Vincenzo Carravetta

Direct, atomic orbital, static exchange calculations of photoabsorption spectra of large molecules and clusters

We outline a method for static exchange calculations of the photoabsorption spectra of large molecules and clusters, with particular attention to near-edge X-ray absorption fine structures. The static exchange matrices are determined directly from one- and two-electron integrals computed in the atomic orbital basis. Expensive storing and retrieving of integrals is thereby avoided and applications on large species possible. As a demonstration we calculate the discrete and continuum near-edge X-ray absorption spectra for molecules in the n-alkylnitrile sequence CH3(CH2)nCN.

Electronic structure of copper phthalocyanine: An experimental and theoretical study of occupied and unoccupied levels

An experimental and theoretical study of the electronic structure of copper phthalocyanine (CuPc) molecule is presented. We performed x-ray photoemission spectroscopy (XPS) and photoabsorption [x-ray absorption near-edge structure (XANES)] gas phase experiments and we compared the results with self-consistent field, density functional theory (DFT), and static-exchange theoretical calculations. In addition, ultraviolet photoelectron spectra (UPS) allowed disentangling several outer molecular orbitals. A detailed study of the two highest occupied orbitals (having a(1u) and b(1g) symmetries) is presented: the high energy resolution available for UPS measurements allowed resolving an extra feature assigned to vibrational stretching in the pyrrole rings. This observation, together with the computed DFT electron density distributions of the outer valence orbitals, suggests that the a(1u) orbital (the highest occupied molecular orbital) is mainly localized on the carbon atoms of pyrrole rings and it is doubly occupied, while the b(1g) orbital, singly occupied, is mainly localized on the Cu atom. Ab initio calculations of XPS and XANES spectra at carbon K edge of CuPc are also presented. The comparison between experiment and theory revealed that, in spite of being formally not equivalent, carbon atoms of the benzene rings experience a similar electronic environment. Carbon K-edge absorption spectra were interpreted in terms of different contributions coming from chemically shifted C 1s orbitals of the nonequivalent carbon atoms on the inner ring of the molecule formed by the sequence of CN bonds and on the benzene rings, respectively, and also in terms of different electronic distributions of the excited lowest unoccupied molecular orbital (LUMO) and LUMO+1. In particular, the degenerate LUMO appears to be mostly localized on the inner pyrrole ring.

Oxidation states of graphene: Insights from computational spectroscopy

When it is oxidized, graphite can be easily exfoliated forming graphene oxide (GO). GO is a critical intermediate for massive production of graphene, and it is also an important material with various application potentials. With many different oxidation species randomly distributed on the basal plane, GO has a complicated nonstoichiometric atomic structure that is still not well understood in spite of intensive studies involving many experimental techniques. Controversies often exist in experimental data interpretation. We report here a first principles study on binding energy of carbon 1s orbital in GO. The calculated results can be well used to interpret experimental x-ray photoelectron spectroscopy (XPS) data and provide a unified spectral assignment. Based on the first principles understanding of XPS, a GO structure model containing new oxidation species epoxy pair and epoxy-hydroxy pair is proposed. Our results demonstrate that first principles computational spectroscopy provides a powerful means to investigate GO structure.

Electronic structure of aromatic amino acids studied by soft x-ray spectroscopy

The electronic structure of phenylalanine, tyrosine, tryptophan, and 3-methylindole in the gas phase was investigated by x-ray photoemission spectroscopy (XPS) and near edge x-ray absorption fine structure (NEXAFS) spectroscopy at the C, N, and O K-edges. The XPS spectra have been calculated for the four principal conformers of each amino acid, and the spectra weighted by the Boltzmann population ratios calculated from published free energies. Instead of the single peaks expected from the stoichiometry of the compounds, the N 1s core level spectra of phenylalanine and tryptophan show features indicating that more than one conformer is present. The calculations reproduce the experimental features. The C and O 1s spectra do not show evident effects due to conformational isomerism. The calculations predict that such effects are small for carbon, and for oxygen it appears that only broadening occurs. The carbon K-edge NEXAFS spectra of these aromatic amino acids are similar to the published data of the corresponding molecules in the solid state, but show more structure due to the higher resolution in the present study. The N K-edge spectra of tryptophan and 3-methylindole differ from phenylalanine and tyrosine, as the first two both contain a nitrogen atom located in a pyrrole ring. The nitrogen K-edge NEXAFS spectra of aromatic amino acids do not show any measurable effects due to conformational isomerism, in contrast to the photoemission results. Calculations support this result and show that variations of the vertical excitation energies of different conformers are small, and cannot be resolved in the present experiment. The O NEXAFS spectra of these three aromatic compounds are very similar to other, simpler amino acids, which have been studied previously.

Core Level Study of Alanine and Threonine

Core level X-ray photoemission spectra (XPS) and near edge X-ray absorption fine structure (NEXAFS) spectra of alanine and threonine in the gas phase have been measured at the carbon, nitrogen, and oxygen K edges and interpreted in the light of theoretical calculations. For the computations, a set of approximations is made which allows sufficiently accurate calculations of several conformers to be performed in reasonable computing time. The accuracy has been checked by comparing results obtained for proline to our previous, higher level calculations. The photoemission spectra at the carbon and oxygen edges are assigned and compared. The nitrogen 1s photoemission peaks show anomalous broadening which we relate to the populations and types of conformers. The carbon K-edge NEXAFS spectra of alanine and threonine are compared with our previous data on glycine and resonances assigned accordingly. The nitrogen K-edge NEXAFS spectra of alanine and threonine do not show measurable effects due to the population of conformers, in contrast to the photoemission results. At the oxygen K edge, the spectra of these amino acids are similar with two prominent peaks assigned to transitions of O 1s electrons from the oxo and hydroxyl groups to vacant pi* and sigma* orbitals and additional intensity for threonine due to the second OH group. Conformer effects are observable in photoemission but appear to be more difficult to resolve in photoabsorption. We explain this by energetic shifts of opposite sign for the core hole states and unoccupied orbitals, which causes partial cancelation in NEXAFS but not in photoemission.

Near-edge core photoabsorption in polyacenes: model molecules for graphite

Abstract We present calculations on near-edge X-ray absorption fine structures (NEXAFS) for pyrene and for molecules in the polyacene series; benzene, naphthalene, anthracene, and tetracene. Results from these model molecules are used to characterize the NEXAFS spectrum of graphite. Calculations are carried out with an ab inition static exchange method recently made applicable to large species. The trends of different NEXAFS features with respect to the site of ionization and with respect to the number of hydrocarbon rings are studied. In contrast to the linear polyene series, which shows a decay in intensity and delocalization of the first π ∗ level with the size of the system, the polyacene series shows a rapid build-up of π ∗ excitons, with constant energy and intensity, conforming with a recent experimental observation of a π ∗ excitonic state for graphite. The excitonic character is though different for the symmetry non-related carbon atoms in the polyacene series. The salient double peak feature in these spectra is firmly established as due to chemical shifts. Except for the end atoms, there is only one π ∗ exciton per site. Possible excitonic character of the σ ∗ resonances in graphite is discussed in terms of trends found for the polyacene series of the NEXAFS spectra.

Core-shell photoabsorption and photoelectron spectra of gas-phase pentacene: experiment and theory.

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 pi* virtual orbitals, and broader structures at higher energy, involving sigma* virtual orbitals. The sharp absorption structures to the pi* 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 pi* symmetry, from the six chemically shifted C 1s core orbitals.

A theoretical study of the near-edge x-ray absorption spectra of some larger amino acids

The near-edge x-ray absorption (NEXAFS) spectra at the carbon K edge of the five amino acids, glycine, phenylalanine, histidine, tyrosine, and tryptophan, have been computed by the ab initio static-exchange method and compared to recently collected experimental data. The analysis of the spectra is carried out on the basis of a building-block decomposition taking advantage of the separate spectra for each carbon center. It is shown that different subunits can be clearly identified by such an analysis and that their spectral features remain largely unperturbed in the different molecules. The results support the suggestion by Boese et al. [J. Electron. Spectrosc. Relat. Phenom. 85, 9 (1997)] that differences in the NEXAFS spectra might be used for mapping proteins which differ in their amino acid content.

Near‐edge core photoabsorption in polyenes

We present calculations on near‐edge x‐ray absorption fine structures (NEXAFS) in polyenes based on a newly derived direct, atomic orbital, static exchange method. The trends of different NEXAFS features with respect to the site of ionization and with respect to the number of ethylene subunits in the oligomer chain are studied. The trends for energies and intensities are found to be regular and alternant. A substantial reduction of the π to σ intensity ratios with the number of ethylene subunits is predicted, indicating delocalization of the screening π electrons. In contrast to NEXAFS spectra of a heterogeneous group attached to hydrocarbon chains, the NEXAFS spectra of oligomer sequences, here the polyenes, converge slowly. The building block principle is less appropriate than the final state rule for rationalizing the present findings.

Molecular photoionization cross sections and asymmetry parameters by L2 basis functions calculations: H2O

A K‐matrix technique using a basis set of square‐integrable functions is applied to the calculation of differential photoionization cross sections in molecules. Continuum orbitals are variationally determined in the static‐exchange approximation of the ion field. Integrated photoionization cross sections and asymmetry parameters β of the three main valence ionization processes in H2O are calculated, in the independent channel approximation, for the photon energy in the range of 14–50 eV and compared with the available experimental data.