their parent species in energy and shape (ie., MDs). Phenylalanine orbitals show strong bonding in the energy range of 14-20 eV, rather than outside of this region. This study presents a competent orbital based fragments-in-molecules picture in the valence space, which supports the fragment molecular orbital picture and building block principle in valence space. The optimized structures of the molecules are represented using the recently developed interactive 3D-PDF technique.

Electron Momentum Spectroscopy of pyrimidine at the benchmark ADC(3) level

An extensive study of the valence electronic structure, ionization spectrum and electron momentum distributions of pyrimidine is presented, on the ground of accurate calculations of one-electron and shake-up ionization energies and of the related Dyson orbitals, using one-particle Green’s Function theory in conjunction with the third-order Algebraic Diagrammatic Construction scheme [ADC(3)]. Comparison is made with results obtained from standard (B3LYP) Kohn–Sham orbitals and recent measurements employing Electron Momentum Spectroscopy. Quantitative insights into the experimental momentum distributions are amenable without resorting to any empirical rescaling of pole strengths, provided band overlaps and relaxation effects are properly accounted for.

Quantum Chemical Study of Conformational Fingerprints in the Photoelectron Spectra and (e, 2e) Electron Momentum Distributions ofn-Hexane

The main purpose of the present work is to simulate from many-body quantum mechanical calculations the results of experimental studies of the valence electronic structure of n-hexane employing photoelectron spectroscopy (PES) and electron momentum spectroscopy (EMS). This study is based on calculations of the valence ionization spectra and spherically averaged (e, 2e) electron momentum distributions for each known conformer by means of one-particle Green's function [1p-GF] theory along with the third-order algebraic diagrammatic construction [ADC(3)] scheme and using Kohn-Sham orbitals derived from DFT calculations employing the Becke 3-parameters Lee-Yang-Parr (B3LYP) functional as approximations to Dyson orbitals. A first thermostatistical analysis of these spectra and momentum distributions employs recent estimations at the W1h level of conformational energy differences, by Gruzman et al. [J. Phys. Chem. A 2009, 113, 11974], and of correspondingly obtained conformer weights using MP2 geometrical, vibrational, and rotational data in thermostatistical calculations of partition functions beyond the level of the rigid rotor-harmonic oscillator approximation. Comparison is made with the results of a focal point analysis of these energy differences using this time B3LYP geometries and the corresponding vibrational and rotational partition functions in the thermostatistical analysis. Large differences are observed between these two thermochemical models, especially because of strong variations in the contributions of hindered rotations to relative entropies. In contrast, the individual ionization spectra or momentum profiles are almost insensitive to the employed geometry. This study confirms the great sensitivity of valence ionization bands and (e, 2e) momentum distributions on the molecular conformation and sheds further light on spectral fingerprints of through-space methylenic hyperconjugation, in both PES and EMS experiments.

Valence ionization spectra of group six metal hexacarbonyls studied by the symmetry-adapted cluster-configuration interaction method

The valence ionization spectra up to 20 eV of group six metal carbonyls, chromium hexacarbonyl, molybdenum hexacarbonyl, and tungsten hexacarbonyl were studied by the symmetry-adapted cluster-configuration interaction (SAC-CI) method. The assignments of the spectra are given based on reliable SAC-CI calculations. The relativistic effects including the spin-orbit effects are important for the ionization spectrum of W(CO)(6). The relation between the metal-CO distance and ionization energies was examined. The lowest ionization energies of the three metal carbonyls are approximately the same because of the energy dependence of the metal-CO length and relativistic effects. In Cr(CO)(6), the Cr-CO interaction significantly increases the lowest ionization energy in comparison with Mo(CO)(6) and W(CO)(6) because of the relatively short metal-CO bond length. The relativistic effect reduces the lowest ionization energy of W(CO)(6) because the effective core potential of 5d electrons is more efficiently screened as a result of the relativistic contraction of the inner electrons.

Electronic Structure of the Azide Group in 3¢-Azido-3¢-deoxythymidine (AZT) Compared to Small Azide Compounds

Theoretical calculations for some structural and electronic properties of the azide moiety in the nucleoside reverse transcriptase (RT) inhibitor 3′-Azido-3′-deoxythymidine (AZT) are reported. These properties, which include geometrical properties in three dimensional space, Hirshfeld charges, electrostatic potential (MEP), vibrational frequencies, and core and valence ionization spectra, are employed to study how the azide group is affected by the presence of a larger fragment. For this purpose, two small but important organic azides, hydrazoic acid and methyl azide, are also considered. The general features of trans Cs configuration for RNNN fragments [1] is distorted in the large AZT bio-molecule. Hirshfeld charge analysis shows charges are reallocated more evenly on azide when the donor group R is not a single atom. Infrared and photoelectron spectra reveal different aspects of the compounds. In conclusion, the electronic structural properties of the compounds depend on the specific property, the local structure and chemical environment of a species.

Correlation effects in the valence ionization spectra of large conjugated molecules: p-Benzoquinone, anthracenequinone and pentacenequinone

A review of an extensive series of theoretical studies of the valence one-electron and shake-up ionization spectra of polycyclic aromatic hydrocarbons is presented, along with new results for three planar quinone derivatives, obtained using one-particle Green's function (1p-GF) theory along with the so-called third-order algebraic diagrammatic construction [ADC(3)] scheme and the outer-valence Green's function (OVGF) approximation. These results confirm both for the π- and σ-band systems the rapid spreading, upon increasing system size, of many shake-up lines with significant intensities at outer-valence energies. Linear regressions demonstrate that with large conjugated molecules the location of the shake-up onset in the π-band system is merely determined by the energy of the frontier (HOMO, LUMO) orbitals. Electron pair removal effects are found to almost compensate the electron relaxation effects induced by ionization of π-levels, whereas the latter effects strongly dominate the ionization of more localized lone-pair ( n) levels, and may lead to inversions of the energy order of Hartree–Fock (HF) orbitals. Therefore, although it increases upon a lowering of the HF band gap, and thus upon an increase of system size, the dependence of the one-electron ionization energies onto the quality of the basis set is lesser for π-levels than for σ-levels relating to electron lone pairs ( n). Basis sets of triple- and quadruple-zeta quality are therefore required for treatments of the outermost π- and n-ionization energies approaching chemical accuracy [1 kcal/mol, i.e. 0.04 eV]. When 1p-GF theory invalidates Koopmans’ theorem and the energy order of HF orbitals, a comparison with Kohn–Sham orbital energies confirms the validity of the meta-Koopmans’ theorem for density functional theory.

Probing molecular conformations in momentum space: The case of n-pentane

A comprehensive study, throughout the valence region, of the electronic structure and electron momentum density distributions of the four conformational isomers of n-pentane is presented. Theoretical (e,2e) valence ionization spectra at high electron impact energies (1200 eV+electron binding energy) and at azimuthal angles ranging from 0 degrees to 10 degrees in a noncoplanar symmetric kinematical setup are generated according to the results of large scale one-particle Green's function calculations of Dyson orbitals and related electron binding energies, using the third-order algebraic-diagrammatic construction [ADC(3)] scheme. The results of a focal point analysis (FPA) of relative conformer energies [A. Salam and M. S. Deleuze, J. Chem. Phys. 116, 1296 (2002)] and improved thermodynamical calculations accounting for hindered rotations are also employed in order to quantitatively evaluate the abundance of each conformer in the gas phase at room temperature and reliably predict the outcome of experiments on n-pentane employing high resolution electron momentum spectroscopy. Comparison with available photoelectron measurements confirms the suggestion that, due to entropy effects, the trans-gauche (tg) conformer strongly dominates the conformational mixture characterizing n-pentane at room temperature. Our simulations demonstrate therefore that experimental measurements of (e,2e) valence ionization spectra and electron momentum distributions would very consistently and straightforwardly image the topological changes and energy variations that molecular orbitals undergo due to torsion of the carbon backbone. The strongest fingerprints for the most stable conformer (tt) are found for the electron momentum distributions associated with ionization channels at the top of the inner-valence region, which sensitively image the development of methylenic hyperconjugation in all-staggered n-alkane chains.

Ionization and double ionization of small water clusters

The valence ionization and double ionization spectra of the water molecule, of the water dimer, and the cyclic water clusters (H2O)3 and (H2O)4 are calculated by ab initio Green's function methods and discussed in some detail. Particular attention is paid to the analysis of the development of the spectra with increasing cluster size. Electronic decay following inner valence ionization is addressed and a crude estimate for the kinetic energy spectrum of the secondary electrons is given for the clusters.

Valence ionization spectra of 4π-electron molecules with low-lying satellites involving n–π* and π–π* transitions§

The valence ionization spectra up to 25–30 eV of the 4π-electron molecules, butadiene, acrolein, glyoxal, methylenecyclopropene and methylenecyclopropane were investigated by the SAC-CI method. Accurate theoretical assignments of the spectra were given and further the natures of the low-lying satellites were examined. Acrolein and glyoxal have the low-lying satellites of n−1π-1π* and n-2π* states and the outermost satellites are lower than the π-2π* state of butadiene. However, their intensities are very small, since they do not effectively interact with the main peaks. The π-2π* state of methylenecyclopropene with constrained π-conjugation was calculated to be much higher than that of butadiene, though the first IP is lower. In these spectra, some split peaks were calculated at 15–16 eV and the continuous shake-up states were obtained in the region higher than ∼18 eV. §Dedicated to Professor Michael A. Robb on the occasion of his 60th birthday.

Probing the Shape and Stereochemistry of Molecular Orbitals in Locally Flexible Aromatic Chains: A Penning Ionization Electron Spectroscopy and Green's Function Study of the Electronic Structure of Biphenyl

We report on the results of an exhaustive study of the interplay between the valence electronic structure, the topology and reactivity of orbitals, and the molecular structure of biphenyl by means of Penning ionization electron spectroscopy in the gas phase upon collision with metastable He*(2(3)S) atoms. The measurements are compared with one-particle Green's function calculations of one-electron and shake-up valence ionization spectra employing the third-order algebraic diagrammatic construction scheme [ADC(3)]. Penning ionization intensities are also analyzed by means of the exterior electron-density model and comparison with photoelectron spectra: in contrast with the lines originating from sigma orbitals, ionization lines belonging to the pi-band system have large Penning ionization cross sections due to their greater extent outside the molecular van der Waals surface. The involved chemi-ionization processes are further experimentally investigated using collision-energy-resolved Penning ionization electron spectroscopy. The cross sections of pi-ionization bands exhibit a markedly negative collision-energy dependence and indicate that the interaction potential that prevails between the molecule and the He*(2(3)S) atom is strongly attractive in the pi-orbital region. On the other hand, the partial ionization cross sections pertaining to sigma-ionization channels are characterized by more limited collision-energy dependencies, as a consequence of rather repulsive interactions within the sigma-orbital region. A comparison of ADC(3) simulations with the Penning ionization electron spectra and UV photoelectron spectra measured by Kubota et al. [Chem. Phys. Lett. 1980, 74, 409] on thin films of biphenyl deposited at 170 and 109 K on copper demonstrates that biphenyl molecules lying at the surface of polycrystalline layers adopt predominantly a planar configuration, whereas within an amorphous sample most molecules have twisted structures similar to those prevailing in the gas phase.

Theoretical fine spectroscopy with symmetry-adapted-cluster configuration-interaction method: Outer- and inner-valence ionization spectra of furan, pyrrole, and thiophene

Theoretical fine spectroscopy has been performed for the valence ionization spectra of furan, pyrrole, and thiophene with the symmetry-adapted-cluster configuration-interaction general-R method. The present method described that the pi(1) state interacts with the pi(3) (-2)pi*, pi(2) (-2)pi*, and pi(2) (-1)pi(3) (-1)pi* shake-up states providing the split peaks and the outer-valence satellites, both of which are in agreement with the experiments. The intensity distributions were analyzed in detail for the inner-valence region. In particular, for furan, theoretical intensities were successfully compared with the intensity measured by the electron momentum spectroscopy. The interactions of the 3b(2) and 5a(1) states with the shake-up states were remarkable for furan and pyrrole, while the 4b(2) state of thiophene had relatively large intensity.

The effect of the Gaunt interaction on the molecular ionization spectra of CO, H2S and TlH

In this work we calculate the outer valence ionization spectra of the CO, H2S and TlH molecules by application of the recently developed Dirac–Hartree–Fock one-particle propagator technique. The molecules hereby serve as representatives of light and heavy systems and the influence of the Gaunt term on the spectral structure is investigated. The Gaunt term is a first correction to the non-covariant Coulomb repulsion term and has a considerable effect on the electronic ground state energy and also affects properties mainly determined by the inner-core region of the wavefunction where the electrons are fast. In order to reveal a possible influence of the Gaunt correction on the valence photoelectron (PE) spectra of heavy systems a consistent theoretical description of electron correlation and relativity is necessary. It was observed that the Gaunt correction plays only a minor role for the valence PE spectra even in heavy systems such as TlH and can safely be neglected.

Theoretical investigation on the valence ionization spectra of Cl2O, ClOOCl, and F2O by correlation-based configuration interaction methods

We report on theoretical valence ionization spectra of molecules relevant in the stratosphere photochemistry obtained by all-electron SAC–CI (symmetry-adapted-cluster–configuration-interaction) calculations. Vertical ionization energies and pole strengths of the one- and two-electron processes of Cl2O and F2O were calculated beyond the energy region so far explored in the few other available theoretical and experimental studies to interpret the electronic structure of these molecules. Early and recently proposed incomplete assignments of the available He I photoelectron spectra are discussed and completed at least up to 20–21 eV binding energy on firmer grounds relying on valuable and accurate results based on different basis sets and an adequate treatment of electron correlations. Our theoretical data predict satellite states of Cl2O starting already in the outer-valence region because of strong correlation effects; the (2b1−1), (8a1−1), and (6b2−1) states interact with the two-electron processes and they split into more-than-two peaks. On the other hand, Koopmans’ picture is valid for the main peaks of F2O and no prominent satellites with strong intensity were found in the outer-valence region. The (4b2−1), (6a1−1), and (1a2−1) were attributed to the second band of F2O, for which different assignments or orderings of the states have previously been proposed in some experimental and theoretical works. Differences of the valence-ionization spectra of Cl2O and F2O for the appearance of the satellites in the intermediate energy region have been discussed with the aid of the calculated ionization potentials and excitation energies. For ClOOCl, we have presented the first theoretical low-energy ionization spectrum and discussed the character of the calculated states referring to the available ionization spectra of ClO radical.

Valence One-Electron and Shake-Up Ionization Bands of Carbon Clusters. III. The Cn(n = 5,7,9,11) Rings

The 1h (one-hole) and 2h-1p (two-hole; one-particle) shake-up bands in the valence ionization spectrum of odd-membered carbon rings (C5, C7, C9, C11) are investigated by means of the third-order algebraic diagrammatic construction [ADC(3)] scheme for the one-particle Green's function. Despite a severe dispersion of the σ- and π- ionization intensity over intricately dense sets of satellites, the present study undoubtedly confirms that structural fingerprints in ionization spectra could be usefully exploited to discriminate the cyclic C5, C7, C9, and C11 species from their linear counterparts in plasma conditions. Such spectra could also be used to indirectly trace very fine details of the molecular structure, such as bond length alternations, out-of-plane distortions, or the strength of cyclic strains. Both structurally and electronically, the cyclic isomers of the C5 and C9 clusters must be described as even-twisted cumulenic tori, whereas the C7 and C11 cyclic species are simply planar polyynic rings. In comparison with their linear counterparts, all species display an intrinsically lower propensity to electronic excitations, marked by a rather significant increase of the fundamental HOMO−LUMO band gap. On the other hand, the lower symmetry of the cyclic clusters, C5 and C9 in particular, permits many more configuration interactions in the cation. The ultimate outcome of these two opposite factors is, overall, a severe enhancement of the shake-up fragmentation of ionization bands, compared with the linear isomers.

Outer-Valence Green's Function Study of Cycloalkane and Cycloalkyl−Alkane Compounds

In this contribution, we report a quantitative analysis of gas-phase ultraviolet photoelectron spectra (HeI) of model compounds such as cyclopentane, cyclohexane, and cycloalkyl−1-alkane derivatives, by means of outer-valence Green's function (OVGF) calculations. These spectra and their OVGF theoretical simulations are also compared with UPS measurements on α-cycloalkyl−ω-alkanethiol monolayers adsorbed on gold. This theoretical study analyses the dependence of valence ionization spectra of saturated hydrocarbons on structural characteristics (configuration, conformation, and torsional flexibility).

Valence one-electron and shake-up ionization bands of carbon clusters. II. The Cn (n=4,6,8,10) rings

The 1h (one-hole) and 2h-1p (two-hole; one-particle) shake-up bands in the valence ionization spectrum of small carbon rings (C4, C6, C8, C10) are investigated up to 40 eV, using the one-particle Green’s Function approach. Calculations have been performed using both the third-order algebraic diagrammatic construction [ADC(3)] scheme and the outer-valence Green’s function (OVGF) approximation. The obtained ADC(3) results indicate a major fragmentation of lines into complex sets of 2h-1p satellites, even for outer-levels of Π-character. The simulated ionization spectra provide striking structural signatures for the carbon rings vs. the carbon chains, that could be usefully exploited to discriminate these two types of clusters in plasma conditions. In general and in spite of the extent of the shake-up spreading, one can rather easily trace from our convolutions the energy degeneracies, and in the outer-valence region, the Σ–Π near-energy degeneracies which characterize the electronic structure of these doubly conjugated rings. In relationship to its doubly antiaromatic (i.e., polyynic) nature and lower symmetry, C8 is subject to stronger many-body effects than the other rings. ADC(3) calculations on first-order saddle point forms of C6 and C10 indicate that thermal averaging of conformations could have a substantial effect on the spectral bands, in particular the shake-up ones.