Alternate-Gradient Focusing of Molecular Beams

Space focusing of molecular beams is an established technique achieved by multipole fields for molecules in states that gain energy with applied field. Very often, however, one wants to focus molecules in states that lose energy with applied field. This latter problem is discussed here, and it is shown that the technique called ``strong'' or ``alternate-gradient,'' AG, focusing is of quite general applicability. The dipole, ``two-wire'' field is shown to be the neutral beam analog of the magnetic or electric quadrupole used for AG focusing of charged beams. Exploitation of circular motion or large energy change to achieve focusing is discussed, although these methods do not seem to be as widely applicable as AG focusing. Some specific field configurations and applications are considered: (1) focusing in a molecular-beam accelerator, (2) ``flop-in'' state selection for positive polarizability or effective moment, (3) beam storage, (4) novel spectroscopy for measuring dipole moments and polarizabilities to high accuracy, that has further applications for high-purity state selection, (5) velocity selection.