Alfred Kastler Biography

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World of Scientific Discovery on Alfred Kastler

Alfred Kastler developed methods for exciting atoms so that they would travel from one sub-level to another in very precise ways, emitting energy in the process. These techniques, called double resonance and optical pumping, later found application in a number of inventions, including the maser and the laser, which use the energy emitted by excited atoms. His research into methods of exciting and controlling this atomic process led Kastler to be awarded the 1966 Nobel Prize in physics.

Kastler was born in the Alsatian village of Guebwiller on May 3, 1902. The village was then a part of Germany, although it was to revert to French control at the end of World War I. His parents were Frederic and Anna (Frey) Kastler. He began his schooling in Guebwiller, but moved with his family to Colmar after World War I began. He studied at the Colmar Oberrealschule (high school), where he became especially interested in mathematics and science courses.

As part of the Versailles peace treaty at the end of World War I, Alsace was ceded to France, and the Oberrealschule became the Lycée Bartholdi. Kastler was not fluent in French; he decided to leave school to become a carpenter. An aunt insisted that he continue his education, however, and he graduated from the lycée a year later. He then traveled to Paris, where he was admitted to the prestigious École Normale Supérieure in spite of having failed the entrance examination. He was admitted under a special disposition offered to residents of the newly regained Alsace territory.

After graduating with a teaching certificate in 1926, Kastler held a series of teaching jobs at lycées in Mulhouse, Colmar, and Bordeaux. He then received an appointment in 1931 as research assistant at the University of Bordeaux, which permitted him to begin his graduate studies there. Five years later, he was awarded his degree of Docteur des Sciences Physiques for his dissertation on the excitation of mercury atoms.

Kastler's first job after receiving his doctorate was at Clermont-Ferrand University, where he was a lecturer in physics from 1936 to 1938. He then accepted an appointment as professor of physics at the University of Bordeaux, where he remained until 1941. Returning to Paris, he became director of the Hertzian spectroscopy group and assistant professor of physics at his alma mater, the École Normale Supérieure. There, he attained the rank of full professor in 1945. After retiring from the École Normale Supérieure in 1968, Kastler served as director of research at the National Center for Scientific Research, a post he held until 1972.

As Kastler was beginning his graduate studies in the early 1930s, scientists were still trying to unravel the problems of the electronic structure of atoms. Niels Bohr had provided a broad, general theory for atomic structure in 1913, one in which electrons were allowed to occupy certain specific energy levels, but not the spaces between those energy levels. Later studies showed that Bohr's model lacked precision, that sub-orbitals existed within any given energy level. Electrons could occupy any one of these sub-levels, their position often being dependent upon the presence of external magnetic fields.

One technique used by scientists to study electron energy levels involved the addition of energy to an atom. By shining light on it, an electron in an atom would then absorb some of that energy and move to a higher energy level. After remaining briefly in that new energy level, the electron would reemit the absorbed energy and fall back to a lower energy level. The emitted energy, in the form of light, could then be studied in order to obtain information about the relative location of the energy levels.

Kastler made two important contributions to this line of research. The first is known as double resonance, because it involves the application of two types of external energy fields, a beam of light and a radiofrequency field. In the first step, energy from a light beam shined on a group of atoms causes some electrons to be excited and to move to a higher energy level. Those electrons distribute themselves unevenly, however, among the sub-levels within that higher energy level.

In the second step, the application of a radiofrequency field, electrons are forced from populated to unpopulated sub-levels. At this point, the excited electrons in all sub-levels emit their absorbed energy, fall back to lower energy states, and give off beams of light that can be analyzed. Kastler reported the results of these studies carried out with his former student and later collaborator, Jean Brossel, at a meeting of La Société Française de Physique on May 30, 1950.

A few months later, Kastler extended his work on double resonance methods to the development of another technique known as optical pumping. In optical pumping, polarized light is shined on a group of atoms. Electrons in one sub-level of the atoms absorb light and move to a higher energy level, while electrons in a second sub-level will not absorb light and remain in their ground state. When the excited electrons return to their original state, however, they distribute themselves between both ground sub-levels, absorbing and non-absorbing.

Because this technique is a way of getting electrons from an absorbing to a non-absorbing level, it is called optical pumping. In later years, optical pumping was used in the development of a variety of practical devices, including the maser and laser, the atomic clock, and the magnetometer. In recognition of his important role in these developments, Kastler was awarded the 1966 Nobel Prize in physics. His award brought rejoicing in France, where Kastler was called the "Grandfather of the Laser" by one national newspaper. It was the first Nobel Prize in physics awarded to a French citizen in thirty-seven years. Kastler married Élise Cosset on December 24, 1924. They had three children, Daniel, Claude-Yves, and Mireille. The two boys became teachers, and the daughter became a physician. After World War II, Kastler became increasingly active in the pacifist movement, opposing both the war in Vietnam and France's occupation of Algeria. He died in Bandol, France, on January 7, 1984.