Paper 2.15: "Hyperfine Structure of 7Li79,81Br by Molecular-Beam Electric Resonance," R. C. Hilborn, T. F. Gallagher, Jr. and N. F. Ramsey, J. Chem. Phys. 56, 855–861 (1972)
Reprinted with permission from Journal of Chemical Physics. Copyright 1972, American Institute of Physics.
Our magnetic resonance experiments on molecules with low moments of inertia were very productive, because few rotational states were excited and the spectral lines could be individually distinguished. The experiments (Papers 2.6 and 2.10) with heavier molecules, like DCl, CO and CH4, gave valuable but less detailed information since the observed spectrum had to be compared with that predicted by averaging over many rotational states. For experiments with even more complex molecules, we needed to select the rotational state by using electric instead of magnetic fields (the effective electric dipole moment of the molecule depends on the rotational state). T. F. Gallagher, R. C. Hilborn, T. C. English and I designed and built a molecular beam electric focusing apparatus with a jet source, electric quadrupole focusing fields giving both parallel and single crossing focusing, a 2.5-m-long resonance region and an iridium hot wire detector. This apparatus, and the initial measurements with it, are described by T. F. Gallagher, R. C. Hilborn and me [J. Chem. Phys. 56, 5972–5979 (1972)] and in Paper 2.1. 7Li35,37Cl and 7Li79,81Br were studied and accurate values obtained for the nuclear electric quadrupole interactions, the spin-rotational interactions and the spin–spin interactions in the J = 1 rotational state and in various vibrational states of the molecules. A. R. Jacobson and I, with the same apparatus, obtained similar results with LiI [J. Chem. Phys. 65, 1211–1213 (1976)].
J. L. Cecchi and I later installed a 2.27 m electromagnet along the resonance region in order to study Zeeman and Stark shifts in the same apparatus [J. Chem. Phys. 60, 53–65 (1974)], the first studies being with 7Li79,81Br. The experiments were done with no external electric field (the effective electric field induced by the motion of the molecule through the magnetic field was sufficient to produce the parity-mixing field that is necessary for observing ΔJ = 0, ΔmJ = ±1 transitions). We accurately measured a number of parameters for these molecules including their nuclear magnetic moments, the tensor terms in their magnetic shieldings and the tensor terms in the diamagnetic susceptibility. R. R. Freeman, D. W. Johnson and I [J. Chem. Phys. 61, 3471–3478 (1974)] used the same apparatus to determine similar quantities for isotopes of LiCl in the zeroth and first rotational states.
Since the original apparatus had an iridium detector, it was limited to molecules containing at least one alkali atom. To extend its use to HBr and similar molecules, D. W. Johnson and I [J. Chem. Phys. 67, 941–947 (1977)] installed an HBr negative ion ionizer to measure the hyperfine interaction parameters of H79Br, H81Br, D79Br, D81Br and D35Cl in the first and second rotational states.
