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New developments in far UV, soft x-ray research at the Canadian synchrotron radiation facility.

New users are encouraged to visit the facility and test an experiment before submitting a formal proposal

In the last decade, synchrotron radiation has revolutionized most pure and applied research which requires far ultraviolet and/or
X-ray photons [ 5eV( 2,500 [angstrom]) eV
(2,500) to 50 keV ( 0.2 [angstrom])] . [1-5] Synchrotron

radiation now plays an important and essential role in many research areas in physics, chemistry, biology, geology, biomedical physics and materials science. Many commercially and industrially important applications have also been demonstrated. Indeed, X-ray lithography using X-rays from synchrotrons is already being used by IBM and Sumitomo Heavy Industries for making 0.25 [micro] m circuits.

Over 10,000 scientists are now using synchrotron radiation routinely at the 39 laboratories in 15 countries engaged in the operation and construction of 54 synchrotron radiation sources for pure and applied research. [6]All the G7 countries (except for Canada), as well as Russia and Sweden have more than one synchrotron radiation source, and countries such as Brazil, China, Taiwan, India and Korea are presently constructing synchrotron sources.

In Canada, over 100 Canadian scientists from academic, government and industrial laboratories have used foreign synchrotron radiation sources. Moreover, Canadian use of synchrotron radiation has increased, and is increasing, dramatically, as reflected in the increase of Canadian synchrotron radiation publications, from three in 1980, to over 120 in 1989-90. The Canadian Institute for Synchrotron Radiation (CISR) was formed recently to co-ordinate and facilitate Canadian synchrotron radiation research. Presently, there are 75 individual members (university faculty or senior scientists from university, government or industry), 15 institutional members (universities and industries) and 22 student members.[7]

Synchrotron radiation is useful due to its high collimation, and its high continuous intensity over most of the high-energy part of the electromagnetic spectrum from the infrared to hard X-rays. For example, the X-ray brightness from synchrotron radiation sources now is over a million times that available from rotating X-ray anodes, and the new "third generation" synchrotrons will improve this by another factor of [10.sup.4] to [10.sup.6].

Such an increase in intensity will always dramatically increase the ability of any technique to enhance resolution (in space, time or angle), sensitivity, and intensity. Techniques which have been transformed with the use of synchrotron radiation include: X-ray absorption spectroscopies such as Extended X-ray Absorption Fine Structure (EXAFS) and Near-edge X-ray Absorption Fine Structure (NEXAFS) of solids (crystalline or amorphous), liquids and solution, gases, surfaces, and interfaces; Photoelectron and Auger spectroscopy of gases, solids and surfaces; X-ray diffraction (XRD) of solids (powders, single crystals, non-crystalline materials) including small angle X-ray scattering (SAXS) from polymers, composites, and biological materials.

Many new spectroscopies, or new detection methods for existing spectroscopies, have been or are being developed using synchrotron radiation, and some of these are bound to be of considerable basic and applied interest.

Most of the above techniques can now be used at the recently expanded Canadian Synchrotron Radiation Facility (CSRF) at the Aladdin Synchrotron Radiation Centre (SRC) outside Madison, WI. CSRF is a national facility, owned and managed by the National Research Council (NRC), (N.K. Sherman, manager), funded by NRC, NSERC, the Ontario Centre for Materials Research (OCMR), and the University of Western Ontario (UWO), and operated by UWO scientists - (G.M. Bancroft, FCIC, scientific director and K.H. Tan, CSRF operations manager) - on behalf of Canadian users. Operation is only possible through the out standing hospitality and co-operation of the SRC and the American National Science Foundation (NSF) neither of whom charge us any fees for using the synchrotron.

CSRF began operation with its first beamline, which gives high intensity, between 20 eV and 1000 eV photons, in January 1986 at die Aladdin ring. Reference 2 describes the Aladdin ring and uses of the beamline. In the last 18 months two new beamlines have been completed and commissioned. It is the purpose of this article to describe:

* The characteristics of the three beamlines;

* The types of chambers that are available for Canadian researchers at CSRF;

* Recent results from the three beamlines which illustrate some of the unique Canadian capabilities of the beamlines and techniques at CSRF.

The three beamlines

Figure 1 shows the continuous spectrum of energies emitted from Aladdin. The linear photon energy used in this plot focuses on the high-energy part of the spectrum. The spectrum is usually plotted with a logarithm photon energy scale, Fig. 2 in reference 2, which shows, even more clearly than Fig. 1, that very high intensity and brightness is available over about four decades of the electromagnetic spectrum (< 1 eV to 5 keV). It has been our intention at CSRF to develop beamlines that will use all the energies in this large photon energy region optimally. Indeed, the existing three beamlines together with a proposed beamline, Table 1, will enable CSRF users to perform state-of-the-art experiments from 20 eV to 4,000 eV.


Figure 2 shows a block diagram of the present three beamlines; some of the properties of these beamlines are summarized in Table 1. One new beamline (093, Fig. 2), with its double crystal monochromator (DCM) was funded by the Ontario Centre for Material Research (OCMR) in 1987. With its present InSb crystals, this beamline gives very high intensity ([10.sub.10] photons/sec) in a small beam (3 mm x 1 mm) between 1750 eV and 3,800 eV.

The photon energy range can be extended to less than 1000eV with beryl and quartz crystals, Fig. 1. This beamline was commissioned in October 1990, began working routinely in early 1991 and it gives at least as good resolution, intensity and stability as any soft X-ray beamline in the world.

This beamline is ideally suited for core level studies using the Si, P, S and C1 K edges, the Sr, Y, Zr, Nb, Mo, Ru, Rh, Pd, and Ag L edges and many heavy metal M edges, Fig. 1. Also, the beamline is ideally suited for small angle X-ray scattering (SAXS) of polymers, clays, composites and biological materials using 4-7 [angstrom] radiation.

The second new beamline (092, Fig. 2) funded by NRC and NSERC has no monochromator or focusing, and yields a large, very intense beam. This beamline is ideally suited for radiation damage studies of polymer photoresists for microscopy and lithography, medical tomography and chemical vapor deposition (CVD).

The original beamline (091, Fig. 2) with its "Grasshopper" grazing incidence monochromator[2], gives very high intensity in a small beam between 21 eV and 1000 eV depending on the grating employed. Between 21 eV and 200 eV this beamline gives very high resolution (< 0.1 eV) which has enabled us to obtain very high resolution Al, 2p, Si 2p, P 2p and S 2p core level photoelectron, photoabsorption (NEXAFS) and Auger spectra of gases and solids. Above 200 eV, the resolution of this beamline (increasing to > 1 eV above 500 eV) is no longer competitive with the latest monochromators. To cover this range at high resolution, we will be applying to NSERC for a new spherical grating monochromator beamline, Table 1.

Experimental chambers

and experiments

The experimental end chambers, Table 2, are available for any qualified Canadian user. The two photoelectron chambers are ideally suited for high-resolution photoelectron and Auger spectroscopy of gases, solids and surfaces; two other chambers - Table 2, No. 3 and 4 - use either total electron yield or fluorescence detection to obtain EXAFS and/or NEXAFS spectra on the two monochromatic beamlines.


Two SAXS chambers are being completed by NRC researchers, and a general purpose chamber is available for microscopy, tomography and reflectivity measurements. CSRF researchers are applying for three other experimental chambers (for electron-electron coincidence, electron-ion coincidence, and high resolution photoelectron spectroscopy of surfaces).

Each of these beamlines has a full time beamline manager, Table 2, employed by the University of Western Ontario but resident in Madison. The beamline managers were responsible for constructing the beamlines. They are now responsible for maintaining the beamlines and providing users with the technical assistance necessary for the success of the experiment.

Formal requests for beamtime are submitted in the form of one-year proposals. The beamlines lines are available to all qualified investigators at no charge just as there is no charge to U.S. researchers at Aladdin. These proposals are submitted via the author to the SRC Program Advisory Committee and the CSRF Users Committee. The proposals are rated and the author develops the schedule with the users committee. The beamtime is granted in one-to three-week blocks.

We encourage new users to visit CSRF and test an experiment for a few days before formulating a complete proposal. A large amount of useful data can often be obtained with a few days beamtime. Please phone or write the present author for further information: Tel: 519-661-3122; Fax: 519-661-3022.

CSRF users and

experimental results

Like most synchrotron radiation user groups, the CSRF user community is very diverse scientifically and geographically, Table 3. Over 30 faculty and senior scientists regularly use the CSRF beamlines, along with about the same number of graduate students and post-docs. Users include chemists, physicists, geologists, and chemical and materials engineers from British Columbia to Quebec. The user community has increased substantially with the commissioning of the new DCM beamline mainly because excellent EXAFS and SAXS measurements can now be made at CSRF for the first time.

In 1990 and 1991, over 40 papers were published from work at CSRF (mainly on the first Grasshopper beamline). As of March 1, 1992, over 20 papers have already been published, accepted or submitted. CSRF recent publications include reports on new instrumental and technique developments[8-11], many NEXAFS and EXAFS studies of surfaces and solids[12-18], polymer and multi-layer studies[19-21], high-resolution photoelectron studies of surfaces[22-24], and high-resolution spectroscopy (photoelectron, photoionization, photoabsorption and Auger) of gas phase molecules[25-30], Table 4.


None of these studies could be performed anywhere in Canada, and many are very important international contributions because of the very high resolution and high signal to noise of the measurements.

A few brief examples will illustrate better the range of research at CSRF. We first look at uses of EXAFS and NEXAFS. In these techniques, the absorption of soft X-rays is monitored as the photon energy is varied from below a core level edge to well above
this edge ( 50 eV above for NEXAFS to
 1000 eV above for EXAFS). These spectra

can be obtained in four basic modes: absorption, total electron yield, fluorescence or Auger electron yield. The latter three measure the electrons or photons emitted after a core electron is excited or ionized and all three modes yield spectra that are closely proportional to the actual absorption spectrum.

At an edge, the absorption, electron yield, or fluorescence is increased dramatically due to the resonant absorption of the core electron to antibonding or Rydberg levels (e.g., the S 1s EXAFS, Fig. 3, or the S 2p NEXAFS, Fig. 4). The EXAFS oscillations (e.g. on the Si 1s level, Fig. 5) arise from scattering of the core ionized electron by adjacent atoms, resulting in constructive and destructive interference on the outgoing photoelectron wave.

The Fourier analysis of the EXAFS oscillations, Fig. 5, gives a radial distribution function which provides the identity of the surrounding atoms, interatomic distances and co-ordination number. EXAFS senses only the local structure around specific atoms, and requires only short range order. As a result, EXAFS is ideal for non-crystalline materials such as glasses, surfaces and biological molecules, where normal diffraction techniques are not useful.

Several CSRF users have been obtaining the Si K edge EXAFS from the DCM beamline to obtain the local structure of Si in various types of amorphous and porous Si; and also the degree of intermixing in strained layer [(Si.sub.mGe.sub.n).sub.p] superlattices, with Si and Ge layers being only a few atoms thick. To properly model and analyze both the Ge K edge and Si K edge EXAFS of these superlattices[14], it is important to record EXAFS of model compounds such as Si[Si[(CH.sub.3).sub.3].sub.4] and GE[Si[(CH.sub.3).sub.3].sub.4] to obtain accurate phase shifts and amplitudes. The top of Fig. 5 shows the gas phase Si K edge EXAFS of the former compound. Fourier analysis of these oscillations yields peaks, bottom of Fig. 5, corresponding to the Si-C and Si-Si bond lengths of 1.80 [angstrom] and 2.45 [angstrom] respectively.

The Si-Ge bond lengths in SiGe superlattices and in Ge [Si([CH.sub.3).sub.3].sub.4] are very similar, as are the Si-Si bond lengths in Si [Si([CH.sub.3).sub.3].sub.4] and in the superlattices. However, the different phase shifts and amplitudes of the EXAFS oscillations for Si-Si and Si-Ge enable estimates of the number of Si and Ge neighbors about the Si and Ge in the superlattices[14], i.e., estimates of the degree of intermixing can be made readily.

The NEXAFS features are usually not as well understood as EXAFS, but NEXAFS is a very sensitive chemical probe for surfaces and non-crystalline materials. Also, the spectra have great angular sensitivity. The peaks which arise from core to unoccupied molecular orbitals in surface species are only strongly observed when the oriented molecules are aligned such that the electric vector of the synchrotron radiation is parallel to the transition moment for the transition of interest, Fig. 3 in reference 1.

To illustrate the angular sensitivity of NEXAFS, the S K edge NEXAFS of S passivated InP(100)-(1x1) surfaces were recently recorded on the DCM beamline[12] to determine the S structural positions on the InP surface. Representative S K edge spectra at different azimuthal angles, Fig. 3a, for a constant polar angle of 90 [degrees]. (The crystal is normal to the beam, and the crystal is rotated while normal to the beam). The intensity of the first resonance, (the so called [sigma.sup.*] resonance, from S 1s [right arrow] [sigma.sup.*] (p) transition) is strongly angular dependent because of the shape of the antibonding orbital of high p character, while the other resonances do not depend on angle.

From theoretical modelling of this dependence, the S forms a bridge bond, Fig. 3b, with two In atoms with an In-S-In bond angle of 100 [degrees]. The atomic position of S is found to be close to the tetrahedral site of a P vacancy.

NEXAFS peaks near threshold often are much more sensitive chemically than competing techniques such as X-ray photoelectron spectroscopy (XPS). The L edge spectra for A1, Si, P and S are especially chemically sensitive at CSRF, because we can obtain total resolutions of < 0.1 eV for A1 and Si, to < 0.2 eV for S. This sensitivity has enabled us to distinguish different chemical forms of S in coal[13] for the first time, and different chemical environments of S and P in anti-wear films (in conjunction with K. Laycock and A. Blahey, ESSO).

The spectra in Fig. 4 illustrate the chemical sensitivity for different S compounds. Many S compounds give a basic triplet structure for the near-threshold NEXAFS, Fig. 5. Not only are these peaks relatively narrow (<0.8 eV) and show marked chemical shifts, but the relative intensities and splittings of the triplet are quite different for different organic S compounds. These spectra obviously lead to a clear distinction of the thiophene and cystine type S in these model compounds, Fig. 5, and similar species in coal. XPS could not be used to distinguish even these two species clearly.

Small angle soft X-ray scattering is being developed at CSRF by J.S Tse, MCIC, and D.D. Klug of NRC. Disordered materials, polymers, clusters and biomolecules often contain large-scale composition or density fluctuations. The use of conventional X-ray sources to study these materials is often limited by the lack of sufficient intensity and tunability of the source. A synchrotron source yields a high-intensity tunable flux of X-rays that is also highly collimated. This source can yield high-resolution, small-angle data in a short period of time.

The apparatus now being developed will fulfil these requirements in addition to having several unique features. The beamline produces a high flux of X-rays in the range of 1-4 keV (3-10 [angstrom]) which is the soft X-ray region of the electromagnetic spectrum. Therefore, a considerable increase in resolution is achievable by using longer wavelength X-rays. Two versions of the apparatus are available. Both can be used with the sample in vacuum or in a helium-filled environment. One version uses film detection for 2-dimensional diffraction patters. Figure 6 gives the SAXS of 0.5 micron polystyrene beads with 7.1 [angstrom] radiation[10]. Particle sizes down to a few hundred [angstrom] could be distinguished from these patterns.

The other apparatus uses a cooled Si-Li detector and a Bonse-Hart double crystal analyzer for high-resolution studies. In addition it has an automated data acquisition system. A primary feature of this apparatus will be the ability to use the tunability of the synchrotron source to explore anamalous scattering studies for elements such as silicon, sulphur and phosphorus by taking diffraction patterns above and below the K edges of these elements. This will enable us to examine the interior structure of heterogeneous particles such as biomolecules and aggregate clusters. The apparatus has also been designed to obtain wide-angle scattering with possible applications to surfaces, interfaces and thin films.

X-ray photoelectron spectroscopy (XPS) became one of the most powerful surface chemical probes about 20 years ago, after development by Siegbahn et al., mainly on gas molecules. The chemical resolution was always rather poor: linewidths of 1 eV were obtained, mostly because of the broad linewidths of the X-ray sources.

The latest commercial XPS instruments use monochromators to decrease the photon widths to close to 0.4 eV. However, monochromatized synchrotron radiation has yielded photon linewidths of < 0.1 eV, and CSRF workers have obtained amongst the narrowest core-level linewidths on surfaces, and the narrowest core-level linewidths on gas phase molecules. In addition, the ability to vary the photon energy enables us to change the valence band cross sections dramatically, so that the s, p, and d character of valence bands in surface phases and gas phase molecules can be readily obtained[24]. A dramatic increase in resolution in any spectroscopy always leads to exciting new fundamental and applied results, and two brief examples illustrate this excitement.

With high resolution XPS, it is now possible to resolve the peaks from surface metal atoms from the bulk atoms: the so-called surface chemical shift. These shifts are of fundamental importance because they are related to quantities like segregation, surface energies and the surface bonding[22]. A variety of measurements of this chemical shift have been reported for Al, yielding widely disparate values from -40 meV to +200 meV. Recent high-resolution measurements (instrumental width 0.1 eV) at CSRF, Fig. 7, and a Swedish group at MAX-I in Lund, show that the surface chemical shift is -96 [+ or -]5 meV. By varying the photon energy to 120 eV (kinetic energy of electron is 45 eV), the surface peak can be enhanced to be larger than the bulk peak, Fig. 7. The Swedish spectra taken at 100 K show an even better resolution, and distinct shoulders due to the two sets of doublets.

High-resolution gas phase core-level spectra[25,26] led to the observation of novel vibrational and ligand field effects on core levels, and the study of chemical effects on inherent linewidths. Figure 8 shows Si 2p spectra of [SiH.sub.4] and [SiD.sub.4] with total linewidths of 130 meV. The ion state vibrational splitting on both the Si [2p.sub.3/2] and Si [2p.sub.1/2] lines are apparent in both compounds. This is the first example of isotope vibrational splitting on core level XPS; the ratio of splittings is exactly that expected from the Si-H and Si-D reduced masses. These symmetric stretching frequencies are much larger than the ground state frequencies, but are very dose to those for the so-called core equivalent species [PH.sub.4.sup.+] and [PD.sub.4.sup.+]. It is now possible to study vibrational splittings on many molecules, and such splitting must now be considered for all photoelectron studies of gases, surfaces and solids.


[1.] A.P. Hitchcock and G.M. Bancroft, Canadian Chemical News; 43, 16 (1991). [2.] G.M. Bancroft, K.H. Tan and J.D. Bozek, Physics in Canada, 43, 113 (1987). [3.] M. Sutton and G.B. Stephenson, Physics in Canada, 44, 131 (1988). [4.] C.R. Catlow and N. Greaves, Chemistry in Britain, 803-852, (1986) (September). [5.] A. Bienenstock and H. Winick, Physics Today, 48-58 (1985) (June). [6.] Synchrotron Radiation News; 4, 23 (1991). [7.] Further information on CISR, Canadian uses of synchroton on radiation, and application forms are available from the author, C.B. Bigham, AECL, or E.D. Crozier, Simon Fraser University. [8.] B.X. Yang, F.H. Middleton, B.G. Olsson, G.M. Bancroft, J.M. Chen, T.K. Sham, K.H. Tan and D.J. Wallace, Rev. Sci. Instr., 63, 1355 (1992); Nucl. Instr. Methods, in press. [9.] R.A. Rosenberg, J.K. Simons, S.P. Frigo, K.H. Tan and I.M. Chem, Rev. Sci. Inst., in press. [10.] J.S. Tse, D.D. Klug, B.X. Yang and X.H. Feng, Applied Physics Letters, submitted. [11.] T.K. Sham, P. Kristof and R.A. Holroyd, Rev. Sci. Instr., 63, 1198 (1992); R. Holroyd, T.K. Sham, B.X. Yang and X.H. Feng, J. Phys. Chem., submitted. [12.] Z.H. Lu, M.J. Graham, X.H. Feng and B.X. Yang, Applied Physics Letter, in press. [13.] J.R. Brown, M. Kasrai, G.M. Bancroft, K.H. Tan and J.M Chen, Fuel, April 1992; M. Kasrai, J.R. Brown, G.M. Bancroft, K.H. Tan and I.M. Chen, Fuel 69, 411 (1990). [14.] P. Aebi, T. Tyliszczak, A.P. Hitchcock, K.M. Baines, T.K. Sham, T.E. Lockwood, J.M. Baribeau and D.J. Lockwood, Phys Rev. B., in press: J. Xiong, T.K. Sham, P. Aebi, A.P. Hitchcock, K. Mueller, K.M. Baines, J.M. Chen, B.X. Yang and X.H. Feng, in preparation. [15.] F.J. Esposto, P. Aebi, T. Tyliszczak, A.P. Hitchcock, M. Kasrai, J.D. Bozek, T.E. Jackman, S.R. Rolfe, J. Vac. Sci. Technology, A9 1663 (1991). [16.] B.M. Way, J.R. Dahn, T. Tiedje, K. Myrtle and M. Karasi, Phys. Rev. B., Submitted. [17.] M. Szymonski, T. Tyliszczak, P. Aebi and A.P. Hitchcock, Phys. Rev. B., in press. [18.] W. Lu, M. Kasari, G.M. Bancroft, M.J. Stillman and K.H. Tan, Inorg. Chem., 29, 2561 (1990). [19.] B.W. Yates, D.M. Shinozaki, A. Kumar, and W.J. Meath, J. Polymer Science, 30, 185 (1992). [20.] B.W. Yates, D.M. Shinozaki, J. Material Research, 7, 520 (1992). [21.] C. Montcalm, Brian T. Sullivan, H. Pepin, J.A. Dobrowlski, G.D. Enright, Proc. SPIE, 1574, 127 (1991). [22.] S. Bushby, P.R. Norton and K.H. Tan, to be published; R. Nyholm, J.N. Anderson, J.F. van Acker and M. Qvarford, Phys. Rev. B., 44, 10987 (1991). [23.] T.K. Sham, Z.F. Liu, and K.H. Tan, J. Chem Phys., 94, 6250 (1991). [24.] Z.H. Lu, T.K. Sham, P.R. Norton and K.H. Tan, Appl. Phys. Letters, 58, 161 (1991). [25.] J.D. Bozek, G.M. Bancroft, and K.H. Tan, Phys. Rev. Lett, 65, 2757 (1990); D.J. Sutherland, G.M. Bancroft and K.H. [25.] Tan, Surface Science Letters, 262, 96 (1992). [26.] J.N. Cutler, G.M. Bancroft, D.G. Sutherland and K.H. Tan, Phys. Rev. Letters, 67, 1531, (1991). [27.] A.D.O. Bawagan, B.J. Olsson, K.H. Tan, J.M. Chen, and B.X. Yang, Chem. Phys., Submitted. [28.] G. Cooper, W. Zhang, C.E. Brion, K.H. Tan, Chem. Phys., 145, 117 (1990) [29.] R.A. Rosenberg, C.R. Wen. K.H. Tan and J.M. Chen, J. Chem. Phys., 92, 5196 (1990). [30.] H. Aksela, S. Aksela, M. Ala-Korpela, O.P. Sairanen, M. Hotokka, G.M. Bancroft, K.H. Tan and J. Tulkki, Phys. Rev. B., 41, 6000 (1990). [31.] Z.F. Liu, J.N Cutler, G.M. Bancroft, K.H. Tan, R.G. Cavell, J.S. Tse, Chem. Phys. Lett., 172, 421 (1990).

Appendix: A guide to acronyms and specialized slang at the SRC and CSRF.
Aladdin The synchrotron radiation source at the University
 of Wisconsin (Madison)
CSRF Canadian Synchrotron Radiation Facility
CISR Canadian Institute for Synchroton Radiation
CVD Chemical Vapor Deposition
DCM Double Crystal Monochromator
EXAFS Extended X-ray Absorption Fine Structure
Grasshopper The common grazing incidence monochromator at CSRF
 and SRC
NEXAFS Near Edge X-ray Absorption Fine Structure
NSF National Science Foundation (U.S.)
NSLS National Synchrotron Light Source at Brookhaven
 National Labs, Upton, NY
OCMR Ontario Centre for Materials Reseach
PAS Photoabsorption Spectroscopy
PES Photoelectron Spectroscopy
PSD Photon Stimulated Desorption
PSL Physical Sciences Laboratory, adjacent to the SRC,
 University of Wisconsin-Madison
SRC Synchrotron Radiation Centre, which houses the
 Aladdin synchrotron
SAXS Small Angle X-ray Scattering
UHV Ultra-high Vacuum
XANES X-ray-Absorption Near Edge Structure. Normally
 used synonymously with NEXAFS
XPS X-ray Photoelectron Spectroscopy
XRD X-ray Diffraction
COPYRIGHT 1992 Chemical Institute of Canada
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Copyright 1992 Gale, Cengage Learning. All rights reserved.

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Author:Bancroft, G. Micahel
Publication:Canadian Chemical News
Date:Jun 1, 1992
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