William A. Arnold

William Archibald Arnold (December 6, 1904 – October 26, 2001) was an American plant physiologist and biophysicist whose work helped establish the physical basis of photosynthesis. He is known for experiments with Robert Emerson that led to the concept of the photosynthetic unit, and for later discoveries including delayed light emission and thermoluminescence in photosynthetic systems.

Early life and education Arnold was born in Douglas, Wyoming, on December 6, 1904, the eldest of four children of William A. Arnold and Nellie Agnes O’Brien Arnold.

Their 1932 papers showed that approximately 2,400 chlorophyll molecules were associated with the production of one molecule of oxygen under optimal conditions.

Delayed light emission and thermoluminescence At Oak Ridge National Laboratory, Arnold and Bernard L. Strehler discovered delayed light emission (delayed fluorescence) in photosynthetic organisms in 1951, demonstrating that absorbed light energy could be stored and later re-emitted.

Together with Helen Sherwood, Arnold later identified thermoluminescence in chloroplasts, showing that previously absorbed light energy could be released upon heating, a phenomenon that became an important tool for studying photosynthetic mechanisms. Arnold also collaborated with Robert Oppenheimer on studies of excitation energy transfer and the electronic properties of photosynthetic processes to understand energy transfer mechanisms in photosynthetic pigments.

Other contributions Arnold worked broadly across physics and biology, including studies of fluorescence polarization, electroluminescence in chloroplasts, and energy transfer mechanisms.

While working in Copenhagen in 1938–1939, Arnold was present during early experiments on nuclear fission and participated in discussions with Otto Robert Frisch, in which he suggested the biological analogy of "binary fission"; Frisch subsequently adopted the term "fission" for the process.

Honors and recognition Arnold was elected to the National Academy of Sciences in 1962.

He received the Charles F. Kettering Award from the American Society of Plant Physiologists in 1963 for applying physical principles to biological systems.

He was also awarded the Charles Reid Barnes Life Membership Award in 1975.

Personal life Arnold married Jean Irving Tompkins on September 11, 1929; they remained married for 68 years until her death in 1997. They had two daughters.

Arnold was known for his emphasis on simplicity and precision in scientific work and for his interdisciplinary approach spanning physics, biology, and chemistry.

He died on October 26, 2001, at the age of 96. In accordance with his wishes, he was cremated and his ashes were buried next to his wife’s at Oak Ridge Memorial Park.