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.
Read more about this topic: Franco Rasetti
Famous quotes containing the words model, atomic and/or nucleus:
“The playing adult steps sideward into another reality; the playing child advances forward to new stages of mastery....Child’s play is the infantile form of the human ability to deal with experience by creating model situations and to master reality by experiment and planning.”
—Erik H. Erikson (20th century)
“Other centuries had their driving forces. What will ours have been when men look far back to it one day? Maybe it won’t be the American Century, after all. Or the Russian Century or the Atomic Century. Wouldn’t it be wonderful, Phil, if it turned out to be everybody’s century, when people all over the world—free people—found a way to live together? I’d like to be around to see some of that, even the beginning.”
—Moss Hart (1904–1961)
“Just see how it glints and sparkles. Of course it is a nucleus and focus of crime. Every good stone is. They are the devil’s pet baits. In larger and older jewels every facet may stand for a bloody deed....”
—Sir Arthur Conan Doyle (1859–1930)