L. Avaldi

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.

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.

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.

Double-excitation and double-escape processes studied by photoelectron spectroscopy near threshold

Recent experimental work using photons to study double excitation, double escape and related processes near their respective thresholds is reviewed. These processes are of significant current interest because they are completely determined by electron-electron correlations. In particular, experimental studies based on the techniques of low-energy electron spectroscopy are described. By using selected examples from coincidence and non-coincidence experiments the various kinds of dynamic and spectroscopic information that can be obtained are illustrated. Recent results, mainly in the rare gases, are described and their interpretation is discussed. Future perspectives are considered.