John Hasbrouck Van Vleck was an American physicist and mathematician.
Background
John Hasbrouck Van Vleck was born on March 13, 1899, in Middletown, Connecticut. He was the only child of Hester Laurence Raymond and Edward Burr Van Vleck, a mathematician.
His father was heir to a comfortable fortune, and the family often summered in Europe. With his parents, and later his wife, Van Vleck traveled over much of the world.
Education
Raised in Madison, Wisconsin, where his father taught at the University of Wisconsin beginning in 1906, he attended public schools and the University of Wisconsin.
An outstanding undergraduate, Van Vleck majored in physics at the University of Wisconsin, but he did not settle on a career in mathematical physics until he began graduate study at Harvard in the spring of 1920.
He completed his doctorate in 1922 and by 1927 was a full professor at the University of Minnesota.
Career
Van Vleck spent most of his career at Harvard (1922-1923, 1934 - 1969), with shorter terms at the University of Minnesota (1923 - 1928) and the University of Wisconsin (1928 - 1934). He was often invited to lecture at other universities in America and in Europe. His first professorship there carried light teaching duties, and it allowed him time to develop his research. Magnetism was what Van Vleck called his first love. His entry into that field was marked by his discovery of the general theory of magnetic (and electric) susceptibilities in gases, in 1927, soon after the advent of quantum mechanics.
Beginning in 1927, he (and his students) delved into molecular spectroscopy. Van Vleck's familiarity with magnetic theory, spectroscopy, and radio frequency techniques (radar and magnetic relaxation) put him in a central position in the development of magnetic resonance techniques in the late 1940's and early 1950's. Although he published comparatively little on resonance topics, his close contact with experimentalists, such as Edward Purcell's nuclear magnetic resonance group at Harvard, frequently expedited their research.
In the middle of his career, Van Vleck established himself as a capable administrator. He directed the theory group at the Radio Research Laboratory (the radar countermeasures laboratory) at Harvard from 1943 to 1945. Also at Harvard, Van Vleck was chairman of the physics department from 1945 to 1949, and from 1951 to 1957, he shaped the interdisciplinary Division of Engineering and Applied Physics, as its first dean. In 1951, Van Vleck became Hollis Professor of Mathematics and Natural Philosophy at Harvard, and in 1952 and 1953, he was president of the American Physical Society.
Van Vleck died at his home in Cambridge, Massachusetts on March 13, 1899.
John's theory of magnetic (and electric) susceptibilities, along with experimental substantiation and extensions to related topics, made up his masterpiece. The book also contained the germ of the crystal field theory, which Van Vleck and his students William Penney and Robert Schlapp developed in 1932 to describe paramagnetic salts, particularly salts containing rare-earth ions. In paramagnetic materials, magnetic moments of electrons in various quantum states align themselves with an external field; the object of any theory is to discover which of those quantum states are occupied.
The crystal field theory considers only energy perturbations of the quantum states of a paramagnetic ion that arise from electrostatic effects, but by 1935 Van Vleck was able to suggest how to account for perturbations from molecular bonding of the ion to the rest of the crystal. Physical chemists have since expanded this suggestion into ligand field theory. The exchange of energy between paramagnetic ions and vibrations in the salt crystal lattice (the phenomenon of paramagnetic relaxation) first attracted Van Vleck's attention in 1939.
He continued to study relaxation and paramagnetic materials, particularly crystals containing rare earths, through 1975. Ferromagnetics were a recurring concern of Van Vleck's from 1931. Ferromagnetic materials (such as iron) have intrinsic magnetic moments, in contrast with the induced moments of paramagnetics. They are, however, far more complex than paramagnetics, and neither Van Vleck nor anyone else achieved a fundamental understanding of them.
Magnetism was not Van Vleck's sole concern. His most notable discovery in molecular spectroscopy (stemming from wartime research on radar, and published in 1947) was a spectral absorption line for the water molecule near 1. 3 cm wavelength. This absorption line made impractical a proposed radar system that would have operated at about that wavelength. Between 1932 and 1935, Van Vleck and his students investigated chemical physics, in particular, the methane molecule. They were able to reconcile the seemingly disparate theories of atomic orbitals and molecular orbitals. Such reconciliations appeared in Van Vleck's work in several areas.
Quotations:
“. .. one can still say that quantum mechanics is the key to understanding magnetism. When one enters the first room with this key there are unexpected rooms beyond, but it is always the master key that unlocks each door. ”
Membership
a member of the United States National Academy of Sciences
Personality
Van Vleck was unathletic, but from his father he learned to enjoy hiking, and he followed college football throughout his life. Van Vleck was a precocious child; memorizing railway timetables was his hobby.
In adulthood he was able to travel without referring to a printed schedule. Van Vleck an open and generous man, was known to most acquaintances as Van. He took care that his collaborators received their share of credit for work done with him, if not more.
Connections
On June 10, 1927, Van Vleck married Abigail June Pearson; they had no children.