Alternate-Gradient Focusing of Molecular Beams

Abstract

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.

A Storage Ring for Neutral Molecules

“…This so-called alternate gradient (AG) focusing technique was first proposed and implemented for charged particles, which are natural highfield seekers [30]. Later, AG-focusing was proposed to focus a beam of neutral molecules [38] which was soon afterwards experimentally demonstrated [71].…”

“…A proposal for a storage ring for neutral molecules can already be found in a paper by Auerbach et al in 1966 [38] although it is very likely that it has even been considered by those working with hexapole focusers before. In this paper [38] alternate-gradient focusing of molecular beams is discussed. This focusing technique is quite general as it is applicable to both high-field seeking and low-field seeking particles (vide infra).…”

Improved alternating gradient transport and focusing of neutral molecules

“…Cylindrical electrodes, which are two-wire field lenses (with r 0 = half-gap), shown in Fig. 3 (a), have long been used to focus molecules in strong-field seeking states [1,2,3,4,5,6,7]. They contain multipoles of all odd orders and of equal strengths ( a 1 = 1, a 3 = −1, a 5 = 1 .…”

“…Molecules in strong-field seeking states have been transported and focused by an alternating sequence of electric field gradient lenses [1,2,3,4,5,6,7] (as have neutral atoms [8]), but it has been neither as successful, nor as widely used, as has quadrupole and sextupole focusing for molecules in weak-field seeking states [7,9].…”

Optimizing the resolution of the alternating-gradientm/μselector

“…This principle is exploited to confine ions in quadrupole mass filters [10,11], in Paul traps [10,12], and in virtually all particle accelerators. The application of AG focusing to neutral polar molecules was first proposed by Auerbach, Bromberg, and Wharton [13] and experimentally demonstrated by Kakati and Lainé for ammonia molecules in hfs states [14]. More recently, AG focusing was applied to the deceleration of polar molecules [15][16][17][18], to the guiding of cold molecules from an effusive source [19], and to the guiding of a supersonic jet of CaF molecules [20].…”