Rudolf Messbauer Physicist Date of Birth: 31.01.1929 Country: Germany

1. Early Life and Education
2. Research on Gamma-Ray Fluorescence
3. Discovery of the Mössbauer Effect
4. Applications of the Mössbauer Effect
5. Nobel Prize and Later Career

Early Life and Education

Rudolf Ludwig Mössbauer was born on January 31, 1929, in Munich, Germany. He was the son of Ludwig Mössbauer, a photography technician, and Erna Ernst Mössbauer. Rudolf's early education took place in local schools, followed by attendance at a non-classical secondary school in Munich, graduating in 1948.

After a brief stint working for an optical company, Mössbauer enrolled at the Technical University of Munich (TUM). He earned a bachelor's degree in 1952, a master's degree in 1955, and a doctorate in physics in 1958. During the academic year 1953-54, he served as an assistant lecturer in mathematics at TUM.

Research on Gamma-Ray Fluorescence

From the 1850s, it was known that certain gases, liquids, and solids, such as fluoride compounds, absorbed electromagnetic radiation (often visible light) and promptly re-emitted it, a phenomenon known as fluorescence. In the specific case known as resonance fluorescence, both the absorbed and emitted radiation have the same energy, wavelength, and frequency.

Mössbauer's interest lay in analogous X-ray fluorescence, where a material excited by X-ray absorption emits X-rays of the same wavelength and frequency. X-ray fluorescence had been discovered and measured between 1915 and 1925 by Charles Barkla and Manne Siegbahn.

Discovery of the Mössbauer Effect

Fluorescence absorption only occurs when the energy of the exciting photon (an electromagnetic radiation particle) is equal to the energy needed to excite an atom or its nucleus. However, a photon's energy depends on the motion of the atom that absorbs or emits it: an approaching atom and photon have increased energy; a receding atom and photon have decreased energy. This complicates the picture since the act of emitting or absorbing a photon determines its motion relative to the atom.

The process of emitting or absorbing a photon conserves both energy and momentum; in other words, the total energy and momentum of the photon and the atom must remain the same before and after the event. This implies that an atom emitting a photon must recoil. The recoil energy is subtracted from the photon's energy, which is therefore slightly less than what it would have been without the recoil.

For visible light photons, which have relatively low energy and momentum, the effect of atomic recoil can be neglected. However, gamma-ray photons have energies that exceed those of visible light by factors ranging from 10,000 to a million, and recoil becomes significant. When an atomic nucleus emits a gamma-ray, the resulting recoil motion of the nucleus causes a noticeable decrease in the photon's energy. As a result, the emitted photon does not have exactly the same energy (or wavelength, or frequency) as photons that can be absorbed by the same type of nucleus. For this reason, resonance fluorescence – where the emitted and absorbed photons must have equal energies – is usually not observed with gamma-rays.

Mössbauer found a way to achieve gamma-ray resonance fluorescence. He used atoms of the radioactive isotope of the metal iridium as his source. Iridium forms a crystalline solid, so both the emitting and absorbing atoms occupy fixed positions within crystals. Cooling the crystals with liquid nitrogen, he observed that fluorescence markedly increased.

Studying this phenomenon, he determined that individual nuclei emitting or absorbing gamma-rays transferred the recoil momentum to the entire crystal. Since the crystal is much more massive than the nucleus, the frequency shift experienced by the emitted and absorbed photons was negligible. This phenomenon, which Mössbauer called "recoilless nuclear resonant absorption of gamma radiation," is now known as the Mössbauer effect. Like any effect occurring in a solid, it depends on the substance's crystal structure, temperature, and even the presence of trace impurities.

Mössbauer showed that suppressing nuclear recoil via the Mössbauer effect enabled generation of gamma-rays with a wavelength that was constant to within one part in a billion (10⁹); other researchers have improved on this, achieving stability to within one part in a hundred trillion (10¹⁴).

Applications of the Mössbauer Effect

Initially, Mössbauer's results, published in 1958, were either ignored or met with skepticism. However, within a year, recognizing the potential importance of the Mössbauer effect, several researchers repeated his experiments, and the results were confirmed.

The fact that recoilless nuclear resonant absorption enables the measurement of an extremely small difference in energy between two systems (as long as it is large enough to prevent resonance fluorescence) led to a technique with a wide range of important applications. Having extremely stable wavelength and frequency, fluorescent gamma-rays serve as a high-precision tool for measuring gravitational, electric, and magnetic fields of minute particles.

One of the first applications of the Mössbauer effect, in 1959, was by R.V. Pound and G.A. Rebka at Harvard University, who used the effect to confirm Albert Einstein's prediction that a gravitational field could change the frequency of electromagnetic radiation. Measuring the change in gamma-ray frequency caused by the difference in the gravitational field at the base and top of a 70-foot tower provided a strong confirmation of Einstein's general theory of relativity.

The Mössbauer effect also allows for information to be obtained about the magnetic and electric properties of nuclei and the electrons surrounding them. The effect has found applications in areas as diverse as archaeology, chemical catalysis, molecular structure, valence, solid-state physics, atomic physics, and biological polymers.

Nobel Prize and Later Career

In 1961, Mössbauer was awarded half of the Nobel Prize in Physics "for his researches concerning the resonance absorption of gamma radiation and his discovery in this connection of the effect which bears his name." With the Mössbauer effect, said Ivar Waller, a member of the Swedish Royal Academy of Sciences, in presenting the award, "it became possible to investigate important phenomena which previously were beyond the reach of even the most refined measuring techniques."

Mössbauer was slated to become a full professor at TUM, but disillusioned by the bureaucratic and authoritarian principles of German university structures, he took a sabbatical to Heidelberg in 1960 and became a research associate at the California Institute of Technology, followed the next year by a professorship there.

However, in 1964, he returned to Germany to become Professor of Physics at TUM, which he remodeled on the lines of American university structures. Some academics jokingly referred to this change in the structure of German academic life as the "second Mössbauer effect." From 1972 to 1977, Mössbauer served as Director of the Institut Laue-Langevin in Grenoble, France.

Mössbauer married Elisabeth Pritz, an interior designer, in 1957. They had a son and two daughters. In his spare time, he enjoyed playing the piano, cycling, and doing photography. Mössbauer was a member of the American, European, and German Physical Societies, the Indian Academy of Sciences, and the American Academy of Arts and Sciences. He received honorary doctorates from Oxford, Leicester, and Grenoble Universities.

In addition to the Nobel Prize, Mössbauer received the Scientific Achievement Award from the Research Corporation of America (1960), the Wilhelm Conrad Roentgen Prize from the University of Giessen (1961), and the Elliott Cresson Medal from the Franklin Institute (1961).