P. Bolognesi

A modular end-station for atomic, molecular, and cluster science at the low density matter beamline of FERMI@Elettra

The low density matter end-station at the new seeded free electron laser FERMI@Elettra is a versatile instrument for the study of atoms, molecules and clusters by means of electron and ion spectroscopies. Beams of atoms, molecules and helium droplets as well as clusters of atoms, molecules and metals can be produced by three different pulsed valves. The atomic and molecular beams may be seeded, and the clusters and droplets may be pure, or doped with other atoms and molecules. The electrons and ions produced by the ionization and fragmentation of the samples by the intense light of FERMI can be analysed by the available spectrometers, to give mass spectra and energy as well as angular distributions of charged particles. The design of the detector allows simultaneous detection of electrons and ions using velocity map imaging and time-of-flight techniques respectively. The instruments have a high energy/mass resolution and large solid-angle collection efficiency. We describe the current status of the apparatus and illustrate the potential for future experiments.

(e,2e) ionization of helium and the hydrogen molecule: signature of two-centre interference effects

Relative (e,2e) triply differential cross sections (TDCS) are measured for the ionization of the helium atom and the hydrogen molecule in coplanar asymmetric geometry at a scattered electron energy of 500 eV and ejected electron energies of 205, 74 and 37 eV. The He experimental results are found to be in very good agreement with convergent close-coupling calculations (CCC). The H2 experimental results are compared with two state-of-the-art available theoretical models for treating differential electron impact ionization of molecules. Both models yield an overall good agreement with experiments, except for some intensity deviations in the recoil region. Similar (e,2e) works were recently published on H2 with contrasted conclusions to the hypothesis that the two H nuclei could give rise to an interference pattern in the TDCS structure. Murray (2005 J. Phys. B: At. Mol. Opt. Phys. 38 1999) found no evidence for such an effect, whereas Milne-Brownlie et al (2006 Phys. Rev. Lett. 96 233201) reported its indirect observation. In this work, based on a direct comparison between experimental results for He and H2, we observe an oscillatory pattern due to these interference effects, and for the first time the destructive or constructive character of the interference is observed, depending on the de Broglie wavelength of the ejected electron wave. The experimental finding is in good agreement with the theoretical prediction by Stia et al (2003 J. Phys. B: At. Mol. Opt. Phys. 36 L257).

Investigation of halogenated pyrimidines by X-ray photoemission spectroscopy and theoretical DFT methods.

The inner shell ionization of pyrimidine and some halogenated pyrimidines has been investigated experimentally by X-ray photoemission spectroscopy (XPS) and theoretically by density functional theory (DFT) methods. The selected targets-5-Br-pyrimidine, 2-Br-pyrimidine, 2-Cl-pyrimidine, and 5-Br-2-Cl-pyrimidine-allowed the study of the effect of the functionalization of the pyrimidine ring by different halogen atoms bound to the same molecular site, or by the same halogen atom bound to different molecular sites. The theoretical investigation of the inductive and resonance effects in the C(1s) ionization confirms the soundness of the resonance model for a qualitative description of the properties of an aromatic system. Moreover, the combination of the experimental results and the theoretical analysis provides a detailed description of the effects of the halogen atom on the screening of a C(1s) hole in the aromatic pyrimidine ring.

Determining the polarization state of an extreme ultraviolet free-electron laser beam using atomic circular dichroism

Ultrafast extreme ultraviolet and X-ray free-electron lasers are set to revolutionize many domains such as bio-photonics and materials science, in a manner similar to optical lasers over the past two decades. Although their number will grow steadily over the coming decade, their complete characterization remains an elusive goal. This represents a significant barrier to their wider adoption and hence to the full realization of their potential in modern photon sciences. Although a great deal of progress has been made on temporal characterization and wavefront measurements at ultrahigh extreme ultraviolet and X-ray intensities, only few, if any progress on accurately measuring other key parameters such as the state of polarization has emerged. Here we show that by combining ultra-short extreme ultraviolet free electron laser pulses from FERMI with near-infrared laser pulses, we can accurately measure the polarization state of a free electron laser beam in an elegant, non-invasive and straightforward manner using circular dichroism.

Pyrimidine and halogenated pyrimidines near edge x-ray absorption fine structure spectra at C and N K-edges: experiment and theory.

The inner shell excitation of pyrimidine and some halogenated pyrimidines near the C and N K-edges has been investigated experimentally by near edge x-ray absorption fine structure spectroscopy and theoretically by density functional theory calculations. The selected targets, 5-Br-pyrimidine, 2-Br-pyrimidine, 2-Cl-pyrimidine, and 5-Br-2-Cl-pyrimidine, allow the effects of the functionalization of the pyrimidine ring to be studied either as a function of different halogen atoms bound to the same molecular site or as a function of the same halogen atom bound to different molecular sites. The results show that the individual characteristics of the different spectra of the substituted pyrimidines can be rationalized in terms of variations in electronic and geometrical structures of the molecule depending on the localization and the electronegativity of the substituent.

Vibrational state dependence of β and D asymmetry parameters: The case of the highest occupied molecular orbital photoelectron spectrum of methyl-oxirane

The beta angular asymmetry and D dichroic asymmetry parameters of the methyl-oxirane highest occupied molecular orbital (HOMO) band have been experimentally investigated with vibrational resolution using synchrotron radiation. A theoretical calculation of the Franck-Condon factors between vibrational ground state and different ionic vibrational states, in the Born-Oppenheimer harmonic approximation, has been performed in order to gain information on the vibrational states mainly involved in the HOMO photoelectron band. The general good agreement between theoretical and experimental results allows a reliable assignment of the major features. The experimental determination of beta and D shows their dependence on the different final vibrational states. This paper reports, for the first time, experimental evidence of the dependence of the dichroic D parameter on the vibrational excitation of the ion.

Threshold photoelectron spectroscopy of SF6

Abstract The threshold photoelectron spectrum of SF 6 has been recorded under good resolution conditions (9–17 meV) over the photon energy range 15.0–28.3 eV using synchrotron radiation and a penetrating-field electron spectrometer. Significant differences have been found between the threshold photoelectron spectrum and the Hell photoelectron spectrum that are ascribed to resonance autoionization processes in the former. The 6t 1u shape resonance molecular orbital of SF 6 appears to play a major role in this regard. This virtual molecular orbital is tentatively located 0.067 eV above the onset of the ionization continuum. The Rydberg series [ 2 A 1g ] n p x, y, z 1 F 1u , for n = 4–7 of SF 6 has been observed by means of resonance autoionization into, most probably, the E ( 2 T 1u ) ion continuum.

The Auger spectroscopy of pyrimidine and halogen-substituted pyrimidines

The C 1s and N 1s Auger spectra of pyrimidine, 2-chloropyrimidine, and 5-bromopyrimidine have been measured in an electron impact experiment at 1000 eV. In the case of the halogen-substituted pyrimidines, also the Cl 2p and Br 3d Auger spectra have been recorded. We have thoroughly analyzed and interpreted all the Auger spectra recorded here with the aid of accurate Greens function calculations with a large basis set. The spectra are extremely complex with thousands of states contributing and almost no single-state feature even near the double ionization threshold. Besides reproducing and explaining with great detail nearly all the main spectral features observed, the calculations have successfully unraveled the interplay among the different C 1s core hole chemical shifts in each molecule and how this affects some fingerprinting details in the composite C 1s Auger spectra.

Effects of nuclear dynamics in the low-kinetic-energy Auger spectra of CO and CO2

The CO and CO(2) carbon and oxygen Auger spectra have been measured by electron impact and compared with accurate theoretical calculations accounting for the effects of the dynamics of the nuclei on the energy and linewidth of the Auger bands. The calculations for CO were previously published [L. S. Cederbaum et al., J. Chem. Phys. 95, 6634 (1991)], while for CO(2) they are new and presented here for the first time. For both molecules, particular attention has been paid to the low-kinetic-energy region of the spectra, which corresponds to doubly charged ion states with the two holes mainly localized in the inner valence region. New bands have been observed. It is shown that a proper consideration of the vibrational broadening and shift of the bands due to the dynamics of the nuclei is needed to assign these features. For CO, very large energy shifts between corresponding features in the C 1s and O 1s spectra have been observed, confirming the theoretical predictions of 1991. The new computed spectra of CO(2) allow a very accurate analysis of the experiments over the whole energy range.

An experimental and computational study of the valence photoelectron spectra of halogenated pyrimidines

The electronic structures of pyrimidine and a selection of its halogen-substituted derivatives have been investigated using ultraviolet photoelectron spectroscopy and ab initio quantum chemical methods. Assignments are proposed for all of the features in the PES spectra by comparison with the vertical ionization energies of the molecular orbitals calculated using the partial third-order quasiparticle approximation as applied to electron propagator theory and a corrected density functional method based on the B3LYP functional. The shifts of the outermost five molecular orbitals of the pyrimidine ring structure in the halogen-substituted derivatives with respect to the binding energies of the equivalent orbitals in the parent pyrimidine molecule are discussed as a function of the identity and ring position of the halogen atom.