Paper 2.2: "The Rotational Magnetic Moments of H2, D2, and HD Molecules," N. F. Ramsey, Phys. Rev. 58, 226–236 (1940)
Reprinted with permission from The Physical Review. Copyright 1940, The American Physical Society.
While studying the nuclear moment resonances of H2 and D2, I discovered a different group of resonances that were associated with the reorientation of the molecular rotational magnetic moments. After deciding that the nuclear resonance studies, including the deuteron quadrupole moment, should be a joint project, we agreed that my Ph.D. thesis should be on the resonances associated with the molecular rotational magnetic moments of the molecular hydrogens. I found 6 resonances for H2, 6 for D2 and 12 for HD. From the rotational magnetic moments and G. C. Wick's theory, I obtained values for the high frequency (or second order paramagnetic) term in the magnetic susceptibility of the molecule.
When I adapted our successful theory of the nuclear resonances to the rotational moment resonances, the agreement between theory and experiment was almost but not quite perfect. I then realized that the orientation of the molecule was changed in the molecular rotational experiments so differences in the molecular diamagnetic susceptibility would shift the resonance frequencies. When I introduced into the theory such a diamagnetic susceptibility term with only one free parameter, the rotational resonances agreed perfectly with theory and I obtained an empirical value for the orientation dependence of the magnetic susceptibility. I later [Phys. Rev. 78, 221–222 (1950)] developed the theory of the orientation dependence of the magnetic susceptibility and derived from our measurement the quadrupole moment of the electron distribution in the H2 molecule. Several years later (Paper 5.3), I showed that this result, combined with values for the magnetic susceptibility, gave the principal second moments of the electron distribution of the H2 molecule.
