Fig. 5.1.2 (a) The ZFC order parameter squared in Fe, Zn,F, as measured at the (100) position with x-rays for five fields and H=OT. For HO, the data are well described by a power-law-like behavior with a broadened transition region. The broadening is modeled by a Gaussian distribution of transition temperatures of width Ozc(H)αAH. (b) The HO data of (a) replotted as a function of the temperature interval away from T (H) as measured in units of H2. This illustrates the rounding of the transition which is attributed to nonequilibrium effects arising from extreme critical slowing down and the univeral scaling behavior of the trompe l'oeil critical phenomena. The inset shows the phase boundary of Fe,Zn,F, as determined from the x-ray fits.

Fig. 5.1.3 Schedmatic picture of the local arrangement in the interior of lipid-DNA complexes. The semiflexible DNA molecules are represented by rods on this molecular scale. The neutral and cationic lipids comprising the membrane are expected to locally der uix with the cationic lipds (red) more concentrated near the DNA.

Stewardship of Nuclear Weapons Stockpile

Research at synchrotron facilities will potentially make significant contributions to the DOE's Science-Based Stewardship Program. In particular, synchrotron-based x-ray diffraction and spectroscopic methods can provide detailed information about aging processes in metallic, polymeric and other organic components of the enduring stockpile. While monitoring the degree of aging is necessary in its own right, such studies can also help to reveal the microscopic mechanisms involved. For example, recent work using x-ray absorption spectroscopy has documented variations of near-neighbor atomic coordinations in plutonium alloys, comparing the effects of aging versus composition. Similarly, spatially variable changes in polymer crystallinity can be quantified using x-ray diffraction. Detailed information of this kind is essential for the reliable prediction of aging phenomena needed for enhanced surveillance.

In addition, synchrotron radiation makes it possible to characterize materials under conditions of very high pressures and temperatures. Thus, the effects of aging on the yielding strengths, equations of state and other thermomechanical properties of materials can now be precisely documented at relevant pressures and temperatures. Though static in nature, such experiments provide an important complement to dynamic measurements, especially when the latter require the use of difficult subcritical underground experiments. Specifically, synchrotron-based high-pressure research can provide essential data both for planning the most effective dynamic measurements and to aid in their interpretation. More generally, the high pressure-temperature experiments carried out at synchrotrons offer some of the most stringent tests of existing materials theories, whether atomistic (e.g., based on quantum mechanics) or more macroscopic and phenomenological in character. Testing and improving such theoretical models of material properties plays a central role in the long-term stewardship program.

5.2 Surface Science

Overview

The field of modern "surface science" was created and defined in the 1960's by the pursuit to understand the fundamentals underlying two important technologies: semiconductor devices and heterogeneous catalysis. The continuing importance of the two areas is reflected by the 1997 world wide annual revenues of the associated industries: $150 billion and $1300 billion, respectively. The initial emphasis in this work was on the development of quantitative analytical techniques and on studying "model systems" consisting of well-prepared, well-controlled and reasonably well defined surfaces, mostly single crystal substrates that were studied under ultrahigh vacuum (UHV) conditions. In addition to studies associated with the two main technologies, work has been aimed at understanding the general principles of chemisorptive and physisorptive bonding, the oxidation and corrosion of surfaces, the magnetic properties of surfaces and interfaces, the structure of liquid surfaces and electrolytic interfaces and the nature of phase transitions in two dimensional systems.

As a result of this work, during the 30 year period from the mid 1960's to the mid 90's our understanding of surfaces has undergone a revolution. At the beginning of this period we could not determine any of the important characteristics of a surface, as for example the chemical identity of surface species, their atomic geometries (structure), their electronic charge distributions (bonding), their magnetic properties and the dynamics of their atomic motions. Improvements in UHV technology and the development of sophisticated experimental and theoretical techniques laid the foundation for the revolution in our knowledge. Most of the important surface properties of "model systems" can now be determined accurately and quantitatively. In parallel with this development, surface analysis (e.g. x-ray photoemission and Auger spectroscopy) has found a

routine use in industrial settings for studying more complex technologically relevant systems. The development of new surface tools appears to be reaching saturation, although refinements and extensions of them will no doubt occur, for example at third generation synchrotron radiation

sources.

From a synchrotron radiation perspective it is important to recognize that the formative years of surface science overlapped with those of synchrotron radiation research. Because of the emphasis of traditional surface science on model surfaces and UHV techniques, surface science research has had a strong historical link with VUV/soft x-ray science. The growth of the surface science community in the 70's therefore paralleled the growth in the VUV/soft x-ray synchrotron radiation community, in particular, the increased utilization and develop ent of photoemission spectroscopy. Up to the early 80's, at the time of the Eisenberger-Knotek and Seitz-Eastman reports, surface science research with synchrotron radiation predominantly involved the use of VUV/soft x-ray radiation. In fact, it is interesting to note that at that time, because of the strong surface science effort, the VUV/soft x-ray synchrotron community produced a comparable number of scientific publications as the entire hard x-ray synchrotron community. Over the last 10-15 years a significant shift has occurred. The overall scientific productivity of the hard x-ray community is now considerably larger and the two energy regimes are contributing about equally to surface science studies, largely due to increased use of various x-ray scattering techniques.

What has been accomplished?

To a large degree, the early scientific goals of understanding the basic properties of "model" surfaces have been accomplished. The structure of clean surfaces, ultra thin films and of chemisorption complexes can now be determined routinely. We have obtained a fundamental understanding of semiconductor surfaces and interfacial junctions, like Schottky barriers, through observation of bulk and surface electronic states. We understand many of the dynamic and kinetic properties of surfaces and the fundamental growth modes of thin films. Through sophisticated imaging techniques we can directly observe the growth of materials down to atomic dimensions. We understand the connection between chemical viewpoints of surfaces, emphasizing the local bonding, and physical viewpoints based on two or three dimensional band structure. Surface science has also established a framework of mechanistic concepts, principles, and insights of surface chemical reactions and of the surface chemical bond which form the basis of heterogeneous catalysis. We have even mastered the ability to move atoms on a surface, one at a time.

Contributions of Synchrotron Radiation Techniques

The revolution in our understanding of surfaces has come from an interplay of many approaches and the application of many techniques. Synchrotron radiation techniques have made many significant contributions. Ultraviolet photoemission spectroscopy (UPS) of solids, pioneered with laboratory sources in the 60's has been used by many groups in conjunction with VUV/soft x-ray synchrotron radiation since the 70's and it has been extensively applied to many kinds of problems. It has directly revealed the electronic "surface states" postulated in conjunction with the invention of the transistor and allowed the observation of molecular orbitals associated with the surface chemical bond. Synchrotron radiation based UPS, especially in its angular resolved mode developed in the 70's, has emerged as the technique-of-choice for the study of the electronic structure of surfaces (and solids) and together with theory has provided the basis of our present understanding of such structure. During the last ten years spin resolved UPS studies have revealed the spin-dependent electronic structure of surfaces. Through observation of spin dependent quantum well states in transition metal multilayers such studies have provided an explanation of the oscillatory exchange coupling in such systems.

Building on the concepts of laboratory based ESCA spectroscopy developed in the 60's, synchrotron radiation based x-ray photoemission spectroscopy (XPS) in the 50-1000eV range has provided valuable element and chemical state specific information of surfaces. It has led to the identification of reaction intermediates at surfaces and at interfaces which could be identified only because of the increased spectral resolution and the enhancement in surface sensitivity afforded by tunable synchrotron radiation. Magnetic dichroism in core level photoemission is presently being used as a surface and element specific probe of magnetism.

Other important contributions have come from the photoelectron diffraction (PD) technique. In contrast to LEED this technique offers elemental and chemical state specificity, a significant advantage in the study of heterogeneous systems. Photoelectron diffraction has revealed detailed information on the structure of chemisorbed atoms and molecules and is

now one of the three most used methods (together with LEED and SEXAFS) for studying surface structures. The closely related photoelectron holography technique has been shown capable of providing three-dimensional images of surface structure with atomic resolution. This technique is being further developed at the ALS.

Surface x-ray diffraction has been used to determine the structure of clean surfaces

and ultra thin films with unprecedented accuracy and precision. Presently, it is replacing LEED as the technique of choice for structure determinations of ordered surfaces when it is available. It has also been applied for in situ studies of thin film growth. Application of this technique to the study of phase transitions and correlations in two dimensional inert gas layers has had an important impact on soft matter physics. Owing to the large x-ray penetration depth in matter, this technique has also given us unprecedented insight into the structure of electrolytic metal surfaces buried under a liquid, and together with STM and AFM has revolutionized our understanding of these technologically important interfaces. Furthermore, this technique has revealed the structure of liquids above such interfaces. We have greatly improved our understanding of polymer surfaces and the structure of liquid surfaces by application of x-ray diffraction and reflectivity measurements.

Surface sensitive x-ray absorption spectroscopy in the extended fine structure regime (SEXAFS) pioneered in the late 70's at SSRL was the first technique to challenge surface structure determinations by LEED. Owing to the high precision of the technique for bond length determinations we have learned that bond lengths at surfaces are similar to those expected from chemical rules established for the bulk. Over the last ten years the technique has provided detailed information on the local dynamics of atomic motions at surfaces and the links between static and dynamic structure. To-date, only LEED has determined more surface structures than SEXAFS, with SEXAFS and PD contributing about equally.

The related NEXAFS technique developed in the early 80's at SSRL has given us detailed information on the structure of molecular chemisorption systems. From detailed studies of the molecular orientation at surfaces in more than one hundred systems we have learned how to link the molecular chemisorption geometry to the geometry and molecular orbital structure of the free molecule. More recently, NEXAFS has revealed the orientation of large organic molecules on surfaces and the orientation and relaxation of molecular groups at polymer surfaces, disordered systems that cannot be studied with diffraction techniques. Owing to its elemental and chemical specificity, its sensitivity to local bonding anisotropies through its polarization dependence, and its applicability to disordered systems, the technique is increasingly being applied for technological surface analysis, often in conjunction with photoelectron emission microscopy (PEEM).

Absorption measurements with circularly polarized x-rays, so-called x-ray magnetic circular dichroism (XMCD) spectroscopy, pioneered at HASYLAB in the late 80's, have significantly impacted our understanding of surface and interface magnetism. A related technique