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CONCLUDING REMARKS
T. Aberg
To cite this version:
T. Aberg. CONCLUDING REMARKS. Journal de Physique Colloques, 1987, 48 (C9), pp.C9-1207-
C9-1215. �10.1051/jphyscol:19879218�. �jpa-00227339�
Colloque C 9 , supplhent au n012, Tome 48, decembre 1987
CONCLUDING REMARKS
Laboratory of Physics, Helsinki University of Technology, SF-02150 Espoo, Finland
Abstract
Future aspects of the physics of x-ray and inner-shell processes are discussed and some scientific advances, reported at X-87 are described.
1. INTRODUCTION
X-87 has covered a sizeable part of atomic, molecular, and solid state physics. All modes of photon-electron interactions including the nucleus as an external agent have been represented among the x-ray processes. We have discussed the inelastic scattering of x-rays including fluorescence, x-ray absorption, bremsstrahlung, multiphoton ionization, collisionally induced x-ray emission, elastic scattering, electron capture and coherent production of x-rays. The concept of inner-shells has refered to atoms with and without outer shells, i.e.
highly charged ions. We have discussed inner-shell processes in various external environments including plasmas, solids, and molecules not forgetting the transient molecular environment of ion-atom collisions. Thus X-87 has had a flavour of an interdisciplinary meeting. Hopefully future conferences will continue to strengthen their role as a common forum for specialists in various fields, related to x-ray and inner-shell processes.
In the following I shall comment on some highlights and likely future trends which, I feel, has been disclosed by this conference. My remarks reflect my own interests and are ment to be illustrative rather than exhaustive.
2. BIG MACHINES
Haensel and Bosch reported on the present status of the two major storage ring (I am not talking about super-colliders, i.e. LEP) projects in Europe. Using undulators the European Synchrotron
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19879218
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Radiation Facility (ESRF) in Grenoble will produce a brilliance which is of the order of 1018 10 keV photons per sec mm2mrad2 in 0.1%
bandwidth. This will require a very small beam size which nevertheless is attainable using modern accelarator technology. The Experimental Storage Ring (ESR) at GSI in Danstadt will produce completely or partially stripped heavy ion beams up to 1 GeV/u TJg2+. The beam will have a very high phase space density due to ingenious stochastic and electron cooling systems.
World wide there is a boom in accelerator based physics. There are about twenty synchrotron radiation sources operating at present and plans for several very sophisticated sources which use undulators to achieve high brilliance. The recently funded Advanced Light Source (ALS) in Berkeley will operate in the soft x-ray and uv range. Most hard x-ray sources are still either parasitic or partially dedicated (NSLS in Brookhaven, SRS in Daresbury, and the Photon Factory in Tsukuba are exceptions), which explains the urgent need for sources like ESRF and the Advanced Photon Source (APS) at Argonne National Laboratory.
In addition to ESR in Darmstadt there are three more atomic physics oriented heavy ion rings presently under construction in Europe. ASTRID in Aarhus, CRYRING in Stockholm and TSR in Heidelberg will all be equipped with an electron beam cooling system. A similar facility (HISTRAP) has been proposed for advanced atomic physics research in Oak Ridge. There are also plans to combine a synchrotron radiation source with a heavy-ion storage ring. This is the proposed atomic physics facility at Brookhaven. To this list one should add a number of operating ion-atom collision physics facilities without storage rings (e.g. GANIL in Caen, SUPER HILAC in Berkeley) including those under construction like the one at the Kansas State University.
Related to this X-ray emission from Tokamak and laser-induced plasmas is also studied at several laboratories. With this enormous activity going on the x-ray and inner-shell physics has definitely become a part of the "big science", although the development of instruments and theory will certainly continue also in small laboratories.
3. DEDICATED INSTRUMENTATION
The development described above puts a demand on the construction of "dedicated" x-ray and electron spectrometers, including beam lines, monochromators, and mirros. This is also true for plasma diagnostics at Tokamaks and for the study of laser-induced plasmas and ions. If we consider the biggest x-ray tube among all "the universe" there is a
according to Amaud.
There has been considerable progress in instrumentation since the last conference in Leipzig. Wigglers and undulators will become an essential part of the synchrotron-radiation technology in all wavelength regions. Whether the undulator in association with multilayer mirrors will lead to an X W free electron laser is an open question. Although the technology exists for wavelengths larger than 100 nm according to Ortega, shorter wavelengths may require entirely new ideas like Scullys suggestion of using a Laser Undulator. With this arrangement one may go down to the 10 nm regime in which observations of x-ray laser lines have already been made from laser- induced plasmas.
The development of x-ray optics and of the multilayers in particular is crucial for the construction of many devices ranging from the x-ray plasma laser to microprobes. For example, using synchrotron radiation the distribution of subpicogram quantities in sample areas of less than 100 square microns can be rapidly detected with the multilayer microprobe of Thompson et a1 at LBL. As stressed by Chevallier the improvement of the beam quality is another essential factor in the development of the trace analysis. Vinogradov described the recent progress in multilayer optics and Schmall the development of miniature zone plates. Their talks also pointed towards a recent trend to combine features of visible optics with diffraction optics in crystals and gratings. Examples are the dispersive multilayer-grating x-ray mirrors and the Bragg-Fresnel crystal reflectors.
The "dedicated" x-ray and electron spectrometers are those instruments which most likely "are described in detail elsewhere" in the literature. The curved crystal spectrometer, which was developed and used by Cauchois to measure faint satellite lines in the 301s, has experienced a renaissance in association with position sensitive detectors and multichannel plates. It is used for measurements of x- ray emission from gaseous samples, excited by synchrotron radiation, from Tokamak plasmas and from ion-atom collisions at accelerators. The multi-grating grazing incidence spectrometer of Nordgren et a1 represents new thinking in the construction of soft x-ray spectrometers. It is transportable and yet combines high sensitivity with high resolution. As reported by Nordgren it has been succesfully used at the soft x-ray wiggler beam line of HASYLAB and at the ECR ion source in Grenoble.
It is also clear that a multitude of synchrotron radiation experiments on inner- and outer-shell photionization of atoms and
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molecules would not have been possible without the construction of versatile electron spectrometers. The threshold electron analyzer of Heimann et a1 is a representative example of current progress. The high-temperature Auger electron spectroscopy of Aksela et a1 and the zero-degree Auger spectroscopy of projectile ions in ion-atom collisions, developed by Stolterfoht et all are also examples of recent innovations.
4. SYNCHROTRON RADIATION AND PLASMA RESEARCH
The developement of synchrotron radiation sources may be compared with the progress in laser technology. Technical advances have in both cases made the study of inelastic photon scattering feasible. There are numerous studies of Raman scattering using lasers but relatively little has been done in the x-ray region. Results reported by Deslattes and his coworkers at X-87 indicate that the use of the tunability of the synchrotron radiation will change this situation drastically. In a series of experiment at BNL they have studied the evolution of inelastic x-ray scattering into fluorescence near the argon K edge and near the chlorine K edge in various molecules. They have demonstrated the narrowing of the resonance lines by using a spectral bandwith of 0.4 eV which is less than the K width of argon and chlorine. They have also shown that the chlorine a and K resonance lines exhibit strong polarization when the linearly polarized synchrotron radiation is used to excite the randomly oriented molecules. They have indicated other conspicious but less understood effects due to electron correlation and fragmentation. It would be of interest to extend these type of measurements to solids in order to correlate the inelastic x-ray scattering near the edge with the x-ray absorption core exciton and near-edge structure (XANES). As became clear from the talks of Natoli, Petiau, and Sawatzky the structure of the x-ray absorption edge contains a wealth of information regarding the local structure of the absorbing atom.
Tunability has played a fundamental role in a number of soft x-ray experiments, discussed at X-87. Krause and Caldwell have succeed to measure both the photoelectron and Auger electron spectrum of vaporized beryllium in the vicinity of th K edge. The results which indicate strong electron correlation effects provide a fundamental test of the theory of photoionization. This is also true for photoionization of laser-excited atoms, described by Ederer et a1 and Sonntag. For example, atoms may be prepared in specific doubly excited states, with characteristic correlated motions of the two excited electrons. The photoelectron satellites at outer shell thresholds of
unexpected features. In neon quartet states seem to be present among the final ls22s22p4n12S+1~ shake-up states, and 1=0 or 2 is favoured over 1=1. The correlation effects involved are not unrelated to the post collision interaction (PCI) problem, discussed by Schmidt. It appears that for high excess energies the angle-averaged photon induced PC1 effect is now well understood from both the semiclassical and quantum-mechanical point of view. However, concidence measurements of the angle-dependent PC1 effect are needed to verify predictions of the theory.
Photoelectron, Auger electron and X-ray spectroscopy of carbon nitrogen, and oxygen containing free or chemisorbed molecules is a field of its own. The central question has been the response of the electrons and nuclei to the creation of the K hole at various photon energies. Coincidence measurements of Eberhardt et a1 have revealed the dependence of the molecular fragmentation on the formation of the initial and final states in the K-hole decay of diatomic molecules. It would be of interest to relate these observations to laser-induced fragmentation by also studying the soft x-ray induced fragmentation of large molecules and clusters. This would bring us into a regime, where a discussion of the fragmentation in terms of potential curves may become meaningless. Statistical concepts must be introduced depending on the size of the molecule and the fundamental question is how and at which stage.
In 1985 several laboratories reported observations of soft x-ray laser transitions in laser-induced plasmas. The wavelengths of the lasing lines are in the 10 to 30nm range and originate from population inversion in hydrogen, lithium and neon like ions. Although the mechanisms for population inversion are not entirely clear recombination population of the upper level in association with radiative cooling of the expanding plasma and collisional excitation seem to occur. The latter mechanism is thought to be responsible for the inverse population of the upper ~~~3~ states of the lasing 3p to 3s transitions in neon-like ions. Recent experiments of Elton et a1 in copper plasmas seem to support this suggestion in contrast to previous measurements on selenium by Matthews et al. Jaegle who is one of the pioneers in the field presented a pertinent review. Altogether the basic understanding of the lasing is based on a fascinating combination of plasma hydrodynamics and physics of highly charged ions as also became clear from the talk of More.
It is not only possible to create "explosions" in thin foils and rods with strong laser pulses but in single atoms as well. According
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to LrHuillier the mechanism for multiple ionization in multiphoton absorption is different from that in one-photon ionization which is a pure electron correlation effect. After much discussion it is now generally believed that the ionization occurs primarily stepwise in single atoms during the laser pulse. Several laboratories are now studying this phenomenon and one of the goals is to extend the observations to a regime in which the atom is exposed to electric field strengths larger than one atomic unit.
In Tokamaks the temperature of the plasma is in general higher but the density lower than in the laser-induced plasma. The spatial and temporal variation of the plasma during the energy confinment time (10 ms-1s) is a complex phenomenon. The time- and position resolved x- ray diagnostics requires special instrumentation due to the "hostile"
operational conditions of a Tokamak. Nevertheless in addition to the deduction of impurity concentrations, ion temperatures, and toroidal plasma velocities it has been possible to study processes which populate excited states of hydrogen- and helium-like impurity ions in various parts of the plasma. This topic as well as future instrumentation was considered by E. Kallne in her talk on the x-ray diagnostics program at JET in Culham.
5. QUANTUM ELECTRODYNAMICS AND HIGHLY CHARGED IONS
An often stated motivation for building heavy ion storage rings and related devices is the production of few-electron systems in which the electrons have relativistic velocities and in which the quantum- electrodynamical (QED) effects are large. This motivation may seem somewhat strange since in general the predictions of QED are considered to be verified. As a consequence, further non-linear generalizations of QED have been developed. The most succesful one is the theory of quark-gluon matter, the quantum chromodynamics (QCD).
The properties of the ordinary matter, nuclei and electrons, are considered to be in principle calculable within the framework of QED, at least up to the fusion temperature, l o 8 K.
I do not think this common view is justified from the point of view of atomic physics. First, in its pure form QED starts from free- particle basis states and considers the interaction between electrons, positrons and photons in a perturbative way. Yet we deal with strong nuclear-electron interactions (QED does not allow point-like charges higher than 137) and even with supercritical fields corresponding to a transient nuclear charge of 2 ~ 1 7 3 in super heavy ion-atom collisions.
This has lead to a non-perturbative reformulation of QED as emphasized by Miiller and coworkers. The prediction that the vacuum may decay into
observations of the e' e+ production in ion-atom collisions. However, the repeatedly observed discrete features in the positron spectrum have remained unexplained-. Hence the reports of Miiller and Meyerhof on the recent observations of e-e+ and 77 coincidences from U-U and U- Th collisions were exciting. As a result it seems clear that somehow neutral objects of atomic dimensions with masses close to 2 M ~ V / C ~ and with lifetimes somewhere between and 10'1° seconds are formed in the transient Coulomb field. Their two-body decay is not a nuclear phenomenon nor can it be associated with the formation of an elementary particle. The search of corresponding resonances in the Bhabha scattering continues phrenetically. As pointed out by Remilleux supercritical fields may also be reached in high-energy channeling, and it may even become possible to observe the decay of the vacuum in strong pulsed laser fields.
Second, it is not possible to construct within the framework of QED a fully covariant Hamiltonian for a many-electron ion in a closed form. Hence one usually treats the electron-electron interaction as in the non-relativistic theory and corrects for the Breit interaction.
Although the resulting Dirac-Fock method does (at least for closed shell atoms) not suffer from the problem of continuum dissolution there remain problems with the radiative corrections. The recipes used today are based on one-particle radiative corrections. The many- electron effects are estimated by invoking the screening which is unsatisfactory. As shown by Marrus everything seems to work beautifully but it may be accidental since many small contributions including the electron correlation energy must be added up to obtain a transition energy for comparison with the experiment. And even if it is not accidental there remains the question, why this approximate scheme does work. The importance of these issues is illuminated by the fact that recently a program on "Relativistic many-body theory, QED, and weak interactions" by Johnson, Mohr and Sucher has been accepted at the Institute of Theoretical Physics at Santa Barbara.
Since our present theoretical quidelines are rather poor regarding a consistent many-body version of QED it is of importance to continue the study of the interaction between photons and highly- charged ions. There is another reason, too. The electron correlation can be monitored by changing the charge of the nucleus in few-electron ions with two to four electrons. The doubly excited states will be especially interesting due to the delicate correlated motion of the two electrons as we have learned from studies of the helium atom and low Z two-electron ions. The understanding of electron transfer in
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collisions between highly charged ions and atoms is also imperative for many purposes like plasma heating. M~ltiple-electr~n capture, described by Niehaus in his talk, is one of the characteristic features of these collisions.
6. SOLID STATE SPECTROSCOPY
Recently one of my colleaques, S. Stenholm, made a relevant remark about the development of solid state physics which seems to apply to X-87 too. In solid state physics there is clearly a trend from the physics of simple metals and semiconductors towards studies of exotic compounds like heavy fermions, high-Tc ceramics, quasicrystals and amorphous semiconductors. The theory of the core level or inner-shell spectroscopies of simple solids has been developed to a state of understanding by Hedin and coworkers, Kotani, and many others. There is no need to quarrel about the "Mahan-Nozi6res-DeDominicis edge effect" any more as we did ten years ago.
Like in the molecular core electron spectroscopy the may-body response of the valence electrons to the core hole is the crucial question in almost all spectroscopical methods (I counted about twenty methods in the Book of Abstract) which are applied to solid state systems. This is true for all materials including the "exotic" solids.
The Bremsstrahlung Isochromat Spectroscopy (BIS) which probes the unoccupied bands is a notable exception as became clear from the illuminating talk of Baer.
It is rewarding how quickly this community has responded to the discovery of the superconducting phase of the R B ~ ( S ~ ) C U ~ O ~ - ~ (y>O) compounds, where R is a transition metal or rare earth atom. However, it appears that any evidence of the superconductivity mechanisms which comes from the core level spectroscopies below and above the critical temperature is rather indirect. Nevertheless in combination with other methods and as a test of the band structure they may be very useful.
For example, the study of excitonic effects in superconductivity requires accurate band calculations and they can be tested using x-ray spectroscopy. It is also important to use all the available information of the measured core level spectra as the following argument shows. Suppose the bonding in the CuO planes plays a crucial role in the formation of the superconductivity (resonating valence- band model). Then it may be of interest to study the response of the valence electrons to the creation of the oxvqen K hole by measuring the accompanying x-ray emission in addition to the photoelectron and Auger electron spectra. It may be that the response of the electrons
reflected in the satellite structure of the K x-ray emission spectrum in analog to simple oxides. This suggestion may be naive but it demonstrates that a succesful application of core spectroscopy requires an understanding of the relationship between the bonding and the response mechanism.
7. YEAR 1996
In November, 1895 Wilhelm Conrad Rantgen discovered the x-rays. He described his first findings in a report of Sitzungsberichte der physikalisch-medizinischen Gesellschaft zu Wurzburg. It was received on 28 December, 1895. The next year Beiblatter zu den Annalen der Physik und Chemie, Band 20 (Leipzig, 1896) contained 400 titles on x- rays. Thus if we continue with three years periods the third conference from now will be the hundred years celebration of the x- rays. If we reach the limit of 400 contributed papers we can still consider the physics of x-ray and inner-shell processes to be a vivid field. In light of X-87 the prospects look good.
