Kastler
NAME: Kastler, Alfred
ACHIEVEMENT(S): Nobel Prize for Physics in1966 for his discovery and development of methods for observing Hertzian resonances within atoms.
NICKNAME/ALIAS: None
DATE/PLACE OF BIRTH: 3 May 1902, at Guebwiller, Alsace, Germany [now in
France]
DATE/PLACE/AGE AT DEATH: 7 January 1984, in Bandol, France. Aged 81
DISCIPLINE: Physicist
FIELD: Physics
LANGUAGE: German and French
PARENTS: Frederic Kastler and Anna Frey
SIBLINGS: ?
SPOUSE: Elise Cosset, married 1924
CHILDREN: Daniel (b. 1926, prof. of physics), Mireille (b. 1928,
ophthalmologist) and Claude-Yves (b. 1936, teacher in Russian)
EDUCATION: Guebwiller Primary School; Colmar Oberrealschule, to 1919; Ecole
Normale Superieure, Paris, 1920-26
MAJOR WORK: 'La Diffusion de la Lumiere par les Milieux Troubles', 1952
LIFE AND TIMES: At the Ecole Normale, Kastler was introduced to the concepts of <influenced by> Bohr's* atom and quantum physics, and in particular to A.J.W. Sommerfeld's book on atomic structure and spectral lines. Here Kastler first read of the principle of the conservation of momentum applied to the exchange of energy between atoms and radiation, a principle that was to guide the whole of his research in spectroscopy.
One technique used to study electron energy levels involved the addition of energy to an atom because when they absorb energy from incident light electrons rise to higher energy levels, and then - after remaining briefly at the new levels - fall back to lower levels, releasing the previously gained energy in the form of emitted light.
(Light, like all electromagnetic radiation, including radio waves, microwaves and X-rays, consists of units of energy called photons: the energy in an absorbed or emitted photon, which is proportional to the frequency of the absorbed or emitted light, equals the energy difference between the two levels that bracket an electron transition).
Since a set of allowed energy levels is unique to the atoms of a particular element, and since excited atoms emit light only at frequencies corresponding to the energy differences between levels, the emitted energy (in the form of light) could then be studied using a spectroscope (which shows the spectrum of emission as a series of coloured lines) in order to obtain information about the relative location of the energy levels, and
then from this to extrapolate the identity of the element and information concerning its atomic structure.
Following the discovery in 1935 of the strong emission of the sodium D line in the twilight luminescence of the sky, in 1937 Kastler began studying the luminescence of sodium atoms in the upper atmosphere.
In 1939-40, he established that the D line of the twilight sky could be easily absorbed by a layer of sodium vapour, and he was able to demonstrate that this line is polarized, and that its emission is an optical resonance phenomenon produced by solar radiation. By experiment, he was then able to show that the twilight D line has the same small polarization as sodium resonance light produced in the laboratory.
In 1949 Kastler studied developments made by a group of Columbia University scientists of a form of radio-frequency spectroscopy known as the atomic beam-magnetic resonance technique, and was using this method to make precise measurements of atomic
energy levels in the ground state (i.e. at the lowest energies).
Generally, the ground state has several magnetic sub-states that can be slightly separated by a magnetic field, within which atoms can then be made to move from one sub-state to another by a stimulating field of the approximate frequency - i.e. one that supplies photons whose energy equals the energy difference between the sub-states - which happens to be in the radio-frequency range.
Using an arrangement of magnets and slits the Columbia group produced a narrow beam of atoms in a few magnetic sub-states, with only atoms in particular sub-states able to reach a detector. When the field was tuned to the correct frequency, the number of atoms reaching the detector changed, indicating that transitions had occurred from which - knowing the photon energy that caused the transitions - the group could calculate the sub-state energy levels. This correspondence between sub-states became known as Hertzian resonance, after the experimental physicist Heinrich Hertz.
The atomic beam-magnetic resonance technique had limitations: the extremely brief average lifetime of an excited-state particle before it emitted energy and returned to the undisturbed ground state (around one ten-millionth of a second); and only a small number of atoms were affected by the resonance-induced transition.
In 1949 Kastler began investigating ways of using light to overcome some of these problems, and produced a method (1950) which subsequently became known as double resonance, because it involves the application of two types of external energy fields - a beam of light and a radio-frequency field.
First, energy from a beam of light is shone on a group of atoms, causing some electrons to be excited and to move to a higher energy level. However, these electrons distribute themselves unevenly among the sublevels within that higher energy level, and so when the atoms fall back to the ground state, light is not emitted equally in all directions, and the light emitted in any one direction is partially polarized.
At this point Kastler then applied an electromagnetic field to the excited atoms of exactly the right frequency (photon energy) to force transitions from populated to unpopulated sublevels. The subsequent changes in the directional distribution and polarization of the emitted light could then be analyzed, as Kastler realized that these changed indicate that the radio frequency is tuned to (in resonance with) the energy difference between sublevels. The method is a precise tool for mapping the sublevels of excited atomic states.
Kastler extended his own work on double resonance methods to the development of another technique, now known as optical pumping. In this method, having directed specially polarized light at a group of atoms (the ground state of which has, for example, two magnetic sublevels), the electrons in one sublevel of the atoms absorb light and move to a higher energy level, while electrons in the second sublevel will not absorb light and remain in their ground state.
However, when the excited atoms emit radiation and return to the ground state, they distribute themselves between both ground sublevels (absorbing and non-absorbing), so the light is said to have "pumped" atoms into the non-absorbing ground state.
During the 1950s and 1960s, optical pumping was used in the development of a variety of practical devices, including the maser and <made possible> laser*, the atomic clock and the magnetometer.
During his later years, Kastler became active in peace movements and in groups opposed to nuclear weapons.
ASSESSMENT: He made a major contribution to the development of the laser.
EXTRA CONNECTIONS: None
ONELINER: Alfred Kastler, who made possible the laser
BIBLIOGRAPHY:
EB, DSB