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:
“When Titian was mixing brown madder,
His model was posed up a ladder.
Said Titian, “That position
Calls for coition,”
So he lept up the ladder and had her.”
—Anonymous.
“The pace of science forces the pace of technique. Theoretical physics forces atomic energy on us; the successful production of the fission bomb forces upon us the manufacture of the hydrogen bomb. We do not choose our problems, we do not choose our products; we are pushed, we are forced—by what? By a system which has no purpose and goal transcending it, and which makes man its appendix.”
—Erich Fromm (1900–1980)
“I could not undertake to form a nucleus of an institution for the development of infant minds, where none already existed. It would be too cruel.”
—Henry David Thoreau (1817–1862)