Initially, his major interest was the theory of the continuous spectrum. His first published paper, in 1926, concerned the quantum theory of molecular band spectra. He developed a method to carry out calculations of its transition probabilities. He calculated the photoelectric effect for hydrogen and X-rays, obtaining the absorption coefficient at the K-edge. His calculations accorded with observations of the X-ray absorption of the Sun, but not helium. Years later, it was realized that the Sun was largely composed of hydrogen and that his calculations were correct.