A new tool for chemical and structural studies Keith 0 . Hodgson and Sebastian Doniach, Stanford University
Electromagnetic radiation is a universal probe for studying the chemical and structural properties of matter. Since the 1960's, new sources providing a unique form of such radiation, called synchrotron radiation, have become increasingly available to researchers. Use of synchrotron radiation, in turn, is having an impact on many experiments that, in essence, involve the interaction of photons with atoms. Synchrotron radiation is produced as a natural consequence of bending the trajectory of very-high-energy electrons or positrons by a magnetic field. The thrust in high-energy physics toward the study of particle interactions at ever higher energies has led to the construction of circular multi-GeV electron and positron accelerators (synchrotrons) and storage rings. In such machines, very intense, extremely broad-band beams of electromagnetic radiation are produced by a process that is a relativistic version of that which produces the normal electromagnetic waves emitted from an antenna. The emitted synchrotron radiation is very sharply collimated in the plane of the electron orbit in the machine. The unusual properties of synchrotron radiation—its broad spectral distribution (extending from x-ray to infrared wave lengths), its high intensity, and its time and polarization structure—make it an attractive light source for use in numerous experiments. During the past decade, scientists have increasingly exploited synchrotron radiation as a photon source. Early experiments with synchrotron radiation date back to work in the 1950's at Cornell University and at the National Bureau of Standards. Beginning in the late 1960's, a relatively small storage ring at the University of Wisconsin called Tantalus I produced synchrotron radiation with usable wave lengths extending down to about 60 A. About the same time, experiments started using a much larger and higher-energy electron accelerator—the DESY electron synchrotron in Hamburg, West Germany. This machine accelerated electrons to above 5 GeV and the resulting synchrotron radiation spectrum extended down to x-ray wave lengths (less than 1 A). Although a successful program of measurements in the far ultraviolet region was conducted at DESY, the rapid movements of the electron beam position inherent in the synchrotron (in which the electrons are being injected, accelerated, and extracted many times a second) made x-ray experiments difficult, because of the small acceptance angle of the x-ray monochromators. In 1974 a more stable source of high-energy synchrotron radiation became available at the Stanford Positron Electron Accelerating Ring (SPEAR) of the Stanford Linear Accelerator Center at Stanford University. SPEAR was designed for studying electron-positron collisions at multi-GeV energies, and its use led to the discovery and analysis of a new family of elementary subnuclear particles. Soon after SPEAR came into operation in late 1972, a committee set up by the National Science Foundation recommended that two national centers 26
C&EN Aug. 21, 1978
The Stanford synchrotron radiation project is housed in the building to the right of the storage ring at Stanford University's linear accelerator center
be operated in the U.S. for synchrotron radiation research, the Tantalus I ring at the physical sciences laboratory at the University of Wisconsin, Stoughton, under the directorship of Ednor Rowe, and SPEAR under the directorship of Sebastian Doniach. By this time, synchrotron radiation research laboratories also had been set up in Italy, France, the U.K., Japan, and the Soviet Union. As experience was gained at the Stanford Synchrotron Radiation Laboratory (SSRL), it became apparent that the excellent properties of a multi-GeV storage ring (as opposed to an electron synchrotron) as a source of photons in an energy range extending into the x-ray region of the electromagnetic spectrum allowed a whole new range of applications to become possible. Tantalus and SPEAR, as well as several other storage ring sources abroad, continue to provide the means for scientists to explore the research and technological applications of synchrotron radiation. Selected examples of such applications, which illustrate the current and future trends in the uses of synchrotron radiation, are highlighted in this article. Photons and chemistry Electromagnetic radiation long has been used to characterize materials of interest to chemists. The color of a material is a measure of its absorption spectrum in the visible wave-length
RAD^TION range. When suitably quantified, color is, of course, an indicator of the electronic structure of the material. The availability of an intense broad-band spectrum of photons immediately extends absorption spectroscopy into regions of energy previously either unattainable or reached with great difficulty using conventional sources, such as discharge lamps or x-ray tubes. By the use of various monochromators, virtually any wave length within the range produced by the synchrotron emission can be selected for experiments. Besides absorption spectroscopy, other kinds of spectroscopy become accessible at photon energies above the ionization threshold of the sample. Photoemission, photoionization, and photodissociation are examples that effectively use the soft x-ray region of the synchrotron spectrum. Application of these techniques is proving important in understanding both electronic structure and chemical processes that occur at surfaces. Moreover, when the wave lengths of the photons are comparable to interatom spacings (of the order of a few angstroms), diffraction effects also become important. Many of these experiments could benefit from the availability of the intense, stable beam of photons produced by electron storage rings. Consider, for example, x-ray diffraction, which is the classical tool for crystallographic measurements used for structure determination. An intense, variable wave-length x-ray beam has the potential to add an extra dimension to crystallography by allowing large changes to be produced in the way atoms scatter x-rays when the incident x-ray wave lengths are tuned to be near the absorption edges (energies at which inner shell electrons become ionized) of selected elements in a crystal. This effect, known as anomalous scattering, can help resolve the well-known phase problem of crystallography. Thus, using synchrotron radiation, it may well be possible to collect x-ray diffraction data more rapidly, on smaller samples, and use the "tunable" nature of the source to make determination of the structure easier. Another kind of spectroscopy that becomes more powerful with the availability of a variable wave-length x-ray and ultraviolet source is fluorescence emission. The use of intense x-ray beams (rather than the electron or proton beams that have been used often in the past) for exciting x-ray emission spectra greatly improves the sensitivity of trace element analysis. Because the electrons in a storage ring circulate precisely in the form of a bunch, the synchrotron light is produced in very stable, subnanosecond pulses that are repeated on a microsecond time scale. This short and very narrow pulse shape (which exhibits no "tailing" when compared with that produced by more conventional discharge sources) is proving important in determining lifetimes of optically excited states. More important, synchrotron radiation extends the capabilities for making these measurements into the vacuum ultraviolet region of the spectrum where laser sources are much less effective or are even unavailable. These are but a few examples of areas in which synchrotron radiation has been usefully exploited. It should be noted that synchrotron radiation will not revolutionize every area of spectroscopy—for example, in the visible region it cannot compete in intensity with laser sources. Nevertheless, scientists in numerous fields—among them chemistry, biology, and physics—have begun to use synchrotron radiation both to en-
Special Report
Synchrotron radiation ranges across visible, ultraviolet, x-ray regions of the spectrum Electromagnetic radiation Wave length, cm
Energy, eV 10- 9
Kind
|
Source
10- 8
Radio waves
|
Transistors
I
&
10- 7
-
10- 6
-
10- 5
-
—
105
- 104 103
tubes FM radio
|
10
Klystrons o
10- 4
Microwaves
— 1 magnetrons Wavelength, A a
I I
IO- 3 10-2
- 10 2
|
]
-
10-1
Infrared
|
Visible light
I
[
1
j
10
106 5 - 10
_
_
Lasers
104
&
103
lamps 102
102 Ultraviolet 103
I I
Synchrotron radiatioh
I
X-ray tubes
10
10 4 105
1
m
x
"ravs
10" 1
10 6
I I
Radioactive sources
~~
10 7
10- 3 Gamma rays
| 10- 4
10 8 Accelerators
10 9 I |
10 10
10-5
&
10- 6
storage rings
10 11
10-7 Cosmic rays
|
10 12 10 13
10-2
!
10- 8 10- 9
Aug. 21, 1978 C&EN
27
Selected synchrotron radiation storage ring sources
1 1
Machine, location
In operation VEPP-3 VEPP-4 Novosibirsk, U.S.S.R. DORIS Hamburg, West Germany SPEAR Stanford DCI Orsay, France Tantalus Wisconsin SURF IE NBS, Washington, D.C. Under construction PETRA Hamburg, West Germany (1978) PEP Stanford (1979) CESR Cornell (1979) NSLS Brookhaven(1981) SRS Daresbury, U.K. (1979) ALADDIN Wisconsin (1980) NSLS Brookhaven(1981)
Maximum electron energy, GeV
Bending radius, meters
Current, mllllamps
Critical energy, 8 keV
Critical wave length, A
Comments
'
2.25 7
6.15 10
100 10
4.2 46.1
2.952 0.269
Not dedicated Not dedicated, current operation at 4.5 GeV
4.5
12.1
50
16.7
0.742
Not dedicated
4.0
12.7
100
11.1
1.117
Dedicated 50% after 1980
1.8
4.0
500
3.63
3.145
Not dedicated
0.24
0.64
200
0.048
258
Dedicated
0.25
0.84
25
0.036
344
Dedicated
18
192
18
67.4
0.184
No synchrotron radiation use currently planned
18
166
10
78
0.159
No synchrotron radiation use currently planned
100
35
0.354
Synchrotron radiation user facility planned
8
32.5
2.5
8.17
500
4.2
2.95
Dedicated
2.0
5.55
500
3.2
3.87
Dedicated
1.0
2.08
500
1.70
7.29
Dedicated
0.70
1.90
500
0.40
31.0
Dedicated
a Critical energy = 2.218 E3/*? where E = circulating electron energy (GeV) and R = bending radius of orbit (meters).
hance the utility of more conventional spectroscopic methods and to open up new areas of photon science. X-ray absorption spectroscopy Since the pioneering work in the early 1960's at the National Bureau of Standards, synchrotron radiation has become a principal tool for studying photoabsorption cross sections of atoms and molecules at wave lengths in the vacuum ultraviolet region of the spectrum. Because of the very poor penetration of ultraviolet radiation in matter, most of these studies must be done in the gas phase at low pressures. Synchrotron radiation in the x-ray region of the spectrum has allowed extension of these studies. Because of the much greater penetration of x-rays in matter, x-ray spectroscopy may conveniently be carried out on solid or liquid samples in an atmospheric environment. Therefore, x-rays can more readily be applied to structural problems arising in the study of macromolecules and solids. X-ray absorption spectra, and the fine structure found above the absorption edges (called extended x-ray absorption fine structure, or EXAFS), have been studied since the 1920's. Because the x-radiation from an electron storage ring is intense and continuous, synchrotron radiation has revolutionized the ability to record such x-ray absorption spectra accurately, especially on dilute samples. This, coupled with advances in the quantum mechanical theory of EXAFS and concurrent development of new techniques of data analysis, has elevated EXAFS studies from a physical curiosity to a powerful new tool for 28
C&EN Aug. 21, 1978
chemical and structural analysis. The white radiation background from a conventional x-ray tube is relatively low in intensity compared to the characteristic emission line intensity of the x-ray tube. Thus, the intensity of photons from an x-ray tube source at noncharacteristic energies is some three to four orders of magnitude lower than the intensity of x-rays from a storage ring like SPEAR. X-ray absorption spectra that take a few minutes to record when radiation from a storage ring is used would require days or even weeks using a conventional x-ray tube and monochromator. Many hundreds of x-ray absorption spectra at energies ranging from 4 keV up to 30 keV have now been taken at SSRL by a number of different groups from all over the U.S. and by visiting scientists from Japan, Canada, and Europe. The use of x-ray absorption data to obtain structural information was suggested in the early 1970's by Edward Stern and Dale Sayers working at the University of Washington, Seattle, and Farrel Lytle at Boeing Co. Early work was carried out on reasonably concentrated samples using an x-ray tube source. The availability of the intense x-rays has allowed collection of much more accurate data on a wide variety of samples in different physical states. Analysis of the x-ray absorption edge and EXAFS data provides electronic and structural information about the element being studied. The local structural environment on the atomic scale around that element can be determined without requiring that the element be in a crystalline lattice. Studies using synchrotron radiation of numerous compounds of known
structure by research groups at the University of Washington, Bell Telephone Laboratories, and Stanford University have shown that EXAFS analysis can measure the distance between an absorbing atom and its near atomic neighbors to an accuracy of a few hundredths of an angstrom and provide coordination numbers (under favorable conditions) to about one atom in five. Information about the type of atoms surrounding the absorber is also available from EXAFS. As the method can be applied to gases, solids, or liquids, it offers an excellent means of directly comparing local environments of metal atoms under different physical conditions. For example, the distance to a liganding atom in solution can be compared with that in a crystalline solid. By adjusting the wave length to a selected absorption edge, one type of atom can specifically be studied in the presence of others. EXAFS spectroscopy has proved particularly powerful for studying the environment of metal atoms in noncrystalline samples extracted from biological systems. An example is rubredoxin, a low-molecular-weight protein involved in biological oxidation-reduction reactions. Rubredoxin contains one iron atom coordinated by four cysteine sulfur atoms. Conventional x-ray crystallographic studies initially suggested that three of the four Fe-S bonds were of normal length (2.34 A), whereas the fourth was shorter (less than 2.05 A). X-ray absorption studies by Robert Shulman and collaborators at Bell Labs and by Edward Stern and his coworkers at the University of Washington have demonstrated that all of the Fe-S bonds are in fact equal in length, 2.26 A, with an uncertainty of about 0.05 A. Further refinement of the x-ray structure has led to lengths agreeing with those determined by EXAFS. Shulman and his collaborators more recently have studied the iron-to-porphyrin nitrogen distance in oxy and deoxy hemoglobin. In contrast to information from crystallographic studies, their EXAFS studies show that the iron atom moves much less than 0.7 A away from the heme plane on deoxygenation. This important observation suggests that the phenomenon of "cooperativity," in which the affinity of hemoglobin for oxygen is dependent on how much oxygen already is bound to the hemoglobin, cannot be explained by a simple model that postulates that cooperativity is mediated by motion of the iron atom relative to the heme plane on oxygenation. These results prove that other factors, remote from the iron site, must be involved. This example also illustrates the usefulness of EXAFS in accurately determining a coordination distance, which would be difficult to obtain by even high-resolution protein crystallographic studies. This is because EXAFS focuses attention exclusively on the metal and its local environment. In macromolecules for which no crystal structure is available, EXAFS can provide information about the coordination environment of a metal ion that can help identify the nature of the active site in metalloenzymes. Recent studies at Stanford, in collaboration with Leonard Mortenson at Purdue University and scientists from the University of Wisconsin and Charles F. Kettering Laboratories, have been directed toward the nitrogenase enzyme system. Nitrogenase is the bacterial enzyme system responsible for the conversion of atmospheric nitrogen into ammonia. The enzyme contains molybdenum and iron atoms, both of which may be components of the active site. EXAFS studies now have clearly established that the molybdenum is bonded to sulfur atoms and not to doubly bound oxygen as had been previously suggested. Further evidence that a second neighbor of the molybdenum is a set of iron atoms provides a new breakthrough in finding the answer to the riddle of the structure of this important enzyme. Efforts are under way to model this site chemically in nitrogenase. X-ray absorption studies also have been used for comparison studies. EXAFS shows that the molybdenum environment in the low-molecular-weight FeMo cofactor derived from nitrogenase is indeed very similar to its environment in the active protein. Thus, the molybdenum site in the cofactor has remained essentially intact during the extraction of the cofactor from the nitrogenase.
EXAFS also has been extended to the study of atoms adsorbed on surfaces and of catalysts. Work by Stern and collaborators at the University of Washington on the EXAFS of bromine molecules adsorbed at monolayer concentration on a graphite substrate, for example, has provided information about the orientation and Br-Br interatomic distance of the adsorbed molecules. And Peter Eisenberger and Paul Citrin of Bell Labs, studying the EXAFS of iodine monolayers adsorbed on single crystals of silver, have measured the silver-iodine bond length—which would be very difficult to determine by other techniques, such as LEED (low-energy electron diffraction). LEED is a useful technique for determining lattice parameters of an adsorbed layer but is difficult to interpret in relation to adsorbed atom-substrate bond lengths. John Sinfelt of Exxon Research Laboratories, Linden, N.J., and Farrel Lytle and collaborators at Boeing have used EXAFS to study still more complex heterogeneous catalysts in which very finely divided metallic mixtures (containing ruthenium, copper, and other metals) are supported on noncrystalline silicon dioxide particles. These catalysts are of interest in the processing of petroleum. Owing to the small size of the catalytic particles (in the tens of angstroms range), it is very hard to determine whether the metallic elements are combined as homogeneous alloys or in more complicated arrangements. The EXAFS studies have led to a structural model for the Cu/Ru system in which microscopic platelets of ruthenium are coated
X-ray spectra compare enzyme with extracted cofactor FeMo-cofactor
Azotooacter Mo-he
Clostridium Mo-Fe
20,000
20,125
20,250
Energy, eV
A comparison of the molybdenum K edge x-ray spectra for nitrogenase Mo-Fe protein from two different bacteria (Clostridium and Azotobacter) with that of the FeMo-cofactor extracted from the Azotobacter Mo-Fe component. Detailed numerical analysis of the data for the two different Mo-Fe components establishes that their Mo environments are virtually identical. The Mo is liganded directly by sulfurs which bridge to iron atoms. The similarity of the data for the cofactor establishes that this low-molecular-weight fragment of the Mo-Fe component contains Mo in essentially the same environment as the intact protein. These results illustrate the use of EXAFS both in obtaining quantitative structural information and in directly comparing a metal environment in different situations. The results are the work of Stephen Cramer and Keith O. Hodgson of Stanford and their collaborators working at SSRL. Aug. 21, 1978 C&EN
29
Synchrotron radiation production and properties Synchrotron radiation is electromagnetic radiation emitted by electrons traveling at relativistic speeds in a curved path. The radiation is a by-product of many high-energy accelerators used for studies in particle physics. During the 1960's, most of the machines producing such radiation were synchrotrons. In a synchrotron, particles such as electrons are injected and are raised in energy rapidly over a period of milliseconds while circulating in the machine's orbit. During this time, they emit synchrotron radiation. Upon reaching the desired energy, the particles are targeted. During each repetition of this cycle, the spectral distribution of the emitted radiation is changing constantly. More recently, especially since the early 1970's, storage rings have come into use for colliding-beam particle studies in high-energy physics. These rings consist of large arrays of bending and focusing magnets enclosing an ultrahighvacuum chamber in which the electrons are stored and circulate in tight "bunches." The energy lost by the electrons (because of emission of synchrotron radiation) as they circulate is compensated by radio frequency accelerating cavities operated in phase with arrival of the bunches. The particles are stored with currents up to a few hundred milliamps and adjusted to the desired energy within the range of the machine. At the high vacuums, of about 10~ 9 torr, at which the rings operate, the lifetime of the stored electron beam ranges from two to 20 hours. When storage rings are used for high-energy physics studies, a counterrotating beam of particles is stored and collided with the first. Under such "colliding-beam" conditions, lifetimes of the order of two to six hours are more common. Storage rings provide an inherently more spectrally and positionally stable source of radiation than do synchrotrons. AH synchrotron radiation that is of value for research is produced by synchrotrons or storage rings in which the particles are electrons or positrons. These are to be distinguished from proton accelerators, in which the emitted synchrotron radiation is of negligible practical interest. This is because the power radiated by a
Electron orbit
—
Acceleration
Arc viewed by observer
Radiation pattern from electrons moving in a circular orbit.
with one or two atomic layers of copper—very far from a homogeneous alloy state. Homogeneous- and polymer-bound catalytic systems also have been studied by EXAFS. Peter Eisenberger, Brian Kincaid, Joseph Reed, and Boon-Keng Teo of Bell Labs first studied the polymer-bound equivalent of Wilkinson's catalyst, [(CeHsJsP^RhCl, and found that the Rh(I) catalyst aggregates to form binuclear chloride-bridged clusters when attached to the 2% cross-linked polymer. When the catalyst is reduced with hydrogen, chloride is lost to give what the Bell scientists suggest to be polymer-bound monomeric Rh(III) chlorodihydride. Additional studies on the bromo Rh(I) polymer-bound catalyst demonstrated that higher cross-linking (20%) substantially reduced the degree of dimerization. These EXAFS studies 30
C&EN Aug. 21, 1978
Radio frequency cavity
Wiggler; magnet'
High-vacuum ion pumps Synchrotron b radiation 1 norts
•Roughing vacuum pump
""Bending magnet Distributed sputter-ion pumps (in eight curved sections)
Sync
Wiggler magnet*""
,_— Inflector Synchrotron radiation ports
Sector valve ^High-vacuum ion pump
Schematic of a small (0.5 to 1.0 GeV) storage ring as utilized for synchrotron radiation. A closed, continuous high-vacuum chamber threads through various ring elements, including bending magnets that bend the electrons in a circle and produce synchrotron radiation (only one magnet is shown); special insertions (optional), such as the wiggler magnets, to produce particularly intense or enhanced radiation; a radio frequency cavity and associated power supply, which replenishes the energy lost by the electron beam into synchrotron radiation; vacuum pumps to evacuate the chamber; an inflector that permits electrons from a separate accelerator (not shown) to be injected; and ports from which radiation is emitted.
particle is inversely proportional to the fourth power of its mass. Thus, for example, a proton would emit about 10~~13 of the synchrotron radiation of an electron. As an example, some of the operating parameters of the SPEAR high-energy electron storage ring are: Accelerating radio frequency Total radio frequency power available (4 klystrons) Pulse duration Orbital period Orbiting electron energy (E)
358 MHz 500 kw 0.2 to 0.4 nanosecond 780 nanoseconds 1.3 to 4.0 GeV
suggest a structural interpretation of how the degree of polymer cross-linking affects activity. As the degree of cross-linking increases, the formation of inactive dimer is inhibited and thus catalytic activity increases, at least up to a point where high cross-linking prevents effective substrate interaction. Further studies of these and other systems may unravel the molecular details of how the catalysts function and establish the relationship between homogeneous and heterogeneous catalytic systems through study of polymer-bound equivalents. Recent experiments at SSRL by David Shirley and his group from the University of California, Berkeley, Robert Bachrach and collaborators from Xerox research laboratory at Palo Alto, Calif., and Joachim Stohr of SSRL have studied absorption edges of elements with low atomic numbers. EXAFS has been
Bending radius (R) Magnetic bending field Electron injection energy Maximum available orbit chamber cooling Maximum achieved stored beam (1976)
12.7 meters 10.5 k oersted at 4 GeV 1.5 to 2.4 GeV 150 kw per beam 225 milliamp at 2.25 GeV
The SPEAR storage ring also may be operated with several electron bunches circulating simultaneously (up to 280). In this way, greater stored currents, limited by the capacity for cooling the orbit chamber, may be achieved than in single beam mode. Synchrotron radiation from storage rings has several properties that enhance its ability as a photon source, including broad-spectrum band width, high intensity, extremely high collimation, sharply pulsed time structure, and plane polarization. Power radiated from a storage ring has a broad-spectral band width in a smooth, featureless continuum without the spikes or fluctuations commonly associated with most light sources. The spectrum is characterized by a parameter called the critical energy, ec, which is given (in keV) by 2.2\E3/R(E is stored electron energy in GeV and R is the bending radius, in meters, of the machine). Typically, useful flux is available out to about four or five times the critical energy. It can be seen that the spectral distribution extends over four or more decades of the electromagnetic spectrum, with the useful flux at the short wave-length limit a critical function of the stored beam energy. Synchrotron radiation is a high-intensity continuum source into the x-ray range. For example, it has four or more orders of magnitude of greater intensity than the most powerful conventional x-ray sources over all wave lengths (except certain elemental emission lines). Although visible synchrotron radiation cannot compete in intensity with laser sources, its intensity extends throughout the ultraviolet and soft x-ray regions, thus opening up a new spectral window in a region difficult to cover continuously with conventional sources. Synchrotron radiation is capable of extremely high collimation. It is sharply folded forward by relativistic effects into a narrow cone which sweeps around the orbit as the electrons circulate. The vertical angle of emission at the critical energy is given approximately by mcZ/Eimc2 is the rest mass energy of the electron). At 3.5 GeV, for example, the half opening angle is only 0.14 X 10~~4 radians. This would produce a beam height of 5.7 mm at 20 m from the source. At high photon energies (harder x-rays), the opening angle decreases; at lower energies (soft x-rays and ultraviolet), it increases. The electrons in the storage ring circulate in bunches (so-called "buckets") as a natural consequence of the frequency of the radio frequency power used to replenish the
observed above the 450-eV Lnjn edge in titanium. These studies extend EXAFS into the ultrasoft x-ray region of the electromagnetic spectrum, where the K edges of the important second- and third-row elements fall. This capability could prove important in showing how small gaseous molecules, such as carbon monoxide, interact with metal surfaces. X-ray imaging X-ray lithography is a process by which a mask containing micron and submicron features (such as an integrated circuit pattern) is multiply copied. Of potential commercial interest is the use of soft x-ray synchrotron radiation to replicate such submicron structures. In a typical experiment, an x-ray sensitive
Photon wave length, A
10,000
1000 .
100
10
,
,
j
1 1
Mg
Ca Cu Mo I
Cu K emission I
0.001
•
I
0.01
•
. •
I
>l
i
0.1 1 Photon energy, keV
i i
I
i
10
\
; ,,,,.!
100
a Number of photons per second per millirad per milliampere per 1 % band width.
Spectral distribution of synchrotron radiation. The effect of a wiggler magnet with the beam energy at 2.0 GeV is also shown. For comparison, the emission spectrum of a typical rotating anode copper radiation source is shown. Typical K absorption edges of several elements are shown by arrows. Photon energy (in keV) can be converted to wave length (in A) by the formula A = 12.3981/keV
synchrotron radiation. This results in a pulsed source with a very high repetition rate. With SPEAR, for example, every 780 nanoseconds a fixed observer would "see" a bunch of photons lasting about 300 picoseconds in duration. Unlike many conventional emission sources, the pulses are terminated precisely, with no tailing. The radiation is 100% linearly polarized in the plane of the electron orbit. When summed over all vertical angles, the source is greater than 7 5 % plane polarized.
polymer (photoresist), such as polymethyl methacrylate, is exposed through the master mask that is to be replicated. The exposed photoresist then is removed by development and the substrate etched where the resist has been removed to produce the desired replica. Eberhard Spiller and his coworkers at the IBM research center in Yorktown Heights, N.Y., have demonstrated the feasibility of this method to reproduce submicron structures through a series of experiments carried out at the DESY laboratory. The prospects of using synchrotron radiation for very large scale integrated circuit fabrication are promising. Current methods using electron beams for circuit fabrication are relatively slow. They have the further disadvantage that secondary effects of the electron beam (which result from x-rays produced Aug. 2 1 , 1978 C&EN
31
EXAFS studies can extend into ultrasoft x-ray region
500
600
700
800
Photon energy, eV
The L„t ui x-ray absorption spectrum of a titanium foil, showing the EXAFS oscillations above the absorption edge, which occurs at about 450 eV. First, second, and fourth neighbor distances appeared on A synchrotron x-ray diffraction topograph of a garnet wafer obtained performing a Fourier transform of these data. This work demonstrateswith white x-radiation in about 1 second. The incident beam is normal the feasibility of using ultrasoft x-ray EXAFS to study thin films. These to the stationary crystal wafer surface. A Polaroid film (without fluoare the results of David Deniey, David Shirley, and their coworkers of rescent screen) was placed behind the crystal to record the transmitted the University of California, Berkeley, working at SSRL front Laue-reflection pattern. The reflections are from different planes in the crystal, which select a wave length from the continuous radiation to satisfy Bragg's law. All possible reflections are recorded simultaby the electron impact) can produce unwanted damage in the neously. To study the detail in the spots, high-resolution film is used photoresist. This reduces definition of the replica pattern. and enlarged optically after development. The individual reflections The basic x-ray lithographic process can use x-ray tubes or each clearly show three large defects (the white dots) on the surface. synchrotron radiation as the x-ray source. The advantage of This topograph was provided by William E. Parrish of IBM research synchrotron radiation is that its high intensity allows very short laboratories at San Jose. exposure time (about 1 second) even when the resist is relatively
insensitive. Furthermore, the high collimation of the synchrotron x-rays makes possible deep parallel grooves with large aspect ratios (the ratio of depth to spacing of the groove). The IBM group further has shown that lithography of macromolecular structures, followed by scanning electron microscopy analysis of the developed and shadowed resist, gives image resolutions that approach 100 A. These applications likely will
prove useful for the study of supramolecular structures, since the contrast of biological structures to soft x-rays is much greater than for similar resolution using electron microscopy. Furthermore, the specimen can be examined under less destructive conditions, such as a helium atmosphere. Another advantage is that visibility of a specific element in a complex sample can be enhanced by variation of the incident wave length around the absorption edge of that element. X-ray topography is another imaging method that has found numerous important applications in quality control of crystalline components where defects, cracks, voids, and dislocations in the crystal lattice need monitoring. As with lithography, the high intensity and collimation of synchrotron light make it especially well suited for topographic experiments. Furthermore, the continuous spectrum of synchrotron radiation facilitates the use of Laue diffraction techniques, which make the orientation of the specimen in the beam less critical. In a crystal topograph the contrast, which shows crystal defects, is achieved because of the difference in reflecting power of the perfect vs. the distorted parts of the crystal. Topographic studies of dynamic distortion effects that occur when a crystal is subjected to stress appear to be possible with synchrotron radiation.
A replica of a submicron structure produced by synchrotron x-ray lithography. The photograph is from a scanning transmission micrograph of a resist exposed through the submicron mask and developed. The Surface studies using photoelectron spectroscopy base width of the vertical slabs is about 1 micrometer. This photograph is an excellent example of the high aspect ratio (depth over width) that In photoelectron spectroscopy, a sample is irradiated with can be obtained using synchrotron x-ray lithography. The photograph, monochromatic light of sufficient energy to induce a phorecorded at DESY in Hamburg, was provided by Eberhard Spiller of IBM toemission. The electrons thereby produced are analyzed by research center at Yorktown Heights, N. Y. their energy distribution, and the electron intensity at each 32
C&EN Aug. 21, 1978
energy increment is measured as a function of emission direction as well as the energy and polarization of the incident beam. If all of these variables are analyzed, such experiments can be used to map the spatial and momentum character of the initial and final electron states. Synchrotron radiation, which is of high intensity at wave lengths much shorter than those currently available from laser sources, has stimulated a renewed interest in the application of photoelectron spectroscopy to study electronic and structural problems associated with solids, surfaces, and gases. Recent photoemission experiments have used synchrotron radiation to study the surfaces of clean crystals. Photoemission is useful for studying surface effects because of the dependence of the mean free path of electrons on energy as they emerge from a solid surface as a result of photoexcitation. Work during the past 10 years has established that as the energy of the photoelectron is increased from a few electron volts up to 50 or 100 eV, the mean free path of the emerging electron drops to dis-
Synchrotron radiation probes oxidation of crystal surface Gallium 3^ level .
Arsenic 3d level
Clean surface
Heavily oxidized surface
tances of a few angstroms because the electron collides with valence electrons in the solid itself. Thus, by having variable wave-length sources in the 50- to 100-eV range, the photoelectrons that are excited emerge principally from the surface atomic layers of the solid. The resulting energy spectra of the photoelectrons are then an indication of the electronic states of surface atoms or those immediately below the surface rather than of the bulk substrate. This information is important to surface physicists and chemists interested in knowing the precise state of bonding or adsorption of small molecules (such as carbon monoxide or nitric oxide) on surfaces and of the state of the surface atoms themselves. It is especially valuable because in many materials the chemical state of the top layer of atoms is appreciably different from that of the bulk of the material. Recent experiments by William E. Spicer and coworkers at Stanford University on the nature of cleaved gallium arsenide single-crystal surfaces have probed the surface properties of gallium atoms and arsenic atoms in relation to oxidation. These researchers have used synchrotron radiation-induced photoemission from inner-shell electrons of the surface atoms (as is normally done with x-ray photoelectron spectroscopy for the bulk material). They determined that oxygen on the [110] crystallographic surface of the gallium arsenide predominantly binds to the arsenic atoms rather than to the gallium atoms. These studies are important to the semiconductor industry for understanding how to control and utilize such oxidation effects in the production of semiconductor devices. Similar studies by Stig B. M. Hagstrom and coworkers from the Xerox research center at Palo Alto have established the nature of the aluminum-oxygen bond in the initial stages of oxidation of aluminum surfaces. Synchrotron radiation sources also have been helping with the study of the electronic states of pure materials—metals and compounds. In these experiments (which also are carried out using conventional ultraviolet sources) the angular dependence of the emitted photoelectrons may be correlated with the photoelectron energy to give information about the energy
Beam of synchrotron radiation produced in the SPEAR storage ring emerges through this tangential takeoff port attached to the ring's vacuum chamber for use in spectroscopic studies. The ring's electron beam travels a path from the far end of the vacuum chamber bending past the port and off to the left
Heavily oxidized surface
Heavily oxidized surface
50
40 30 20 10 Photoelectron binding energy, eV
Photoelectron spectra of the 36 level from clean and oxidized single crystal [110] surfaces of a gallium-arsenic semiconductor. The spectra clearly show a shift in the As 36 peak upon oxidation while the Ga 36 peak remains essentially unaffected. This provides evidence that the oxygen is predominately interacting with the As. Increasing the incident photon energy probes more deeply into the substrate (as the mean free path of the electron increases). This results in an increase in the weight of the normal As 36 level relative to the 36 level of the surface-oxidized As atoms. This result suggests that the oxidation is a surface effect under these conditions and does not affect the bulk of the crystal. The results are from the work at SSRL by Piero Pianetta and William E. Spicer and their coworkers at Stanford University.
X-ray absorption spectroscopy: a new method for studying molecular structure The classical approach for determining the structure of a molecule is by x-ray diffraction. The intensity of x-rays diffracted by a crystal depends in a precise way on how the atoms of a material are arranged within the crystal lattice. The intensities can be deciphered by various numerical methods to give the spatial arrangement of all of the atoms in the crystal. Such studies can provide the structure of small ionic structures as well as macromolecules containing hundreds of atoms. This powerful technique requires, however, that the sample be crystalline. For some materials, such as proteins, crystallization often can be quite difficult; for others, such as amorphous powder and glasses, it is impossible. X-ray absorption spectroscopy is a technique that depends on the absorption of x-rays by a selected type of atom within the material. Because of the absorption phenomenon, rather than diffraction, the sample need not be crystalline; it may be a gas, liquid, amorphous solid, or crystal. As the energy of the incident x-ray photon increases, the x-rays become more penetrating and the background absorption decreases. At an energy that is unique for each element, an abrupt rise in the absorption coefficient occurs. This threshold is observed when the incident photon energy is sufficient to ionize an inner shell electron from the absorbing atom—in the case where the initial state is a 1s electron, it is called K shell absorption. Above the absorption edge, the x-ray absorption behavior can be separated conceptually into different areas that can be analyzed to provide different types of information about the absorbing species. The energy required to ionize the photoelectron will depend on the charge the electron "sees" as it leaves the atom. The edge position is sensitive to the charge on the ion, which is, in turn, related to the oxidation state of the absorber. One unit change in oxidation state of a metal ion will produce a small change (typically 1 to 3 eV) in the position of the edge. The smooth transition into the continuum is almost always convoluted with atomic transitions between the inner core state and outer bound electron states which reflect themselves as bumps or shoulders on the edge itself. In certain cases, these transitions can provide electronic information about the absorbing metal ion. For example, in Cu(ll) complexes a weak 1s --* 3d transition is observed, whereas in Cu(l) complexes in which the d-electron shell of the Cu is fully occupied, the transition vanishes since no unoccupied d-states are available. (The 1s -•-* 3 d transition, forbidden by selection rules
versus momentum dependence of electronic states in valence and conduction bands. Using the Tantalus storage ring, this kind of analysis was first performed by Neville V. Smith of Bell Laboratories, and G. J. Lapeyre of Montana State University. Extension of this technique to investigate the full three-dimensional band structure of metals such as copper has been carried out using SPEAR by David A. Shirley and his group at the University of California, Berkeley. A detailed understanding of the way gas molecules adsorbed on surfaces give rise to photoelectrons can provide information about the spatial relationship of the molecule to the substrate. Vacuum ultraviolet studies in 1971 by Dean Eastman and J. K. Cashion at the IBM laboratories in Yorktown Heights using a helium resonance lamp as a source showed that photoelectron spectroscopy may be used to observe changes in the electronic energy levels of a molecule, such as carbon monoxide, when it is bound to a metallic substrate. Work by Ward Plummer, Torgny Gustaffson, and John 34
C&ENAug. 21, 1978
for an isolated atom, is believed to be allowed, albeit weakly, by vibrational distortions of the symmetry of the Cu binding site.) As some of the transitions are forbidden by the rules of symmetry and vibronic interactions are important in determining their intensity, site symmetry information can be obtained as well. Above the absorption edge region, the absorption coefficient is never a smoothly varying function, except in the case of a simple monoatomic gas. The observed modulations in the absorption coefficient are referred to as extended x-ray absorption fine structure—EXAFS. The origin of this phenomenon can be qualitatively understood as follows. Above the absorption edge in energy, the escaping photoelectron can be viewed as a spherical wave moving outward from the absorbing atom. The photoelectron wave will be scattered by each shell of neighboring atoms that surround the absorber. The new waves are scattered back to the absorber and are either in phase or out of phase with the outgoing initial wave. The interference of the outgoing and incoming waves gives rise to the modulation in the absorption coefficient. In more quantitative terms, the absorption coefficient involves both the initial state (1 s) and the final state wave functions, but the final state wave function involves components of both the outgoing and backscattered waves. If they are in phase, the increased amplitude at the absorber results in a large absorption coefficient. When the relative phase relationship between the outgoing and incoming photoelectron waves is changed by varying the energy (and thus wave length) of the incident x-ray photons, then the result is the observed modulation called EXAFS. This leads to a description of the EXAFS in which one shell of atoms surrounding the absorber will give rise to a single modulated sine wave. If a second shell of neighbors is present, the EXAFS of the absorber will have two different sine wave components, and so forth. The effect extends out to around 3 to 5 A from the absorber, beyond which the mean free path of the electron and thermal effects damp out the modulation. Besides the distance information, the data also can be analyzed to give the number and types of neighbors surrounding the absorber. Analysis of the frequency components of the EXAFS provides distances to the neighboring shells of atoms since the phase relationship of the outgoing and incoming waves is mediated by the distance they travel. Edward Stern, Dale
Freeouf of the University of Pennsylvania at the Tantalus ring in 1975 provided further information on the way small molecules interact with clean metal surfaces. For example, they established that carbon monoxide binds to the platinum surface, with the carbon attached to the platinum atoms and the carbon monoxide molecule oriented perpendicular to the surface plane. It happens that this kind of information is difficult to get from LEED analysis of surfaces. Although LEED is very useful for determining the atomic arrangement of adsorbates on a surface plane, it is difficult to use for determining the way adsorbed atoms are arranged along the axis perpendicular to the surface. Thus photoemission spectroscopy is a powerful complement to LEED and more conventional electron spectroscopy in studying adsorbates and their surface chemistry, especially since synchrotron sources have made available highly polarized, intense radiation extending into the region of the spectrum where the emitted photoelectrons have their shortest mean free path and hence their greatest surface sensitivity.
Sayers, and Farrel Lytle first pointed out in 1971 that Fourier theory could be applied to obtain distance information. This method, coupled with other approaches to analysis by curve fitting that have been developed at Stanford and by Brian Kincaid, Peter Eisenberger, and others at Bell Laboratories, has now been used to provide accurate and reliable structural information. Radial distances are determined to about ±0.02 A for neighboring shells of atoms out to distances of 3 to 4 A. In cases of a single scattering shell, even more accurate determination can be made. The magnitude (amplitude) of each wave is related to two factors. The total number of atoms in each shell determines the overall amplitude of each wave. Secondly, an exponential thermal effect called the Debye-Waller factor causes the amplitude to damp out because thermal motion effects a change in the phase relation between the absorbing and scattering atoms. If the Debye-Waller factor is known (or assumed within certain reasonable limits), then the number of atoms in a neighboring shell can be extracted by analysis of the overall amplitude of the wave. Each scattered wave has an absolute phase origin, which depends in a systematic way on the atomic number of the scattering atom. Analysis of this quantity can provide information about the type of the scattering atom—in other words, each different element has a different phase signature that can be used to identify it. It is not possible to distinguish between scattering atoms of similar atomic number (such as carbon and nitrogen) but it is possible to distinguish an atom in one row of the periodic table, such as nitrogen, from one in the next, such as sulfur. The situation can become quite difficult if two different types of scattering atoms fall at nearly the same distance from the absorber. X-ray absorption spectra (and their EXAFS) can be recorded at the edge of any element whose energy falls within range of the available spectrometer. Currently at SSRL, the K edges (ionization of the inner 1 s electrons) of elements from potassium through about silver can be reached. The L edges (ionization of the 2s and 2p electron shells) of elements through uranium can be studied. (It should be noted the L edge EXAFS analysis can be more complicated because of the more complicated symmetry of the core state from which the outgoing wave is generated.) The sample can be in any physical state and the element of interest needs to be present in a concentration of only about 10 millimolar or greater in order to obtain reasonable ratio of signal to noise.
X-ray-induced fluorescence
Thefluorescenceof an atomic relaxation that occurs following a photoionization event also can be monitored. In this case, it is convenient to place the x-ray detectors in such a way that background from x-rays directly scattered by the sample (Compton scattering) is minimized and thereby the sensitivity of detection is improved. In an elegant series of experiments that illustrate this method, a group from Oak Ridge National Laboratory headed by Cullie Sparks recently demonstrated that x-ray-stimulated fluorescence can be an extremely powerful tool for trace element analysis. This group was searching for superheavy elements thought to exist in microscopic inclusions in mica samples from Madagascar. Tentative evidence for the existence of such elements from the high atomic number "island of stability" (elements having atomic numbers of 114 to 127) had come from proton-induced fluorescence studies. The Oak Ridge group used a special focusing graphite-crystal
Pre-edge
Edge
EXAFS Energy
»
A typical x-ray absorption edge shows three phenomenologScaUy different regions of the spectrum.
monochromator to irradiate the few micrometer-sized mica inclusions by the monochromatic x-ray beam from the SPEAR storage ring. They established a new sensitivity of trace element analysis of better than 108 atoms in a region the size of a few micrometers. The advantage of using a tunable x-ray source was that fluorescence x-rays from trace elements such as antimony and cadmium could be distinguished from x-rays from the superheavy elements because the former require different x-ray energies to excite the appropriate inner shell atomic level. By tuning the energy of the exciting x-rays, the group established that the small bumps seen in the fluorescence spectrum on proton irradiation of the samples were, in fact, not the result of superheavy elements. In doing so, they found that they could detect quantities of trace elements that were about one order of magnitude smaller than had previously been possible with proton-induced fluorescence. Moreover, the trace element search could be localized within a small region of a sample (determined by the focal spot size of the irradiating beam or its Aug. 21, 1978 C&EN 35
X-ray stimulated fluorescence detects traces of elements in mica 106
Th
Th U .
ORNLdata
macromolecule. For example, the lifetime of the tryptophan fluorescence in proteins containing two tryptophans can be a measure of the tryptophan-tryptophan distance. This is because one tryptophan can provide a radiationless decay path for another, thus shortening the lifetime. The effect depends on distance. In this way, conformational changes as a function of chemical or physical environment of a large protein in solution can be studied by monitoring the change in the fluorescence lifetime of the tryptophan probe. Similarly, fluorescent groups that have been covalently attached to proteins can be used to monitor structural changes in the protein by studying the distance relationships between various attached groups. In another application of "time-resolved" spectroscopy, groups from the University of California, Berkeley, and the Navy laboratories at China Lake, Calif., have determined fluorescence lifetimes of excited-state molecules (excimers) such as Xe2 and photoproduction cross sections of excimers, such as xenon fluoride, which are of interest in the development of laser systems. To 1984 and beyond
FSU data
During the next five years, facilities for carrying out experiments such as those we have described, as well as numerous other experiments using synchrotron radiation, will increase. \ j Following the recommendations of a 1976 panel of the National Academy of Sciences, NSF and the Department of Energy have begun to implement a plan to expand facilities to meet the research communities' needs for synchrotron radiation during ORNL data the 1980's. Three major facilities are being planned in the U.S.: an expanded SSRL at Stanford, the Aladdin ring at the University of Wisconsin (both supported by NSF), and two rings 102 at Brookhaven National Laboratories called the National 16 20 24 28 12 Synchrotron Light Source (funded by DOE). Along with smaller Energy, keV facilities at Cornell University and NBS, these facilities are expected to provide some 60 experimental stations in the x-ray Comparison of x-ray fluorescence spectra from a giant halo Inclusion range and 40 in the ultraviolet and soft x-ray range. In the words In Madagascar mica excited by a proton beam at Florida State Uniof George Pimentel, deputy director of NSF, "When the current versity (FSU) and by synchrotron radiation at the SPEAR ring by scibuilding phase is completed, the U.S. will have one of the entists from Oak Ridge National Laboratory (ORNL). The experiments world's strongest research capabilities in the synchrotron rawere performed as a part of a search for a superheavy (atomic number diation field." = 126) element. Each line In the spectrum Is Identified with a chemical Together with this new machine design and construction will element In the sample. The concentration of the trace elements (for example, the small tin peak) Is about 10™ atoms per Inclusion. The come increased brightness (flux per unit area per unit solid sensitivity of the detection technique using synchrotron radiation Is a angle) for future synchrotron radiation sources. One important technological advance will be the use of wiggler magnets to infactor of 10 greater than that obtainable from proton Irradiation studies. crease the effective energy of synchrotron radiation emitted by Examination of the spectrum at the energy predicted for the superheavy element established that any such element must be present in con- the electron beam in a storage ring. A wiggler magnet is a secentrations of less than about 10* atoms. These results were obtained quence of high-field electromagnets of alternating polarity that by Cullie Sparks and his collaborators at ORNL in experiments carriedcauses minute but sharp distortions in an electron's orbit as it passes through the device. The net effect is to shift the spectral out at SSRL. distribution to a higher critical energy without severely altering the basic orbital geometry of the storage ring. These various points are illustrated by considering the SPEAR storage ring. Currently, for example, about 10 12 photons per second per eV at 10 keV are available from an x-ray collimation). Thus, x-ray fluorescence, using a storage ring beam port at SPEAR when the ring is operating at 3.5 GeV, 20 source, becomes a form of analytic microscopy for trace element milliamps beam current with colliding beams. When SPEAR analysis of heterogeneous samples. It might be noted that this is being used to produce synchrotron radiation, about one to two concept of spatial elemental analysis had been established orders of magnitude higher flux are obtained under the same earlier by Paul Horowitz of Harvard when he constructed a monochromator conditions. (At present, SPEAR's operation scanning x-ray microscope with an irradiation spot size a few is principally determined by high-energy physics needs, so micrometers in diameter at the Cambridge Electron Acceleraoperation dedicated solely to synchrotron radiation research tor. is limited. However, when the new positron-electron accelerator The precisely repeating, highly stable, sharp pulses of synat the SLAC facility becomes operational in late 1979, it is exchrotron radiation allow the fluorescence lifetimes of excited pected that 50% of SPEAR's operations will be dedicated to atomic states to be measured on the subnanosecond time scale synchrotron radiation research, with photons available for the and permit the study of states whose excitation energies fall in other 50% on a "symbiotic" basis, as at present.) The use of more the ultraviolet and soft x-ray region of the spectrum. Lifetime complex wiggler magnets, such as a type called the "helical" measurements using a storage ring source were pioneered by wiggler, could, in principle, increase this photon flux up to as Ian Munro and Ricardo Lopez-Delgado at the ACO storage ring high as 10 17 photons per second per eV (at least for photon in Paris, France. Fluorescence lifetime spectroscopy is of parenergies up to 2 keV). ticular interest to biochemists studying fluorescent groups in large molecules because it provides a "spectroscopic ruler" for Other developments involving electron beam interactions measuring distance relationships between various groups in a with wiggler magnets offer the possibility of laser action. Recent Predicted Z = 126
36
C&ENAug. 21, 1978
experiments by John Madey and collaborators at Stanford in the infrared spectral region, using a helical wiggler magnet and a tuned optical cavity, have shown the possibility of using an electron beam to pump a laser (the device also is called a free electron laser) and suggest that at least down to optical frequencies, a storage ring could be used to provide an extraordinarily intense source of light. Most important will be the new science and technology associated with these increased capabilities. A few examples are illustrative. Currently, EXAFS can be used routinely to study elements when their concentrations are at or above about 10 millimolar. With the increased flux available at SPEAR or NSLS, it will be feasible to do experiments on submillimolar concentrations. If detector systems are developed to record the complete EXAFS spectrum simultaneously, it might then be possible to try to record a complete EXAFS data set on a submillisecond time scale. New developments in monochromator technology will allow EXAFS studies of elements whose ab-
sorption edges occur in the range of 3 to 0.5 keV. Higher source brightness also will enhance the ability to make elastic x-ray scattering measurements. Recording diffraction patterns on a submillisecond time scale may become feasible. Because of higher source brightness, the use of smaller samples for diffraction measurements will be possible. Increased resolution, achieved through new monochromators, will allow for expanded study of nonlinear optical phenomena, as well as Raman and Compton scattering. Although even more applications could be delineated, it is clear that the interest in synchrotron radiation as a scientific research tool extends over many decades of the electromagnetic spectrum and spans many scientific disciplines. To paraphrase a comment of Peter Eisenberger of Bell Laboratories, one of the pioneers in the application of synchrotron x-rays to the study of matter, in the period beyond 1984, we can look forward to "a brave new world of photon science." •
Selected readings General Synchrotron Radiation Brown, F. C , Solid State Phys. (F. Seitz, D. Turnbull, H. Ehrenreich, eds),
29, 1 (1974). Robinson, A. L, Science, 190, 1074, 1186 (1975). Rowe, E. A., Weaver, J. H., Sci. Amer., 236, No. 6, 32 (June 1977). "Synchrotron Radiation Research," Hodgson, K. O., Winick, H., Chu, G., eds, Stanford Synchrotron Radiation Laboratory, Stanford, Calif. 1976. "Synchrotron Radiation—A Perspective View for Europe," European Sciences Foundation, 1977. X-Ray Absorption Spectroscopy Cramer, S. P., Hodgson, K. O., Gillum, W. O., Mortensen, L. E., J. Am. Chem. Soc, 100, 3398 (1978). Eisenberger, P., Shulman, R. G., Brown, G. S., Ogawa, S., Proc. Nat Acad. Sci. U.S., 73, 491 (1976). Fluorescence and Timing Measurements Lopez-Delgado, R., Tramer, A., Munro, I. H., Chem. Phys. (Netherlands), 5,72(1974).
Keith O. Hodgson (left) and Sebastian Doniach
Matthias, E., White, M. G., Poliakoff, E. D., Rosenberg, R. A., Lee, S-T., Shirley, D. A., Chem. Phys. Lett., 52, &39 (1977). Monahan, K. V., Rehn, V., J. Chem. Phys., 68, 3814 (1978). Trace Element Analysis Sparks, C. J., Raman, S., Yakel, H. L., Gentry, R. V., Krause, M. O., Phys. Rev. Lett., 38, 205(1977). X-Ray Lithography Feder, R., Spiller, E., X-Ray Lithography in "X-ray Optics," H. J. Queisser, ed, Springer-Verlag, 1977. Feder, R., Spiller, E., Topalian, J., Broers, A. N., Gudat, W., Panessa, W., Zadunaisky, P. A., Science, 197, 259 (1977). X-Ray Topography Hart, M., J. Appl. Cryst, 8, 436 (1975). Ultraviolet and Soft X-Ray Studies Koch, E. E., Kung, C , Sonntag, B., Physics Reports, 29, 153 (1977). Spicer, W., Lindau, I., Helms, C. R., Res./Develop., 28, 12, 20 (1977).
Dr. Keith 0. Hodgson, 30, is assistant professor of chemistry at Stanford University. He earned a B.S. from the University of Virginia in 1969 and received his Ph.D. in chemistry at the University of California, Berkeley, in 1972. He spent a year as a NATO postdoctoral fellow at the Swiss Federal Institute of Technology before joining the Stanford faculty in 1973. At Stanford, he has been working on problems in bioinorganic and structural chemistry. He has been involved in the applications of synchrotron radiation at the Stanford Synchrotron Radiation Laboratory since its inception in 1974, using the radiation for both x-ray diffraction and x-ray absorption studies of the structure of metal ions in macromolecules and proteins. Dr. Sebastian Doniach, 44, is professor of applied physics at Stanford. Doniach holds a B.A. from Cambridge University and a Ph.D. in theoretical physics from the University of Liverpool. He taught in England for nine years before joining Stanford in 1969. Doniach was coordinator of a group of Stanford faculty who proposed the creation of the Stanford synchrotron radiation project in late 1972. He served as director of the project from 1973 to 1977. His research interests are the theoretical physics of condensed matter and the study of biophysical problems using synchrotron radiation.
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