Biographical Sketch of G. Klingelhöfer
Reprinted from the May 2006 edition of the Mössbauer Spectroscopy Newsletter, published as part of Volume 29, Issue 5 of the Mössbauer Effect Reference and Data Journal

Dr. Göstar Klingelhöfer was born 2 October 1956. He received his Diploma in Physics (atomic physics, instrument development) in 1984 and his Ph.D. in Physics (nuclear solid state physics, surface science) in 1990, both from the Darmstadt University of Technology. From 1990 to 1993 he served as a Postdoctoral Research Fellow at the Darmstadt University of Technology, and from 1993 to 1999 as Hochschulassistent (Assistant Professor) at the University of Darmstadt. In 1999, he joined the Institute for Inorganic and Analytical Chemistry at Johannes Gutenberg-University Mainz, where he currently serves as Senior Researcher. Dr. Klingelhöfer specializes in Atomic Physics, Nuclear Solid State Physics, and Instrumentation, and also works in Solid State and Nuclear Physics, Planetology, and Environmental Sciences.

Dr. Klingelhöfer is a member of the German Physical Society (DPG), the European Physical Society, the American Geophysical Union (AGU), the Exobiology Study Group of the European Space Agency (ESA) (1998-2000), and the Science Advisory Group for the ESA Mars Mission ExoMars 2009/2011.

In 2005, he was awarded the Eugen Sänger Medal by the Deutsche Gesellschaft für Luft- und Raumfahrt (DGLR – the German Association for Aviation, Aeronautics, and Spaceflight) for his “crucial contributions to the results of the NASA Mars-Exploration-Rover missions Spirit and Opportunity which made it possible to identify with his instrument water bearing minerals, a clear evidence of aqueous processes on Mars in the past.”

In 2006, Dr. Klingelhöfer will be awarded the first IBAME (International Board on the Applications of the Mössbauer Effect) Award in recognition of his exceptional contribution to Mössbauer spectroscopy, especially for the design and construction of miniaturized instrumentation which, amongst other things, has enabled unique information on the composition of soils and rocks to be recorded from the NASA exploration of Mars during 2004 and 2005. Dr. Klingelhöfer will receive this Award on the occasion of the 6th Seeheim Workshop on Mössbauer Spectroscopy, to be held June 7-11, 2006.

Dr. Klingelhöfer’s current research interests include:

  • Experimental studies of planetary surfaces, including the development of a Mössbauer spectrometer for space applications, the development of X-ray spectrometers for the chemical analysis of extraterrestrial surfaces, and the development of neutron spectrometers for solar system exploration. Grants from the German Space Agency DLR have been awarded for those projects.
  • Principal Investigator for the Mössbauer spectrometer MIMOS II on the ATHENA mission to Mars (Mars Exploration Rovers MER 2003, NASA) and the Beagle 2 lander of the ESA Mars Express mission.
  • Principal Investigator of the APX spectrometer for the ROSETTA mission.
  • Principal Investigator for the Mössbauer spectrometer MIMOS II on the ESA ExoMars Mission to Mars in 2011.
  • Principal Investigator for the Mössbauer spectrometer MIMOS II on the Russian Space Mission ‘Phobos-Soil’ to the Mars Moon Phobos in 2009, including sample return.
  • Laboratory weathering studies of minerals under extraterrestrial conditions, particularly Mars conditions.
  • Field experiments with planetary Rovers (FIDO, Rocky-7) developed by NASA/JPL, Pasadena, California, USA.
  • Investigations and field experiments on air pollution (in situ measurements) in Vitoria area, Brazil, in cooperation with Dr. Paulo A. de Souza of Companhia Vale do Rio Doce, Brazil.
  • In situ monitoring of iron mineralogy and oxidation states by Mössbauer spectroscopy in the field: the green rust mineral in hydromorphic soils. 

The main field of the work of Dr. Klingelhöfer in recent years has been the exploration of planetary surfaces, in particular Mars, culminating in the detection of water-bearing minerals at the Mars Exploration Rover landing sites by use of the Mössbauer spectrometer MIMOS II, developed in his research group. These findings have been crucial in respect to the question of the presence of water on Mars in the past. Other important research activities have been the physics of hyperfine interactions and its related phenomena, as well as the development of instrumentation to address the relevant questions and problems in solid state, soils, rocks, and mineral weathering research.

Mössbauer spectroscopy has been used as the central method to address fundamental physics problems and applications to exotic topics, in particular Mars and planetary exploration. Different Mössbauer techniques have been applied (CEMS, DCEMS, LEEMS, and TMS) and/or developed for applications (LEEMS, DCEMS). Other applied methods in his group are X-ray fluorescence spectroscopy and X-ray diffraction.

In the following, the main scientific achievements of Dr. Klingelhöfer will be summarized.

I. In Situ Mössbauer Mineralogical Studies of the Surface of Mars

In 2003, the newly developed instrument MIMOS II was launched as part of the payload of the NASA Mars Exploration Rover (MER) twin mission to Mars. Both Rovers, Spirit and Opportunity, landed successfully on the surface of Mars in January 2004. The two Mössbauer spectrometers, and the whole payload and both Rovers, have operated extremely successfully for more than two Earth years. The recorded Mössbauer spectra have contributed significantly to the scientific results obtained so far during the mission.

The Mössbauer instrument on board Spirit detected the iron mineral goethite in the Columbia Hills, which is clear mineralogical evidence for the presence of water at the time of formation. It also detected olivine in the soils at Gusev Crater, which in general is of basaltic composition, indicating the very small degree of chemical weathering in the plains of Gusev Crater.

The Mössbauer instrument mounted on Opportunity detected the Fe-sulfate jarosite, which forms under very acidic conditions in the presence of water, and is therefore a direct mineralogical evidence for the presence of large amounts of water in the past at the Opportunity landing site.

These highly exciting results have been well received and acknowledged and have been published in numerous articles in Science. The first appeared in Volume 305 on August 6, 2004, comprising the results from the Mössbauer spectrometer on Spirit; results from the Mössbauer spectrometer mounted on Opportunity appeared in Volume 306 on December 3, 2004. For comparison, a dozen jarosite samples collected in different locations around the world are currently measured by Mössbauer spectroscopy in the laboratory of Dr. Klingelhöfer.

The outstanding success of MIMOS II participation in the present Mars missions has already led to the decision that the miniaturized Mössbauer spectrometer of Dr. Klingelhöfer will be part of the analytical instrumentation of future space missions in 2009 and 2011.

The Mars Mössbauer Group at the Johannes Gutenberg-University Mainz, taken from their Web site.

II. Weathering Studies of Fe-Bearing Minerals under Extraterrestrial Conditions

As part of the effort to explore the surface of extraterrestrial planets, in particular the planet Mars, Dr. Klingelhöfer has started a research program to study the weathering of Fe-bearing minerals under extraterrestrial conditions. The focus has been on weathering under Venus surface conditions and under Martian conditions, which are very different from each other.

The detailed experimental study of the kinetics and mechanism of pyrite (FeS2) and chemical weathering under Venus surface conditions showed that pyrite is thermodynamically unstable on the surface of Venus and will spontaneously decompose to pyrrhotite (Fe7S8), which on continued heating forms more Fe-rich pyrrhotite, later being oxidized to form magnetite, which converts to maghemite and then to hematite. This information is very important to an understanding of the Venus sulfur cycle, and therefore the surface-atmosphere interactions.

Dr. Klingelhöfer and his coworkers have also studied experimentally the basalt oxidation and formation of hematite on the surface of Venus to understand and explain the results obtained by the Russian Venera landers, which show red color at different landing sites. These Mössbauer studies showed that hematite, instead of magnetite, is present on the surface of Venus.

Besides these experimental studies, Dr. Klingelhöfer performed a theoretical analysis of the possible use of Mössbauer spectroscopy for in situ mineralogical analysis of the surface of Venus, showing that even at such extreme environmental conditions important information concerning the mineralogy could be obtained. These studies have had positive implications with regard to the use of Mössbauer instrumentation on future spacecraft missions to Venus.

Professor Gerhard Wortmann and Dr. Göstar Klingelhöfer enjoying lunch during ICAME 2005 in Montpellier, France.

In preparation for the analysis and interpretation of mineralogical data obtained in situ during landed missions on Mars, Dr. Klingelhöfer started an experimental laboratory program to study the weathering of Fe-bearing minerals under Martian conditions, which are very different from the Venus surface. These studies, which are still ongoing, show the importance of analyzing the weathering of individual minerals, like olivine or pyroxene, in order to understand the weathering of more complex mineral assemblages, such as, for example, basalt.

III. Development of a Miniaturized Mössbauer Spectrometer for Planetary Missions and Terrestrial Applications

Dr. Klingelhöfer’s main activity in the past several years has been the development of a miniaturized Mössbauer spectrometer for in situ analysis of planetary surfaces. This project was initiated by the invitation of a Russian group from the Space Research Institute in Moscow, to participate in a Russian space mission to Mars. This project resulted in the design of the MIMOS I instrument, having the size of a 0.5 liter soft-drink can, and which now is on permanent display in the German Museum in Cologne (Köln). The next generation of the miniaturized spectrometer, called MIMOS II, was reduced further with respect to MIMOS I by about a factor of two. This instrument, which is unique worldwide, was part of the payload of the NASA space mission Mars Surveyor 2001, which was cancelled a few months before scheduled launch. It became, however, part of the scientific payload of the NASA 2003 Mars Exploration Rover mission, and also part of the payload of the European ESA Mars Express Beagle 2 lander, indicating the high interest of the scientific community in this instrument and method.

Besides these extraterrestrial applications, this portable Mössbauer instrument has been used in several terrestrial applications which became possible only because of the miniaturization, the portability of this instrument, and/or the backscattering geometry that allows non-destructive measurements. Examples of terrestrial applications include the study of rock paintings in the field, the investigation of archaeological artifacts, the in situ monitoring of Fe-oxidation state and mineralogy in areas with intensive farming, and the in situ monitoring of air pollution in industrial areas. There are a whole variety of other applications.

Dr. Klingelhöfer (center) discusses matters with colleagues at the ISIAME conference in Norfolk, Virginia, USA (2000).

IV.  Spin-Polarized Conversion Electron Mössbauer Spectroscopy

The hyperfine field at the Fe-57 nucleus is mainly caused by the net spin density of the ns-electrons, polarized by the spin-dependent exchange interaction with the 3d electrons. The contribution of the individual ns shells to the net spin density at the nucleus is different in sign and amplitude. The contribution of the different s-shells to the total magnetic hyperfine field, which is to a great extent proportional to the conversion coefficient alpha, can be determined by measuring the relative intensities of the spin-polarized conversion electrons of the highly converted M1 transition of the 14.4 keV Mössbauer nuclear state in Fe-57.

In the magnetically split CEMS (Conversion Electron Mössbauer Spectroscopy) spectrum, the ∆mI = +1 transition (-3/2 → -1/2) involves only s-electrons with initial spin up; the ∆mI = -1 transition (+3/2 → +1/2) involves only s-electrons with initial spin down. Therefore, in the ns CEMS spectrum the relative difference (asymmetry δns = N-/N+ -1) of the intensities of these transitions (no. 6 (=N-) and 1 (N+)) should give the individual contributions of the ns shell to the hyperfine field. Theoretical results obtained with various methods indicate differences of about δ1s = 10-5 for the 1s electrons, and about 10-2 for the 2s, 3s, and 4s electrons, with different sign.

Göstar Klingelhöfer and Mira Ristic at a poster session held during ISIAME'00 in Norfolk, Virginia.

These small differences require an extremely high precision of the experiment. In particular, the accuracy and the knowledge of the Mössbauer drive velocity has to be extremely high, in the order of less than 0.1%. Also, the statistical quality of these data has to be extremely high. To achieve these requirements, the experimental setup was significantly improved by developing an electron detection system with high detection efficiency, and by optimizing the energy resolution of the orange-type magnetic spectrometer, which is used to select the 1s, 2s, 3s, and 4s conversion electrons.

In addition, the quality of the Mössbauer drive system was increased such that the non-linearity has become well below 0.1%, and it is reproducible, so the remaining deviations can be accounted for in the data analysis program. This high transmission Mössbauer DCEMS (Depth Selective CEMS) setup was used to acquire ns electron CEMS spectra with extremely high quality never obtained before.

Data were acquired for two different Fe compounds, α-Fe (metallic foil) and α-Fe2O3 (hematite), and in contrast to all theoretical predictions and in contrast to earlier experiments (measuring only part of the Mössbauer spectrum and assuming 1s asymmetries to be due to their experimental setup), one has found in both cases a difference of about 1% for the 1s electrons. If this would be due to a difference in the electron densities for spin-up and spin-down, it would result in a huge magnetic field, several orders of magnitude larger than observed.

The result of this high precision and extremely time-consuming unique experiment shows that the final state with the spin of the remaining 1s electron parallel to the 3d spin is more probable than anti-parallel spins. This may be due to a dynamic coupling of the atomic core with the conversion electron. The conversion process is much more complex than assumed, and cannot be used directly for the measurement of the s-shell polarizations. This complexity is expressed also by the presence of the very low energy electrons (a few eV) emitted simultaneously with the conversion electrons. The nature of this type of electron, and their possible use in Mössbauer spectroscopy using electron spectrometers, was investigated intensively in the laboratory of Dr. Klingelhöfer.

This work has resulted in a significant improvement of the experimental setup, the new “orange”-type spectrometer, operating now at ultrahigh vacuum (below 10-9 mbar), which is used for the study of the very top surface (first few monolayers).

V.    Low Energy Electron Mössbauer Spectroscopy and Shake-Off Effect

As described above and in the literature, resonant very low energy electrons are produced with high intensity as part of the internal conversion process after resonant Mössbauer absorption. The main contribution comes from the so-called “shake-off effect,” the emission of low energy electrons when the depth of the Coulomb-potential seen by the electrons of the Fe-atom changes nearly instantaneously during the internal conversion process. There is also a contribution of resonant low energy electrons (true secondary electrons) generated by the whole electron spectrum emitted by the Mössbauer absorber (K-, L-, M-conversion-, KLL-, KLM-, and other Auger-electrons, etc.).

Dr. Klingelhöfer was able to show that the mean free path of the shake-off electrons is in the order of 40 Angstroem, much less than the mean free path of the “true secondary electrons,” and therefore these electrons are very surface sensitive. It could also be demonstrated that those low energy electrons can be selected by electron spectrometers (demonstrated for the orange-type magnetic spectrometer) with high intensity, enabling studies of the topmost surface layers within short measurement times.

This variant of Mössbauer spectroscopy, which was named LEEMS (Low Energy Electron Mössbauer Spectroscopy), has been frequently used since then and has been extended (in cooperation with the University of Gent, Belgium) to an energy-integral version (not discriminating for electron energies) called ILEEMS (Integral LEEMS). By using these new variants of Mössbauer spectroscopy, one could study, for example, the surface-atmosphere interactions of metal surfaces and other Fe-bearing compounds.

The Editors wish to thank Prof. Philipp Gütlich for providing the Curriculum Vitae and Research Summary from which the information contained in this article was compiled. We would also like to thank Dr. Göstar Klingelhöfer for providing many of the photos used herein, including the photo used on the cover.

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