Editing Robert S. Mulliken (section)

===Graduate and early postdoctoral education===
Mulliken got his doctorate in 1921 based on research into the separation of [[isotope]]s of [[Mercury (element)|mercury]] by [[evaporation]], and continued in his isotope separation by this method. While at [[Chicago]], he took a course under the [[Nobel Prize in Physics|Nobel Prize-winning physicist]] [[Robert A. Millikan]], which exposed him to the [[old quantum theory]]. He also became interested in strange molecules after exposure to work by [[Hermann I. Schlesinger]] on [[diborane]].

[[File:Hund,Friedrich 1929 Chicago.jpg|thumb|300px|Robert Mulliken, Chicago 1929 (third from right)]]
At Chicago, he had received a grant from the [[United States National Research Council|National Research Council]] (NRC) which had paid for much of his work on isotope separation. The NRC grant was extended in 1923 for two years so he could study isotope effects on band spectra of such diatomic molecules as boron nitride (BN) (comparing molecules with B<sup>10</sup> and B<sup>11</sup>). He went to [[Harvard University]] to learn spectrographic technique from Frederick A. Saunders and quantum theory from [[E. C. Kemble]]. At the time, he was able to associate with [[J. Robert Oppenheimer]] and many future Nobel laureates, including [[John H. Van Vleck]] and [[Harold C. Urey]]. He also met [[John C. Slater]], who had worked with [[Niels Bohr]].

In 1925 and 1927, Mulliken traveled to Europe, working with outstanding spectroscopists and quantum theorists such as [[Erwin Schrödinger]], [[Paul Dirac|Paul A. M. Dirac]], [[Werner Heisenberg]], [[Louis-Victor de Broglie|Louis de Broglie]], [[Max Born]], and [[Walther Bothe]] (all of whom eventually received Nobel Prizes) and [[Friedrich Hund]], who was at the time Born's assistant. They all, as well as [[Wolfgang Pauli]], were developing the new [[quantum mechanics]] that would eventually supersede the old quantum theory. Mulliken was particularly influenced by Hund, who had been working on quantum interpretation of band spectra of diatomic molecules, the same spectra which Mulliken had investigated at Harvard. In 1927 Mulliken worked with Hund and as a result developed his [[molecular orbital]] theory, in which electrons are assigned to states that extend over an entire molecule. In consequence, molecular orbital theory was also referred to as the '''Hund-Mulliken theory.'''