Slowing of supersonically cooled atoms and molecules by time-varying nonresonant induced dipole forces Bretislav Friedrich Department of Chemistry and Chemical Biology and Department of Physics, Harvard University, Cambridge, Massachusetts 02138 Received 17 August 1999; published 14 January 2000 We describe a versatile method for slowing molecules or atoms which relies on high-field-seeking states created by the polarizability interaction with a nonresonant laser field. A pulsed supersonic beam expansion is employed to precool the molecules internally and to narrow their velocity spread. The molecules are scooped at right angles by a nonresonant laser beam steered by a scanner and decelerated on a circular path by gradually reducing the beam’s angular speed. PACS numbers: 32.80.Pj, 33.80.Ps, 33.55.Be, 39.10.+j The scope of magnetic trapping has been recently ex- panded to include most paramagnetic atoms and molecules. This feat was made possible by the development of the buffer-gas cooling technique 1,2 which relies on elastic collisions with a cryogenically cooled He gas and, therefore, unlike laser cooling 3, is not limited to species with par- ticular energy level patterns. Buffer-gas loading combines naturally with magnetic trapping as both make use of cryo- genics. While many atoms are paramagnetic about two- thirds of the ground-state atoms in the periodic system 1, few ground-state molecules have the requisite magnetic di- pole moment of 1 Bohr magneton or greater 2. Moreover, magnetic trapping relies on low-field-seeking states that can relax to high-field-seeking states of lower energy 4. Once this occurs, the high-field seekers are ejected from the mag- netic trap and thus are lost. Here we describe a method for slowing molecules and atoms based on the nonresonant interaction of a laser field with molecular or atomic polar- izability, which offers the means to overcome both limita- tions. The method makes use of a time-varying electric field produced by a laser beam steered by a scanner with a vari- able angular speed. A pulsed supersonic beam expansion precools the molecules internally and narrows their velocity spread. The molecules are scooped at right angles by the nonresonant laser beam and brought to a halt by gradually reducing the beam’s angular speed. The arrested molecules can then be evaporatively cooled 4 and accumulated in a nonresonant light trap 5–8 or another suitable trap 3. The interaction of a nonresonant laser field with molecu- lar polarizability occurs for any molecule no matter whether polar or paramagnetic as all molecules are polarizable, and increasingly so as their size increases 9. Moreover, for mol- ecules of any other than spherical symmetry the nonresonant interaction creates an induced dipole moment that is coupled to one of the molecular axes 8. This has been shown 8,10– 14 to give rise to directional states termed pendular in which the molecular axis librates about the electric field vec- tor. The directionality of pendular states arises from hybrid- ization linear superposition of the field-free rotor states. The hybridization enhances the induced electric dipole mo- ment over that of a spherical species i.e., an atom or a spherical top molecule of the same average polarizability. Since the nonresonant polarizability interaction is purely at- tractive, the energies of the corresponding eigenstates de- crease with increasing field strength laser intensity and, therefore, the states are all high-field seeking. At the same time, focused radiation in free space gives rise to a maximum of field strength and thus produces a trap for the high-field seekers. In short, trapping based on the nonresonant polariz- ability interaction is versatile, lacks relaxation losses, and lends directionality to nonspherical species. In previous work with atoms, the nonresonant light trap far-off-resonance trap was loaded using optical molasses 5 or by transfer of a trapped atomic ensemble from a mag- netic or magneto-optic trap 6,15. Magnetic or magneto- optic traps were also used as stages for producing molecules, via either photoassociation 16,17 or ternary collisions 18 of the ultracold atoms. The resulting translationally cold molecules could then be trapped in a nonresonant light trap 18. A possible experimental setup which implements the slowing scheme based on the time-varying nonresonant po- larizability interaction is shown in Fig. 1. The setup consists of three main parts: a pulsed supersonic beam, a steerable FIG. 1. Schematic diagram of a possible experimental setup see text. PHYSICAL REVIEW A, VOLUME 61, 025403 1050-2947/2000/612/0254034/$15.00 ©2000 The American Physical Society 61 025403-1