At Chicago, he had received a grant from the 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 B10 and B11). 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 many future Nobel laureates, including J. Robert Oppenheimer, John H. Van Vleck, and Harold C. Urey. He also met John C. Slater, who had worked with Niels Bohr.