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Synchrotron radiation: a unique, rapidly developing tool for chemical research.

Synchrotron radiation (SR) has become essential not only in chemistry, but also in nearly all of the physical and biological sciences, and in nearly all of the physical and biological sciences, and in several important industrial technologies. This article is a brief report on the status of synchroton radiation research with emphasis on chemical applications.

Fro those readers who are considering entering the field of synchrotron radiation research, or simply for those who are curious, there will be a four day symposium, Chemical Applications of Synchrotron Radiatin, and an associated workshop, Towards a Canadian Synchrotron Radiation Source at the 74th Canadian Chemical Conference and Exhibition in Hamilton, Ont., June 2-6, 1991.

What is Synchrotron Radiation?

A fundamental property of accelerated charged particles is that they lose energy by emitting electromagnetic radiation (light). When a charged particle experiences acceleration at relativistic speeds (close to the speed of light) as in particle accelerators such as Ge V electron synchrotrons (1 GeV = [10.sup.9] electron volts), the radiation is highly polarized, sharply concentrated in the direction of particle travel and the spectrum has a very wide, smooth energy distribution with extremely high intensity in the vacuum ultraviolet, soft- and hard X-ray regions. Initially, the accelerators used for panicle physics experiments considered synchrotron radiation a nuisance. However, in the late 1960s it was realised that the synchrotron radiation spectrum provided a unique, high-intensity light source which could be very useful. SR can be extracted either at the bending magnets, which maintain the circular orbit of the electrons (or positrons), or from insertion devices (undulators or wigglers) located along linear sections of the accelerator. Typically SR is 3 to 5 orders of magnitude more intense than competing laboratory sources such as rotating anode X-ray generators. More importantly, SR covers all X-ray energies not just one (see Figure 1).

To get an idea of the impact SR is having on many areas of science, imagine taking one of the best and most widely-used analytical techniques in your area of chemistry, and improve its capabilities a million fold. It is not difficult to imagine the new ways you could use this capability to enhance resolution (in space, angle or time), sensitivity or throughput. The SR applications, likely to be the most important in the long term, are those based on phenomena which can only be investigated with SR. Two examples are X-ray scattering from magnetic structures (a phenomenon previously widely believed to be accessible only with neutron beams) and nuclear resonance absorption (Mossbauer spectroscopy). In many ways, synchrotron radiation research in the 1990s is like laser research in the 1970s - one has a new, very powerful, general-purpose tool. Finding novel uses is a large part of the challenge and the fun of SR research.

Starting mostly with parasitic activities at existing accelerators, the rapid growth in demand for SR has led to the development of a number of facilities completely dedicated to synchrotron radiation generation and exploitation. At present there are 30 operating SR rings and 17 under contstruction. Each facility has 10-100 experimental stations (called beam lines), each of which is used by a number of scientific groups, typically between three and 20 each year. Overall there are at least 10,000 scientists worldwide using SR. This is a very sizeable community with very diverse research interests (medicine, biology, chemistry, physics, geology, materials, electronic device fabrication, etc), who share a common need for a high-performance facility. While the capital costs of an SR facility are large ([$10.sup.7] - [$10.sup.8]), because of the very large number of users who share each facility, the cost per researcher is very much in line with amounts spent on departmental instrumentation (for example). On this basis and also in view of the mind-set of the typical SR scientist, SR research is better considered as 'small' rather than 'big' science.

While this article emphasizes chemical applications, it is important to realise that SR provides new opportunities for research and technical applications in many areas of science and technology. Two areas which may turn out to have the largest impact are clinical medicine and semiconductor device fabrication. In both areas, chemical aspects play an important role.

SR X-ray absorption with imaging agents (typically I-containing dyes) is used for diagnostic medical imaging procedures, such as angiography. The ability to examine coronary circulation non-invasively, on a time scale short compared to heart-muscle contractions, promises to greatly benefit diagnosis of heart disease. One NSLS beam line is dedicated to clinical angiography. Schemes for radiation therapy through selective heavy element labeling of cancerous cells and SR exposure are being explored.

X-ray lithography with SR is driving the development of commercial SR sources (Sumitomo Heavy Industries, Oxford Instruments, Maxwell-Brobeck). This is widely believed to be the most economical way of producing high-volume electronic devices (such as 1 Mb memory chips) at ultra-scale levels of integration (ULSI-feature sizes below 0.25 [micrometer]). Many of the multi-user research facilities have strong lithography programmes with major industrial involvement (eg. the programme funded by Sematech at SRC, Madison, WI). IBM (NY) is currently commissioning a $500-million dedicated SR facility for X-ray lithography. Hitachi and NTT in Japan are also building dedicated facilities.

Chemical Applications of Synchrotron


X-rays are traditional structural probes, so it is not surprising that the greatly improved properties of SR are revolutionizing X-ray scattering and crystallography. The very high-intensity means that single crystal diffraction patterns can be obtained in a very short time - milliseconds or shorter - enabling time-dependent diffraction studies. The high intensity and brightness allow high-quality diffraction photographs of macromolecules to be obtained before radiation damage destroys the sample. X-ray diffraction, with wavelenghts close to the absorption edge of a specific element, allows solution of the phase problem through anomalous scattering and facilitates direct structure methods. Small and wide angle X-ray scattering are being used to study larger structure elements such as those found in partially crystallized polymers.

In addition to scattering, structural information can be obtained from details of the X-ray absorption spectrum. In fact, extended X-ray absorption fine structure (EXAFS, see Figure 2) has been a major impetus to the development of dedicated SR facilities. EXAFS senses only the local structure (< 5 [angstrongs]) around specific atoms which the user selects by the choice of X-ray energy. Because it requires only short range order, it can be applied to materials for which scattering and diffraction techniques provide little or no information.

An area of rapid development at present is real-space imaging using X-ray microscopy. This holds considerable promise for studies of biological specimens which can provide spatial resolution greater than optical microscopy in an aqueous environment in which the samples are stable. One class of X-ray microscopes uses both spatial and energy analysis of photo-ejected electrons thus enabling both chemical mapping and analysis of sub-micron areas by electron and X-ray absorption spectroscopy.

Synchrotron radiation is being used for dynamic and kinetic studies on a wide range of time scales. Fast-scan EXAFS, dispersive-EXAFS and time-resolved powder diffractometry provide information about structural evolution on a millisecond time scale. Combining lasers with SR allows studies of short-lived species, such as molecular fragments and electronically excited states. The pulsed structure of SR (typically sub-nanosecond pulse widths with a MHz repetition rate) can be combined with pulsed laser sources and delayed coincidence, or boxcar detection techniques, to probe kinetics and molecular dynamics on a nanosecond time scale. At an even faster time scale, studies of the decay of electronically excited and ionized states - involving inner and valence-shell electronic levels - is providing new insights into photoionization dynamics on a femto to picosecond time scale.

The tunable character of the SR spectrum is ideally suited to a wide range of spectroscopies, being applied to matter in all states. The near-edge region of X-ray absorption spectra (NEXAFS) provides a map of the unoccupied electronic structure at a user selected atom. The lowest energy unoccupied molecular orbitals (LUMO) have long been recognized of importance in describing chemical bonding and reactivity through the frontier orbital model. The polarization dependence of NEXAFS provides a simple way of determining molecular orientation in solids and surface adsorbates (see Figure 3). Both aspects of XAS (NEXAFS and EXAFS) have many practical applications such as research on polymers, catalysts and surface/interface chemistry.

Photoelectron spectroscopy (PES) maps the distribution of occupied orbitals (bands). The combination of PES and quantum chemistry is a very effective tool for studying molecular bonding. The tunability and high polarization of SR has greatly expanded PES. For example, tuning to photon energies 20 to 50 eV above the binding energy allows enhanced surface sensitivity and has enabled routine detection of surface-bulk chemical shifts. Angle-resolved photo-electron spectroscopy provides both electronic band mapping and structural studies via photoelectron diffraction. The recent development of very high resolution in the soft x-ray region (100 - 1000 eV) is revolutionizing core level probes such as photoelectron and photoabsorption spectroscopies for both gas phase and solid-state applications.

The SR spectrum also includes considerable flux in the infrared region. In fact, it is more intense than conventional (glow-bar) IR sources below 1000 [cm.sup.-1]. This, combined with the structureless spectrum, has led to recent developments of Fourier transform infrared reflection-absorption spectroscopy IIRAS) for surface studies.

Very recently ultra-high resolution X-ray monochromators have been developed to provide sufficient flux in a narrow band width (5 meV at 14.4 ke V) in order to enable spectroscopy of nuclear energy levels (Mossbauer). When implemented at the most advanced SR sources, the range of elements to which Mossbauer spectroscopy can be applied will be greatly expanded.

Elemental analysis, using white beam SR excitation and X-ray fluorescence detection (SXRF), provides very wide range capabilities with better than ppb detection limits. SXRF has a combination of flexibility and sensitivity that rivals neutron activation analysis and proton induced X-ray emission for non-destructive analysis. Synchrotron X-ray fluorescence is well suited to trace elemental analysis of biological matter with very exciting possibilities when carried out on a spatially-resolved manner on living specimens.

Overview of SR Rsearch programmes of

Canadian Scientists

In January 1990, a group of 50 Canadian scientists met in Ottawa to form the Canadian Institute for Synchrotron Radiation (CISR), an organization pioneered by Bruce Bigham at AECL Chalk River. CISR's mandate is to coordinate and facilitate Canadian SR research, and to improve access to SR facilities. At this meeting and a subsequent one in September 1990, a vision emerged in which research, requiring very high brightness in the hard X-ray, would be carried out on third-generation SR sources abroad. At the same time, a concerted effort would be made to develop in Canada, a general-purpose, multi-beam line SR facility with state-of-the-art performance at lower energies (below 10 keV perhaps) and some lower flux capability on the hard X-ray. This facility would be open on a peer-review basis to both Canadian and non-Canadian scientists, with preference in beam-time allocation being given to thosoe who participate in the development of the source and its beam line instrumentation. Some of the research programmes, currently being carried out by Canadian scientists at foreign facilities, would be shifted to this facility. Much of the research, however, would be new projects stimulated by the greater flexibility which is a consequence of being an owner-operator rather than an outside user of a beam line.

Currently, CISR has 55 individual members (university faculty members or senior scientists from university, government or industry), and 15 institutional members. The information below is taken from data supplied by CISR members. A booklet will be available soon from CISR with more details of Canadian SR research, and how to get involved. If you have not yet joined CISR, please write or phone one of the authors, and application forms will be mailed to you. Alternatively, forms will be available at the Synchrotron Radiation symposium in Hamilton.

Including graduate students, post-doctoral candidates, research associates and research scientists, well over 100 Canadian researchers have used SR at foreign sources. As in all developed (and some developing) countries, the growth of SR research by Canadian scientists has been phenomenal in the last few years. One indicator of this growth is the number of SR publications by Canadians from 1980 to 1990 (see Figure 4). Prior to 1984, only a handful of Canadian faculty were conducting SR studies, and only a few papers were published. In 1989 and 1990, more than 115 papers based on SR research were published by CISR members. These numbers will continue to increase dramatically. Faculty whose primary interest is SR research, have been hired in the last three years at five universities. A number of excellent Canadian graduate students and post-docs (future university faculty and industrial scientists) are currently performing outstanding SR research outside of Canada. Moreover, these trends have occurred without a Canadian synchroton! While the current situation is strong, Canadian activity is at most one-quarter of that in the US or Europe on a per-capita basis.

Several important general observations can be made concerning Canadian SR research. First, Canadian SR researchers come from most of the physical and biological sciences: geology, chemistry, physics, materials science, materials engineering, biology, biochemistry and medical imaging are all represented. Second, the research interests span the fields outlined above. For example, a large number of scientists from diverse areas are involved in structural studies: from crystallographic studies of proteins, polymers, catalysts and solid state phase transitions; to EXAFS studies of semiconductors, glasses, metals, minerals, superlattices, and interfaces. In the area of time-dependent phenomena, research ranges from time resolved X-ray diffraction studies of solid-state phase transitions on the millisecond time scale to studies of ionic and electronic decay of core-excited species on the [10.sup.-15] second time scale. In the area of spectroscopy, studies range from photoelectron spectroscopic investigations (mostly at unprecedented resolution) of semiconductors, metal interfaces, gas phase atoms and molecules; to NEXAFS investigations of sulphur in coal, and antiwear films on stainless steel, minerals, organic species on surfaces, and gas molecules. Although many studies are quite fundamental in nature, some of the studies (eg. semiconductors, coal and antiwear films) have great industrial interest and promise.

Third, many different foreign facilities have been used in the US, Japan and Europe. The majority of soft x-ray users have worked at the Canadian Synchrotron Radiation Facility (CSRF) which operates three beamlines at the Aladdin facility outside Madison, Wisconsin. Monochromatic photons from 20 eV to 4000 eV are available on a grazing incidence monochromator beamline (funded by NSERC and NRC), and a double crystal monochromator beamline (funded by the Ontario Centre for Materials Research (OCMR)), while a third beamline gives filtered unmonochromatized SR for imaging and tomography studies. In the hard x-ray region (energies above 5000 eV), Daryl Crozier (SFU) operates a crystall monochromator beamline at SSRL for diffraction and EXAFS studies. There are strong links between University of Laval researchers and LURE, the French SR facility at Orsay. Other CISR researchers (eg. Adam Hitchcock, FCIC, A. McLean, T.K. Sham, MCIC; T. Tiedje; M. Sutton; and J.S. Tse, MCIC) have strong contacts with CHESS at Cornell and/or NSLS at Brookhaven.

Highlights for the 74th CCC

With support from CISR, Sumitomo H.I. and Maxwell Laboratories, the symposium, Chemical Applications of Synchrotron Radiation, is being held at the 1991 Canadian Chemical Conference (2-6 June, Hamilton, Ont.). Topics which will be explored in the first six sessions (Monday through Wednesday) include: spectroscopic applications in biochemistry and solids; surface studies through VUV absorption, NEXAFS and PES; chemical imaging with soft X-ray microscopy; X-ray diffraction and scattering studies; chemical kinetics and dynamics; PES of gases and solids; and a variety of biological and materials research applications. The two sessions on Thursday will form the technical component of the CISR workshop, Towards a Canadian Synchrotron Radiation Source. Thursday morning speakers will describe the capabilities of American SR facilities and opportunities for Canadian participation at them, either as general users or at a higher level of involvement through participating research teams responsible for building, operating and maintaining one or more beam lines (as at CSRF). Thursday afternoon will deal with proposals for a SR facility to be constructed in Canada, either by a team of accelerator physicists and engineers assembled for that purpose, or by purchase of those services. In the evening there will be a reception, banquet and a keynote tal, "The Future of Synchrotron Radiation Research", by Herman Winick, deputy director, Stanford Synchrotron Radiation Laboratory.

This workshop will provide the Canadian synchrotron radiation research community an opportunity to reach contsensus about the desirable parameters of an SR facility this country. The achievement of this facility would then be worked towards under the leadership of CISR.


General overviews of Sr

Research in Canada:

G.M. Bancroft, K.H. Tan and J.D. Bozek, Physics in Canada, 43, pp. 113-120 (1987).

M. Sutton and G.B. Stephenson, Physics in Canada, 44, 131 (1988).


C.R. Catlow and N. Greaves, Chemistry in Britain, pp. 803-852 (September 1986).

A. Bienenstock and H. Winick, Physics Today, pp. 48-58 (June 1983).

The bi-monthly journal, Synchrotron Radiation News (Gordon & Breach, NY), started in 1988, gives timely reports on developments in SR facilities and technology.
COPYRIGHT 1991 Chemical Institute of Canada
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Copyright 1991 Gale, Cengage Learning. All rights reserved.

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Author:Hitchcock, Adam; Bancroft, G. Michael
Publication:Canadian Chemical News
Date:May 1, 1991
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