K. Le Guen

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.

New setup for angular distribution measurements of Auger electrons from fixed in space molecules

A new experimental setup for measurement of the angular distributions of energy selected Auger electrons emitted from fixed in space molecules is presented. The system is based on two identical ion detectors with a small angular acceptance placed at 0° and 90° relative to the polarization axis of the incident radiation, and a high luminosity double-toroidal electron analyzer combined with position sensitive detection. This setup allows selection of the molecular alignment for σ and π ionization channels simultaneously and provides an energy and ejection angle measurement of the outgoing electron. The performance in terms of energy and angular resolution, as well as the calibration procedure, are discussed. Sample results obtained on the carbon monoxide ionized above the C 1s threshold are presented.

Photofragmentation study of hexamethyldisiloxane following core ionization and direct double ionization

X-ray photoelectron spectroscopy and Auger spectroscopy studies of gas-phase hexamethyldisiloxane (HMDSO) are presented. The photodissociation of this molecule is studied using various experimental coincidence techniques. We compare the fragmentation pathways observed after core ionization followed by Auger decay and after valence double photoionization of the molecule. A strongly selective production of the doubly charged tetramethyldisiloxane ion is observed in the low binding-energy regions. Theoretical calculations are carried out to tentatively explain the stability of the produced dication.

Optical, chemical, and depth characterization of Al/SiC periodic multilayers

We present the characterization of Al/SiC periodic multilayers designed for optical applications. In some samples, a thin layer of W or Mo is added at the SiC-on-Al interfaces. We use x-ray reflectivity (XRR) in order to determine the parameters of the stacks, i.e. thickness and roughness of all the layers. We have performed x-ray emission spectroscopy (XES) to identify the chemical state of the Al and Si atoms present within the structure from an analysis of the shape of the Al Kβ and Si Kβ emission bands. Finally, time of flight secondary ion mass spectrometry (ToF-SIMS) is used to obtain the depth profile of the different elements present within the studied stacks. A fit of the XRR curves shows that the Al/SiC multilayer present large interfacial roughness (up to 2.8 nm), which is decreased considerably (down to 1 nm or less) when the refractory metal layers are introduced in the periodic structure. The combination of XES and ToFSIMS allows us to conclude that in these systems the roughness is a purely geometrical parameter and not related to chemical interfacial reactions.

Influence of formation path on the CH2BrCl2+ dissociation dynamics.

To get further insight into the CH2BrCl site-selective fragmentation previously observed upon inner-shell ionization, we have performed high-resolution Br 3d and Cl 2p Auger and spin-orbit resolved Br 3d Auger spectra, and studied the dissociation properties of the CH2BrCl2+ dication formed at threshold by means of threshold electron pair-ion coincidence measurements. The key point is that the origin of site-specific bond breaking is found in the Auger decay itself, as it preferentially populates selected dication states. Whereas the predominance of the C-Br bond breaking is observed in both threshold and inner-shell studies, no signature of selective C-Cl rupture is reported for the dication formed at threshold.

Observation of an asymmetrical effect when introducing Zr in Mg/Co multilayers

We have developed Mg/Co, Mg/Zr/Co, Mg/Co/Zr, and Mg/Zr/Co/Zr periodic multilayers and measured at 25.1 nm a reflectivity (R) highly sensitive to the material order within the period. To understand why Mg/Co/Zr is a more efficient mirror (R=50%) than Mg/Zr/Co and Mg/Zr/Co/Zr (∼40%), we have probed the interface quality through time-of-flight secondary ion mass spectrometry and nuclear magnetic resonance measurements. The Zr-on-Co interface is found quite sharp while a strong intermixing process is evidenced between the upper Co and lower Zr layers, responsible for the decrease in optical contrast and subsequent R loss.

X-ray spectroscopic application of Cr∕Sc periodic multilayers

The use of Cr∕Sc multilayer interferential mirrors (MIMs) in optical systems such as x-ray microscopes or telescopes have been reported for the water window (between C K- and O K-absorption edges). However, their possibilities in spectroscopic application have never been described in the literature. The purpose of the paper is to report for the first time on the performances of Cr∕Sc MIMs as Bragg dispersive devices for the analysis in wavelength dispersive spectrometry of samples containing N or Sc atoms. The possibility to distinguish the chemical state of the emitting N or Sc atoms is evidenced by using Johan-type and double-crystal spectrometers.

Spontaneous soft x-ray fluorescence from a superlattice under Kossel diffraction conditions

The present study gives the proof of principle of a technique that is an extension of Kossel diffraction both from crystals to superlattices and toward the soft x-ray region, allowing the characterization of the interfaces within a periodic structure. We measure the intensity of the Co Lα and Mg Kα characteristic fluorescence emissions from a Mg/Co superlattice upon soft x-ray excitation. The observation is made so that the angle between the sample surface and the detection direction is scanned around the first and second Bragg peaks of the fluorescence emissions. Clear modulations of the emitted intensities are observed and well reproduced by simulations based on the reciprocity theorem and assuming a perfect stack. The present work gives evidence that such a superlattice plays the role of an optical cavity for the spontaneous emission generated within the stack. This should also be the case for stimulated emission, which when combined with pumping free electron laser, will open the road to innovative x-ray distributed feedback lasers.

Thermal effects on Co/Mo2C multilayer mirrors studied by soft x-ray standing wave enhanced photoemission spectroscopy

Here is presented the spectroscopic study of the evolution of the first buried interfaces of a B4C capped Co/Mo2C multilayer mirror induced by thermal treatment up to 600°C. This kind of study is typically performed to simulate the response of multilayer optics working in extreme conditions, as for instance when irradiated by new high brilliance sources as Free Electron Lasers. In fact, the efficiency of multilayers is related to the optical contrast between the alternating high and low density layers, and then to the degree of interdiffusion and the creation or evolution of interface compounds. The analysis has been performed at the Co L23 edge with different soft x-ray spectroscopic techniques including diffuse and specular reflectivity, total electron and fluorescent yield at the BEAR beamline at Elettra (Trieste) (http://www.elettra.trieste.it/elettra-beamlines/bear.html). The presentation is focused on the spectroscopic results obtained by soft x-ray standing wave enhanced photoemission (XSW) from the Mo 3d, B 1s, C 1s, O 1s core levels by using a photon energy close to the Co L23 edge and corresponding to the first Bragg peak of the multilayer. The experimental results have been compared with simulations to obtain information both on the chemical state (e.g. oxidation state) and interface morphology in terms of profiles of distribution of elements and interdiffusion of B, oxidized B and C in the interface region. In summary, it is possible to conclude in favour of a good stability of the multilayer in the investigated temperature range, as confirmed by the good performance in terms of reflectivity. These results confirm the usefulness of XSW for this kind analysis of multilayer optics.

Origin of step-like behavior in the Co/Si system

A systematic investigation of the structure, nature of the interface and their possible connections with magnetic properties for the as-deposited Co/Si/Co trilayer system has been carried out. X-ray reflectivity, cross-sectional transmission electron microscopy and x-ray emission measurements performed on the Co/Si/Co trilayer system show that when the Si layer thickness is less than ∼ 20 Å, the full Si layer is converted into a cobalt silicide layer whereas when the Si layer thickness > 20 Å along with the silicide layer. the pure Si layer also remains. A comparison of magneto-optical Kerr effect and magnetoresistance measurements reveals the absence of antiferromagnetic coupling in these samples. Double-step-like magnetization, in the case of Si layer thickness > 20 Å between two Co layers, is explained by magnetization reversal of two ferromagnetic layers having different coercivities, independent of each other.