Franco Rasetti - Raman Spectroscopy and The Model of The Atomic Nucleus

Raman Spectroscopy and The Model of The Atomic Nucleus

After the discovery of Raman scattering by organic liquids, Rasetti decided to study the same phenomenon in gases at high pressure during his stay at Caltech in 1928-29. The spectra showed vibrational transitions with rotational fine structure. In the homonuclear diatomic molecules H₂, N₂ and O₂, Rasetti found an alternation of strong and weak lines. This alternation was explained by Gerhard Herzberg and Walter Heitler as a consequence of nuclear spin isomerism.

For dihydrogen, each nucleus is a proton of spin 1/2, so that it can be shown using quantum mechanics and the Pauli exclusion principle that the odd rotational levels are more populated than the even levels. The transitions originating from odd levels are therefore more intense as observed by Rasetti. In dinitrogen, however, Rasetti observed that the lines originating from even levels are more intense. This implies by a similar analysis that the nuclear spin of nitrogen is an integer.

This result was difficult to understand at the time, however, because the neutron had not yet been discovered, and it was thought incorrectly that the 14N nucleus contains 14 protons and 7 electrons, or an odd number of (21) particles in total which would correspond to a half-integral spin. The Raman spectrum observed by Rasetti provided the first experimental evidence that this proton-electron model of the nucleus is inadequate. After the discovery of the neutron in 1932, Werner Heisenberg proposed that the nucleus contains protons and neutrons, and the 14N nucleus contains 7 protons and 7 neutrons. The even total number (14) of particles corresponds to an integral spin in agreement with Rasetti's spectrum.