M. de Simone

Time dependent density functional theory of core electrons excitations

Abstract The Time Dependent Density Functional Theory (TD-DFT) method implemented in the ADF program has been extended to treat core electrons excitations. The scheme consists to reduce the complete one-electron excited configurations space to the subspace where only the core electrons are excited. The scheme has been applied to the Ti 1s, Ti 2p and Cl 2p core excitations of TiCl4, employing different basis sets and exchange correlation potentials. The comparison with the experimental data is good, especially for the Ti 2p shell which cannot be described even qualitatively by the too simple Kohn–Sham method. Also the Cl 2p shell, dominated by Rydberg features, is properly described. The method is computationally economic, and can be applied to larger and less symmetric systems. Further extensions to the relativistic case, with explicit spin–orbit treatment, are suggested to improve the description of spin–orbit splitting and intensity redistributions.

The high resolution Gas Phase Photoemission beamline, Elettra

Abstract The Gas Phase Photoemission beamline at Elettra has been commissioned with outstanding success. All photoabsorption spectra taken between 90 and 900 eV have shown resolution which is equal to or higher than any published spectra. The monochromator is a variable angle spherical grating instrument (plane mirror and grating between entrance and exit slits), with an undulator as the source. Some of the problems encountered in commissioning and their solutions are discussed. In particular the calibration is complicated by the fact that an infinite number of angle pairs of the mirror and grating exist for a particular photon energy, whereas only one angle pair gives the highest resolution. A second problem is that the resolution is so high that above about 80 eV, it is much smaller than the lifetime broadening of any known absorption resonance, making any determination of resolution difficult. The experimental chambers available for users are described together with some examples of spectra which have been taken.

The gas-phase photoemission beamline at Elettra

This paper reports the present stage of commissioning of the gas-phase photoemission beamline at Elettra, Trieste. The beamline is designed for atomic and molecular science experiments with high-resolution and high-flux synchrotron radiation. It consists of an undulator source, variable-angle spherical-grating monochromator and two experimental stations. The design value of the energy range is 20 to 800 eV with a specified resolving power of over 10000. The procedure adopted for calibration of this type of monochromator is discussed. At present a resolving power up to 20000 and a range up to 900 eV have been measured. Absorption spectra taken at the argon LII,III-edge and at the nitrogen, oxygen and neon K-edges are as sharp as, or sharper than, any reported in the literature. The instrumental broadening is well below the natural line-width making it difficult to quantify the resolution; this problem is discussed.

A CRITICAL COMPARISON OF SELECTED 1S AND 2P CORE HOLE WIDTHS

Abstract The linewidths of a series of core excited atoms and small molecules have been measured in photoabsorption with ultra high resolution. The widths of transitions to the antibonding π and Rydberg states from the SF 6 1s levels of carbon, nitrogen and oxygen in CO, CO 2 , N 2 , N 2 O and O 2 were measured. In addition the Ne 1s, Ar 2p and sulphur 2p Rydberg states of SF 6 have been measured. The lifetime widths of this range of chemical states and neutral excited states are compared critically with values from the literature derived from x-ray absorption and other spectroscopies.

Vibrational structure of core to Rydberg state excitations of carbon dioxide and dinitrogen oxide

The near-edge x-ray absorption spectra of dinitrogen oxide and carbon dioxide have been measured with ultra high resolution and high statistical accuracy at the N 1s and O 1s edges, and at the C 1s and O 1s edges in N2O and CO2, respectively. The assignment of the peaks in the spectra is achieved via the analysis of the quantum defects of the Rydberg states, the use of the core-equivalent model and a comparison with previous photoabsorption and fragment ion spectra. New high-resolution spectra are provided for the O 1s excitations of CO2, and several new states are observed at the oxygen edge of N2O.

Vibrationally resolved oxygen K→Π∗ spectra of O2 and CO

Abstract A near-edge X-ray absorption fine-structure (NEXAFS) study of the O K→Π ∗ excitation in CO (534 eV) and O 2 (531 eV) molecules has been carried out. A Franck–Condon analysis of the transitions has been accomplished assuming a Morse potential energy curve for the core hole states. The vibrational constants of O 2 are in agreement with values expected from the equivalent core molecule OF. The values of the lifetime widths of the Π ∗ excited states are Γ O 2 =149±10 meV and Γ CO =158±8 meV. The measured lifetime width of O 2 is an upper limit to the true value as unresolved spin–orbit structure is believed to be present, which may cause an additional broadening of ∼5 meV. In spite of this, it is substantially smaller than the hitherto accepted value of 180 meV, necessitating a re-evaluation of most previously published data.

Electronic state resolved PEPICO spectroscopy of pyrimidine

Photoionization techniques, such as photoelectron spectroscopy (PES) and photoionization mass spectrometry (PIMS), are well-established and powerful methods for studying the spectroscopy of isolated bio-organic molecules and their fate under vacuum ultraviolet (VUV) irradiation. Measuring the energy selected electron leaving a molecular ion in coincidence with other particles, such as ions, can provide even deeper insights into the mechanisms of the interaction of molecules with ionizing radiation. We have thus implemented the electronic state resolved photoelectron photoIon coincidence (ER-PEPICO) technique in our laboratory. Here, we report our newly constructed apparatus, and its application for characterizing fragmentation processes occurring in pyrimidine. Ionization of the two highest molecular orbitals (MOs) of the valence band does not lead to fragmentation of the resulting ion. The third band observed in photoemission is due to the ionization of two MOs, and leads mainly to the formation of the parent ion. The next three electronic states are not resolved experimentally and appear as a single band; their ionization leads to fragments of mass to charge ratio m/e= 53 (C3H3N+), while ionization of deeper lying MOs leads mostly to m/e= 26 (C2H2+). We compare our data with previous non-coincidence photoionization results and describe the problems encountered and their solutions.

The C 1s and N 1s near edge x-ray absorption fine structure spectra of five azabenzenes in the gas phase

Near edge x-ray absorption fine structure spectra have been measured and interpreted by means of density functional theory for five different azabenzenes (pyridine, pyridazine, pyrimidine, pyrazine, and s-triazine) in the gas phase. The experimental and theoretical spectra at the N 1s and C 1s edges show a strong resonance assigned to the transition of the 1s electron in the respective N or C atoms to the lowest unoccupied molecular orbital with pi(*) symmetry. As opposed to the N 1s edge, at the C 1s edge this resonance is split due to the different environments of the core hole atom in the molecule. The shift in atomic core-level energy due to a specific chemical environment is explained with the higher electronegativity of the N atom compared to the C atom. The remaining resonances below the ionization potential (IP) are assigned to sigma or pi [corrected] orbitals with mixed valence/Rydberg [corrected] character. Upon N addition, a reduction of intensity is observed in the Rydberg region at both edges as compared to the intensity in the continuum. Above the IP one or more resonances are seen and ascribed here to transitions to sigma(*) orbitals. Calculating the experimental and theoretical Delta(pi) term values at both edges, we observe that they are almost the same within +/-1 eV as expected for isoelectronic bonded pairs. The term values of the pi(*) and sigma(*) resonances are discussed in terms of the total Z number of the atoms participating in the bond.

Inner shell excitation spectroscopy of the tetrahedral molecules CX4 (X = H, F, Cl)

The near edge x-ray absorption spectra of CX4 (X = H, F, Cl) have been measured with high resolution at the 1s edges of carbon and fluorine, and the 2p edge of chlorine. The absolute photoabsorption cross section has been determined at the resonances and in the nearby continuum. In accord with previous work, the antibonding valence peaks of the halides do not show any vibrational structure but sharp Rydberg states are observed at the C 1s edge. We report the first detailed study of the CCl4 Rydberg states. The vibrational frequency of the CF4 Rydberg states is anomalously high, which we attribute to a Fermi resonance. The C 1s line widths were compared to theoretical predictions of ion state line widths and reasonable values are found for CH4 and CCl4, but for CF4 the measured value is higher than the theoretical value. The discrepancy is assigned to mixing of valence band character in the Rydberg states, which is especially strong in CF4, and to multi-centre autoionization. Finally the integrated oscillator strength below threshold is shown to vary in the order CF4 > CH4 > CCl4.

Synchrotron radiation photoemission study of some π-conjugated alkynes in the gas phase: Experiment and theory

Abstract C1s core level photoelectron studies have been performed in the gas phase for some conjugated organic molecules, namely phenylacetylene, diethynylbiphenyl, diphenylbutadiyne and para -nitrophenylacetylene. The investigated systems consist of alkyne molecules with aromatic substituents and have been used as precursors for the synthesis of organic π-conjugated polymers. The experimental results were interpreted with the help of theoretical calculations: the ionization potentials have been predicted for all the chemically different carbon atoms of the organic molecules by means of ΔSCF quantum chemical calculations and compared with the measured binding energies of the various features present in the experimental spectra. A good agreement between experiment and theory was found.