A. Naves de Brito

X-ray photoelectron, Auger electron and ion fragment spectra of O2 and potential curves of O22+

The O22+ ion has been studied by means of electron-impact-induced and photon-induced Auger electron spectroscopy and oxygen ion fragment spectroscopy of O2. The oxygen ion kinetic energy spectrum was recorded by inverting the relevant potentials of an electron spectrometer for the detection of positive particles. The 4 Sigma - and 2 Sigma - O 1s initial core hole states have been studied using monochromatised X-ray photoelectron spectroscopy. Potential energy curves for a number of electronic states of the O22+ dication have been calculated with the complete active space SCF (CASSCF) and multireference contracted CI (MRCCI) methods with a one-particle basis set of medium size ((8s, 6p, 2d)). An analysis of the O2 Auger electron spectrum based on the computed potential curves of O22+ is presented. The autoionisation satellites are analysed and these lines correspond to molecular singly ionised final states. One line at 510.7 eV, however, is associated with an atomic-like transition. Two shake-up Auger satellites are identified by a comparison with a recent O 1s shake-up spectrum from O2.

A theoretical study of x‐ray photoelectron spectra of model molecules for polymethylmethacrylate

We explore the usefulness of the delta self‐consistent‐field (ΔSCF) approximation in connection with high‐resolution x‐ray photoelectron spectra for component and structural analysis of organic compounds. Results for core electron binding energy shifts for model molecules of the polymethylmethacrylate polymer are presented. A previously devised method for proper self‐consistent‐field solutions for core hole states in molecules is evaluated. The results indicate that chemical shifts can be obtained within a few tenths of an eV. A discussion is presented on the inherent errors in the ΔSCF approximation, the proper corrections for zero‐point vibrational energies, and the role of relaxation of core orbitals.

The electronic structure of free water clusters probed by Auger electron spectroscopy

(H2O)(N) clusters generated in a supersonic expansion source with N approximately 1000 were core ionized by synchrotron radiation, giving rise to core-level photoelectron and Auger electron spectra (AES), free from charging effects. The AES is interpreted as being intermediate between the molecular and solid water spectra showing broadened bands as well as a significant shoulder at high kinetic energy. Qualitative considerations as well as ab initio calculations explain this shoulder to be due to delocalized final states in which the two valence holes are mostly located at different water molecules. The ab initio calculations show that valence hole configurations with both valence holes at the core-ionized water molecule are admixed to these final states and give rise to their intensity in the AES. Density-functional investigations of model systems for the doubly ionized final states--the water dimer and a 20-molecule water cluster--were performed to analyze the localization of the two valence holes in the electronic ground states. Whereas these holes are preferentially located at the same water molecule in the dimer, they are delocalized in the cluster showing a preference of the holes for surface molecules. The calculated double-ionization potential of the cluster (22.1 eV) is in reasonable agreement with the low-energy limit of the delocalized hole shoulder in the AES.

Vibronic and electronic states of doubly charged H2S studied by Auger and charge transfer spectroscopy and by ab initio calculations

Doubly ionic states of H2S are investigated by means of Auger and double charge transfer spectroscopy. From the kinetic energy distribution of H− ions arising from double charge‐transfer of protons impinging on gaseous H2S several singlet state energies of H2S2+ have been resolved in the 30 to 50 eV energy region. The most intense experimental peak is narrow proving that the doubly ionized ground state is stable or quasi‐stable. The LII,IIIVV Auger electron spectrum exhibits a number of well‐defined structures which exhibit vibrational fine structure in the outermost bands. The assignments of the charge transfer states and of the Auger bands are given by ab initio MCSCF electronic structure calculations. We also present vertical double ionization energies, optimized geometries and normal coordinate analysis for the neutral, single and double ionized states. A vibrational analysis of the resolved Auger bands is carried out by employing a recently derived theory for vibrational decay of short‐lived core hol...

The vibrationally resolved participator Auger spectra of selectively excited C 1s(2σ)−12π1 vibrational states in carbon monoxide

The fully vibrationally resolved participator Auger spectra originating from the decay of the C 1s(2σ)−12π1 resonance in CO are presented. The C 1s(2σ)−12π1 v’=0 resonance has been excited with a 75 meV monochromator bandpass, i.e., in Auger resonant Raman conditions, and the participator Auger spectrum observed. The C 1s(2σ)−12π1 v’=1 resonance is also excited and the corresponding participator Auger spectrum observed with a monochromator bandpass slightly larger than the inherent width. The results are compared to theoretical simulations using coherent lifetime‐vibrational interference theory which accounts for the details of the spectrum. We have observed an interference shift on the transitions to different vibrational sublevels in the final state. A high resolution C 1s photoelectron spectrum of CO is also presented. The lifetime width of the C 1s core–hole state is determined to be 97(10) meV, whereas the C 1s(2σ)−12π1 resonance is measured to have a width of 86(10) meV.

Performance of the modified SX‐700 plane grating monochromator at the Finnish beamline in MAX‐lab

A soft x‐ray beamline designed principally for gas phase measurements has been constructed and tested at the 550‐MeV MAX I electron storage ring in Lund, Sweden. The beamline uses synchrotron radiation from a short 35‐period undulator with a magnetic period of 24 mm. The photon energy range of the undulator is about 60–600 eV covering, e.g., the important C, N and O 1s ionization regions. The beamline is based on a modified SX‐700 plane grating monochromator with a plane elliptical focusing mirror. The beamline has been tested by measuring total ion and electron yield spectra from the rare gases Kr and Ar in the resonance excitation regions just below the Kr M4,5 and Ar L2,3 ionization thresholds. These results show that the monochromator has very high resolution, E/ΔE≊4800 at 244 eV and ≊7600 at 91 eV, comparable with the best spherical and plane grating monochromators. The beamline is equipped with an effective differential pumping system for the gas phase experiments combined with a refocusing mirror i...

Gas‐phase x‐ray photoelectron spectroscopy of model molecules relating to the thermochromism in poly(3‐alkylthiophene)

Previously, a geometrical model of the thermochromism in poly(3‐alkylthiophene) was proposed, based upon an analysis of optical absorption spectra as well as ultraviolet and x‐ray photoelectron spectra (UPS and XPS). In the present contribution, the shake‐up features in the XPS C(1s) spectra of thiophene, hexyl‐substituted thiophene, and bithiophene molecules in the gas phase, are compared with the shake‐up features in previously published XPS C1s spectra of poly(3‐hexylthiophene) and poly(3‐butylthiophene). An analysis of these gas phase molecular data confirms the geometric model of electronic localization in the polymer materials proposed previously.

A synchrotron beamline for delivering high purity vacuum ultraviolet photons.

We report on the current status and performance of the toroidal grating monochromator beamline at the Brazilian Synchrotron Light Laboratory (Laboratorio Nacional de Luz Sincrotron). This beamline provides photons in the vacuum ultraviolet and soft x-ray regions from 12 to 330 eV with three interchangeable gratings. We report on the improvement, which allows the possibility of choosing the light polarization degree from linear to almost circular. Here, we also describe the development of a new apparatus, namely, the mirror-inserted harmonic attenuator and calibrating-device with a long length (MIRHACLLE). All beamlines based on diffraction gratings suffer from the problem of high harmonics contaminations to some extent. The MIRHACLLE provides a way to efficiently suppress high harmonics from 25% to 1 ppm in a grazing incidence bending magnet beamline. Its principle of operation relays on the absorption of the high energy photons in a gas phase region. It allows negligible high harmonics contamination for photon energies ranging from 12 eV to the gas first ionization threshold, 21.6 eV, in the case of neon. We also demonstrate the possibility to use this device for energy calibration and resolution evaluation together with any experiment needing its filtering capabilities. The device is also very cost effective compared to other filters presented previously in the literature.

The x‐ray excited Auger electron spectrum of NO and potential curves and photodissociation of the NO2+ ion

A study of the NO2+ ion by means of Auger spectroscopy, fast ion beam laser spectroscopy and ab initio calculations is reported. The photon induced Auger spectrum of NO was recorded. Potential curves for a number of electronic states of NO2+ were calculated by the complete active space SCF method in order to facilitate an analysis of the Auger spectrum. A photoabsorption spectrum of NO2+ was observed by means of photofragment kinetic energy spectroscopy and assigned to the A 2Π←X 2Σ+ transition. The two different experimental methods both give a value of 38.6 eV for the appearance energy of NO2+, which is entirely consistent with recent photoionization and double charge transfer results.

Development of a four-element conical electron lens dedicated to high resolution Auger electron–ion(s) coincidence experiments

A four-element conical electron lens has been developed in view of its integration to a double toroidal electron energy analyzer (DTA) dedicated to Auger electron–ion coincidence measurements. The lens design, using electron trajectory numerical simulations, was entirely guided by the perspective of analyzing energetic electrons with high resolution in the multicoincidence regime. The design, construction, and experimental characterization stages of this electron optics system are described in this article. Emphasis is put on the importance of third generation synchrotron radiation sources when performing such multicoincidence experiments.