Fig. 1: Soft x-ray microXAFS at the Mn L-edge can directly determine chemical composition in complex materials, left, by comparison to standards, right.
High resolution x-ray absorption spectroscopy has many potential applications in environmental sciences. We are exploring several avenues of application in collaboration with a microbiologist (Prof. K. Nealson), a soil chemist from ETH (Klaus Pecher) and a geochemist (Prof. T. Grundl). Here is a partial list of the types of investigations we have initiated:
- Biomineralization: The issue is to determine the relative abundance of different Mn compounds, and particularly the distribution of valence charge, in mineral deposits created by microbial activity in the environment. The relative abundance of Mn(II), Mn(III), and Mn(IV) in these deposits is an important clue to the understanding of the metabolism of these bacteria, which are believed to be one of the major sources of insoluble transition metal compounds in both marine and freshwater seas. Wet chemistry techniques have provided one answer, but it is an indirect one, since it measures the average valence of the Mn, and not the distribution. In preliminary measurements on the HERMON (see Fig. 1), we have found that there is excellent discrimination between the valences in the Mn series, given the high energy resolution of the HERMON instrument.
- Microcrystal chemistry in soils: The chemistry of soils is heavily influenced by catalytic reactions that are induced by microcrystals of alumino-silicates (clays). We are studying clays doped with transition metals, specifically Fe to start. We have developed a ‘wet cell’ method that allows us to use transmission soft x-ray spectroscopy to study the transition-metal valence in fully hydrated crystallites, while maintaining the vacuum of the spectrometer. These measurements are the first of their type, since all previous high resolution spectroscopy at the Fe L-edge has been done on dry specimens. We need to determine the Fe valences in the natural state, since Fe chemistry is strongly influenced by the presence of water (an understatement if there ever was one).
- Bioremediation: We are studying various pathways for the removal of transition metal compound contamination from soils. Both direct (microbially induced) and indirect (resulting from catalytic action on inorganic particles) methods are under investigation. An example of one of these types of study is to determine the chemical composition of a transition metal compound which is deposited on an environmentally relevant substrate (such as a Mn or Fe oxide particle). This study involves primarily Cr and U as the ‘model’ contaminants.
- The “Green Rust” problem: This one sounds horrible to physicists, but it is very exciting to soil geochemists and other geoscientists. “Green rust” is a phase of iron oxide that is responsible for driving a number of reactions in the soil. It’s structure is not well understood—it has not been solved yet. In addition, there appear to be several different types of “green rust”. We believe we are the first group to identify a unique spectrum of the Fe L-edge from green rust (comparing it to goethite and other oxide minerals). We use special techniques to ensure that the sample is never in contact with an oxidizing atmosphere.
- The “EEGLE” project; sediments from Lake Michigan: This one also makes physicists shudder. It is a study of the chemical composition of surface layers on suspended sediment particles taken from Lake Michigan. These suspended sediments are involved in the transport of heavy metals, toxic organic compounds, and radioactive materials between agricultural and industrial sites that border Lake Michigan, from north of Milwaukee, through Chicago and into Michigan. The sediments were observed originally by satellite imagery. The nature of the coating, which is responsible for the transport of contaminants, is not yet known. We are applying the whole arsenal of surface science (IR, XPS, XAS, etc.) to the problem, but XAS is one of the more important components of the study.
This research is in great need of a high resolution database of transition metal L-edge spectra, in the photon energy range from 500-900 eV, for which the HERMON is ideally suited. It is generally true that the Mn example shown above is the ‘best case’ of charge-state speciation: most of the other spectra are not as easy to identify in terms of valence. We need time to accumulate high quality spectra on ‘real world’ samples as well as models, so that we can push this analytical technique a little harder. With spectra of sufficient quality, theory indicates that we can get information about ligand coordination, in addition to the chemical state information.
This is clearly ‘bread-and-butter’ research, but it is also the foundation of several new avenues of environmental ‘surface’ science.
Indeed, since we started on these projects about two years ago, the entire nature of the effort has changed. We find that L-edge spectroscopy is perfectly matched to transition edge studies (particularly Mn and Fe, which play a dominant role in the geochemistry we are studying), as well as for side-by-side studies at the C and O edge (C has been problematic on HERMON in the past, but should be possible now).
We have been gratified to find that support has followed the hard work of establishing successful experimental protocols.
We now have funding from DoE-NABIR to support the research on the mineral-microbe interface. We have funding from NOAA for the EEGLE (Episodic Events Great Lakes Experiment) project. We have funding from NASA for general exploration of soft x-ray spectroscopy in environmental applications.
We anticipate that over the next 3 years, the environmental surface science program, using soft x-ray spectroscopy, will overtake and replace the research on more traditional surface problems that we have done (successfully, too) in the past. Because of this, we expect that the importance of certain beamlines to this research, in particular the HERMON, will grow over time.
PUBLICATIONS OF WORK DONE AT SRC (1995-97)
1.“Magnetic properties of Mn/Cu and Mn/Ni c(2x2) surface alloys,” W. L. O'Brien and B. P. Tonner, Phys. Rev. B 51, 617 (1995).
2.“X-ray magnetic circular dichroism study of Fe-V multilayers,” G. R. Harp, S. S. P. Parkin,. W.L. O’Brien and B. P. Tonner, Phys. Rev. B (Rap. Comm.) 51, 3293 (1995).
3.“Magnetic and structural properties of ultrathin Mn and Fe films on Ir(111),” W. L. O'Brien and B. P. Tonner, J. Vac. Sci. Technol. A 13, 1544 (1995).
4.“Application of photoelectron spectromicroscopy to a systematic study of toxic and natural elements in neurons,” G. De Stasio, D. Dunham, B. P. Tonner, D. Mercanti, M. T. Ciotti, P. Perfetti and G. Margaritondo, J. Synchrotron Rad. 2, 106-112 (1995).
5.“Induced Rh magnetic moments in Fe-Rh and Co-Rh alloys using x-ray magnetic circular dichroism,” G. R. Harp, S. S. P. Parkin, W. L. O’Brien and B. P. Tonner, Phys. Rev. B (Rap. Comm.) 51, 12037 (1995).
6.“Synchrotron radiation photoelectron emission microscopy of chemical vapor deposited diamond electron emitters,” J. D. Shovlin, M. E. Kordesch, D. Dunham, B. P. Tonner and W. Engel, J. Vac. Sci. Technol. A 13 (1111) 1995.
7.“Magnetic phases of ultrathin Fe films on fcc Co(001),” W. L. O’Brien and B. P. Tonner, Surface Science 334, 10-18 (1995).
8.“Development of Electron Spectromicroscopy,” B. P. Tonner, D. Dunham, T. Droubay, J. Kikuma, J. Denlinger, E. Rotenberg and A. Warwick, J. Elec. Spectrosc. and Rel. Phenom. 75, 309-332 (1995).
9.“Room temperature magnetic phases of Fe on fcc Co(001) and Ni(001),” W. L. O’Brien and B. P. Tonner, Phys. Rev. B 52, 15332 (1995).
10.“Effects of ferromagnetic capping layers on the perpendicular anisotropy of Ni/Cu(001) films,” W.L. O'Brien and B.P. Tonner, Applications of Synchrotron Radiation Techniques to Materials Science, Materials Research Society Symposium ProceedingsVol. 375, 77-80 (1995).
11.“Laterally resolved measurements of cesium iodide quantum yield,” T. dell’Orto, J. Almeida and B. P. Tonner, J. Vac. Sci. Technol. A 13, 2787 (1995).
12.“Microanalysis surface studies and photoemission properties of CsI photocathodes,” J. Almeida, A. Braem and B. P. Tonner, Nuclear Instruments Meth. Phys. Res. 367, 337 (1995).
13.“An electron imaging approach to soft x-ray transmission spectromicroscopy,” G. De Stasio, G. F. Lorusso, T. Droubay, M. Kohli, P. Muralt, P. Perfetti, G. Margaritondo, T. F. Kelly, and B. P. Tonner, Rev. Sci. Instrum. 67, 737-741 (1996).
14.“New magnetic phases of Fe on fcc Co(001) and Ni(001),” W. L. O’Brien and B. P. Tonner, J. Appl. Phys. 79, 5629-5631 (1996).
15.“Anomalous perpendicular magnetism in Ni/Cu(001) films and the effects of capping layers,” W. L. O’Brien and B. P. Tonner, J. Appl. Phys. 79, 5623-5625 (1996).
16.“Zinc uptake by brain cells: surface versus bulk,” G. De Stasio, S. Pochon, G. F. Lorusso, B . P. Tonner, D. Mercanti, M. T. Ciotti, N. Oddo, P. Galli, P. Perfetti, and G. Margaritondo, J. Phys. D: Appl. Phys. 29, 2209-2215 (1996).
17.“Photon energy dependence of valence band photoemission and resonant photoemission of polystyrene,” J.Kikuma and B.P.Tonner, J. Electron Spectrosc. Rel. Phenom. 82, 41-52 (1996).
18.“XANES spectra of a variety of widely used organic polymers at the C K-edge,” J.Kikuma and B.P.Tonner, J. Electron Spectrosc. Rel. Phenom 82, 53-60 (1996).
19.“Transitions in the direction of magnetism in Ni/Cu(001) ultrathin films and the effects of capping layers,” W.L. O’Brien, T. Droubay and B. P. Tonner, Phys. Rev. B 54, 9297 (1996).
20.“A photoemission microscope with a hemispherical capacitor energy filter,” B. P. Tonner, D. Dunham, and T. Droubay, J. Elec. Spectrosc. Rel. Phenom. 84 (1997), 211-229.
SRC Publications in Press
“Magnetic ordering in submonolayer Mn films on fcc Co(001), and the effects of oxidation,” W. L. O’Brien and B. P. Tonner, submitted to Phys. Rev. B.
VITA(One page only)
Brian P. Tonner
EDUCATION:
Cornell University, Ithaca, NY, Post doctoral fellow (1983)
University of Pennsylvania, Philadelphia, PA, Ph.D. Physics (1982)
University of Pennsylvania, Philadelphia, PA, M.Sc. (1978)
Brown University, Providence, RI, B.Sc. (1976)
CURRENT POSITION:
Director, Laboratory for Surface Studies
University of Wisconsin-Milwaukee
PAST EMPLOYMENT:
Senior Scientist (visiting) 1996-97
Lawrence Berkeley National Laboratory, sabbatical leave from UW-M.
Associate Director, 6/91-8/95.
Synchrotron Radiation Center, University of Wisconsin-Madison.
Professor, 6/92-present.
Dept. of Physics, University of Wisconsin-Milwaukee.
Associate Professor, 6/88-6/92.
Dept. of Physics, University of Wisconsin-Milwaukee.
Assistant Professor, 9/83-6/88.
Dept. of Physics, University of Wisconsin-Milwaukee.
Research Fellow, 6/82-9/83.
Physics Department, Cornell University, Ithaca, NY
PROFESSIONAL RECOGNITION:
Co-founder, Surface and Interface Magnetism Group, American Vacuum Society, 1996.
Chair of AVS Surface Science Division Executive Committee, 1995-96.
Award for Excellence in Research, UWM Graduate School, 1994.
Post-doctoral Research Fellowship from IBM-Watson Research Center, 1982.
American Vacuum Society Scholarship, 1981.
University Fellow, University of Pennsylvania, 1980-1982.
Honors in Physics, Brown University, 1976.