Sergei Leonidovich Mandelstam (born 22.02.1910) is a Soviet physicist and a corresponding member of the USSR Academy of Sciences (1979). He is a son of L. I. Mandelstam. He was born in Odessa. He graduated from Moscow University (1931). In 1931 — 35 he worked at the Research Institute of Physics of Moscow University. From 1935 he was head of a laboratory at the Physical Institute of the USSR Academy of Sciences and from 1968 director of the Institute of Spectroscopy of the USSR Academy of Sciences. In 1944-47 he was also professor at the Moscow Institute of Steel and Alloys, and from 1947 professor and from 1957 head of a department at the Moscow Institute of Physics and Technology.
His main work is in the field of atomic spectroscopy and its applications and of extra-atmospheric astronomy. He obtained and studied the spectra of highly ionised atoms both under laboratory conditions and in the spectra of solar flares. He studied the conditions of ionisation and excitation of atoms and ions in a plasma and the broadening and shift of spectral lines. He was the first to measure the temperature of lightning and he developed a hydrodynamic theory of the spark discharge. He carried out extensive research on the theory and practice of spectral analysis and on its introduction into industry. He made a comprehensive study of the X-ray radiation of the Sun and established that it is mainly thermal in nature and consists of a quasi-steady and a slowly varying component. He detected the polarisation of the radiation and studied the spectra, the structure and the localisation of X-ray flares.
USSR State Prizes (1946, 1977). The D. S. Rozhdestvensky Prize (1977).