Notice of PLEIADES

The high performances expected in the soft X-ray energy range from the 2.75 GeV SOLEIL storage ring generate considerable interest from the atomic and molecular physics community. Indeed, the increase of several orders of magnitude of the available photon flux in a narrow bandwidth will permit studies totally impossible up to now, focusing on electronic properties determination of very diluted samples (positive or negative ions in various states of charge, mass selected clusters, short lived core excited states) as well as on dynamical studies by using high resolution conditions in both the excitation (photon bandwidth) and analysis (electron or fluorescence photon: energy and emission angle) channels, taking advantage of the so-called Resonant Auger Raman regime.
 
The very general purpose of this beamline is to study the interaction of diluted species (atoms, molecules, ions, clusters and adsorbed molecules) with high energy radiation (VUV, soft X-rays). The broad interest of such studies is focused on fundamental aspects of photon interaction (and the associated decay processes) with matter as detailed later, but is also closely connected to related fields like - chemistry : dissociation can be seen as a half-chemical reaction process - surface chemistry : photon induced reactions - radiation damage : specific and controlled energy deposit in complex molecules - plasma studies : ions spectroscopy - astrophysics and astrochemistry : atoms and isolated molecules in the presence of photons - chemical reactivity on clusters and surfaces.

Multiple ionization of an atom through a one photon process is of fundamental interest because it is directly related to electron correlations which determine the electronic description of both discrete and continuum states. However, such experiments are very difficult to perform due to the rapidly decreasing cross section as the ionization degree increases. A third generation machine like SOLEIL will be very well suited for developing such experiments. Dichroism methods based studies, which represent a severe test of theoretical approaches, will be of special interest. Electron correlations are also very important in « hollow » atoms. Indeed, in such species where several inner electrons are excited, correlations cannot be treated as a perturbation and a collective description is necessary. The use of PLEIADES, with its high resolution and high flux, is crucial in order to perform the spectroscopy of such species in the frame of a one photon process.
Despite their interest for plasma study, there is a considerable lack of experimental data (absolute cross-sections, electron spectroscopy data…) on ions, due to the very low density with which they can be produced. Photoionization experiments of ions have been basically limited up to now to total ionization cross sections on low charge species. An originality of this beamline is to propose a dedicated optical branch and endstation to perform electron spectroscopy on ions (positive and negative) produced by an efficient ECR source. The combination of high flux and high resolution expected on the beamline is a key point of this project.

An additional degree of freedom is present in the case of molecules interacting with SR due to the nuclear motion, either in bound or dissociative states. Three characteristic time scales have to be accounted for in order to understand the various excitation and decay processes: the interaction time with the incident photon, the vibration time of the nuclei and the lifetime of the inner hole. An interplay leading to interference effects is likely to occur when these time scales are equivalent. Such effects can be studied when using the so-called Resonant Auger Raman (RAR) regime where both photon excitation and electron analysis are performed at better resolution than the lifetime broadening (typically 0.1eV). Obviously, an ultra high brilliance beamline is needed to ensure fulfillment of such conditions.
The localized character of core orbitals allows selection of a specific atomic site to be excited in a complex system. Multiple ionization following the core hole decay is thus a unique tool to induce a selective fragmentation of a specific chemical bond. Understanding such fragmentation processes requires coincidence measurements between the ejected electrons and the ionic fragments to determine the fragmentation pattern as a function of the internal energy of the ion. In the case of small molecules, special emphasis will be given to dynamics studies using high resolution electron spectroscopy and coincidence techniques. Vector correlations between the emitted particles (electron, ions) are key measurements to be obtained in such studies; they will be developed with unprecedented efficiency on PLEIADES taking advantage of the simultaneous high spectral resolution and high flux with an excellent spot size (50 mm x 50 mm). Moreover, coincidence techniques also allow selection of a multiply charged ion in order to study its reactivity in a second step. This aspect will be developed on doubly charged ions (created through Auger decay or through double photoionisation at threshold) whose chemical reactivity (unknown) is expected to be very high.

Excited atomic and molecular states represent a particularly interesting case to deeper understanding of the electronic correlations and of the interaction with the ionic nucleus. Investigations of electronically excited states allow modifying the electronic interactions as compared to those present in the ground state. Vibrational and rotational excitation makes possible to study the influence of the molecular geometry changes, following a photoexcitation or a photoionization process. A selective and controlled way to induce such changes is provided by the two-photon pump-probe experiments combining SR and lasers. In atomic physics, for example, the high brilliance of SOLEIL can be used to perform experiments on very-short lived autoionizing states. An intense laser can induce a strong coupling between two autoionizing states. Only predicted theoretically, a strong modification of the relaxation dynamics is expected leading to drastic changes in the widths and profiles of the resonances. The extension to more than one laser pulse would give access to more versatile excitation schemes and, in particular, to the study of high angular momentum states.
For the understanding of many complex reactions a precise knowledge of the time dependence of such processes, i.e. the distinction between sequential or concerted fragmentation, is extremely important. The use of a pulsed laser, synchronized to the SR pulses will enable us to follow the molecular dissociation in a time-resolved way within the time-interval between 30 ps (temporal width of SR pulses from SOLEIL) and 145 ns (time period of SOLEIL “time structure” mode). Moreover, precise information on the symmetry and geometry of the dissociating states can be obtained by high-resolution laser spectroscopy. Finally, the polarization of the laser and of the SR can be brought into play for the study of orientation and alignment processes of electronic states as well as of the dichroism and/or magnetic dichroism.

The interaction of atoms and molecules with surfaces plays a considerable role in a wide range of scientific fields: catalysis, microelectronics, material design and growth, surface properties, corrosion, etc. The heterogeneous reactions coming into play in such processes are, for a great majority, not understood at the atomic level. Synchrotron radiation-based experiments on 3rd generation sources undoubtly provide unique tools to reach a better understanding of this crucial chemistry at the atomic scale. In particular, the X-ray absorption spectroscopy technique in the soft X-ray range has shown its ability to give valuable information on the adsorption mode of the adatoms (chemical state, coupling strength, orientation). The PSD-NEXAFS spectroscopy (Photo Stimulated Desorption-Near Edge X-ray Absorption Fine Structures) will be used to give electronic and structural information, with chemical selectivity and extreme sensitivity to the surface (about 2 Å) as compared to conventional NEXAFS (about 100 Å). The high photon flux delivered by PLEIADES gives the opportunity to strongly develop this technique. PSD-NEXAFS will be applied on heterogeneous reactions that are involved in the atmospheric chemistry. Indeed, most of the chemical reactions in the earth atmosphere are related to heterogeneous cycles at the surface of high altitude icy clouds. Implications can be also found in interstellar physical chemistry. PSD-NEXAFS will be also used on other systems such semiconductors, oxides, etc. Photochemical reactions will be studied as well, and compared to electron-induced reactions, to which they are intimately related.
The second component of this project is more prospective. It consists in coupling this experiment with a scanning tunneling microscope (STM). The general goal of this project is to progress in the understanding of nano-objects, both for applications and at a fundamental level as well. We propose to study, by using the X-ray absorption spectroscopy, the elementary processes involved in the manipulation of atoms by the tip of the STM. We also intend to study sets of nanostructures using both spectroscopic tools and STM imaging. Last but not least, the SR beam will be used to produce nanostructures by photochemical reactions, the STM being used to probe them at the atomic scale; the photo-induced process can be followed at this scale by imaging the adsorbate during the reaction.
The promoters of the PLEIADES beamline project have estimated that the scientific topics using the SOLEIL time structure operation mode represent about 30 % of the beamline program.

Beam Line Proposal 2 : High Resolution (soft X-ray) Photoionization of Diluted Species. There is a clear need for a high resolution Soft X-ray beamline at SOLEIL, for gas phase physics and chemistry, as well as for surface chemistry. The proposal is a good project, promoted by an important, very active community of users. The energy range 10 eV – 1 keV is chosen to support a strongly coherent ensemble of studies, including inner-shell and valence-shell excitation. A good compromise between high flux and useful resolution has been defined at low energy (~ instrument resolution) and high energy (~ core-excited resonance width). A number of strong topics are outlined, such as photoionization of ions, studies combining laser and SR, highly resolved studies of photo-fragmentation and reactivity based on coincidence techniques, nanostructures on surfaces studied with combined SR and scanning tunneling microscopy.
The SAC makes the following comments and recommendations on the proposal:

• The extension of the energy range below 30 eV is motivated by the interest of associating valence- and inner-shell excitation, especially in negative ions, clusters, dications and surface chemistry. This holds as far as the flux in the 30 eV – 1 keV range remains reasonably close to an optimum (see SAC recommendation for BLP1). The proponents are urged to make sure that it is indeed the case.
  
  
• Resolving power of 25000 in the full high energy range (40 eV – 1 keV) is today hardly feasible. Proponents have to check if this can be achieved in due time for SOLEIL.
  
  
• Proponents should check that a 50 μm focal spot size for the two side lines is the optimal definition for the KB optics (tighter focusing - on one side line – would, for example, give a brighter source for dispersed fluorescence measurement, allowing extension to UV).
  
  
• The opportunity of combining the positive and negative ions studies is valuable. On account of the particular geometry of the ions/SR beam interaction, permanent installation of the ion source is required.
  
  
• Experiments combining laser + SR will become a common scheme for other programs (e.g. two-color excitation or time-resolved pump/probe scheme in vector correlations and reactivity studies). Fixed installation for laser is therefore justified.
  
  
• On account of strong common interest for charged particle imaging in coincidence experiments, proponents could consider whether a common apparatus including state-of-the-art detectors would serve their programs – or not - besides specific setups.
  
  
• The SAC remarks that BLP2 demand for 30% of time structure mode in operating the machine might exceed the overall demand.

Most of the proponents have already a strong experience of working on 3rd generation SR centers. They are encouraged to maintain or strengthen these links, in order to keep the definition of the line and its scientific case at the best level.