Rudolf Walther Ladenburg (6.06.1882-3.04.1952) was a German physicist. He was born in Kiel. He graduated from the University of Munich (1906). In 1906-24 he worked at the university in Breslau (professor from 1909), in 1924-31 he was head of a department at the Institute of Physical Chemistry and Electrochemistry (Berlin — Dahlem), and in 1931-50 professor at Princeton University.
His research concerned spectroscopy, anomalous dispersion in gases, the theory of radiation, electric discharges in gases, nuclear physics, viscosity and gas dynamics. He was the first to construct (1921) a quantum theory of dispersion, in which he showed that negative dispersion can exist alongside negative absorption. He discovered experimentally (1928) negative dispersion, which was indirect proof of the existence of stimulated emission, and he formulated the condition for detecting stimulated emission, pointing out that a special selective excitation is needed for it.