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Physical Review Letters

Deflection of Neutral Molecules using the Nonresonant Dipole Force

Henrik Stapelfeldt, Hirofumi Sakai, E. Constant, P. B. Corkum

Published Oct 13, 1997• 198 citations
Publisher

Abstract

The ac Stark shift produced by nonresonant radiation creates a potential minimum for a ground state molecule at the position where the laser intensity is maximum. The gradient of this potential exerts a force on the molecule. We experimentally observe this force when a beam of ${\mathrm{CS}}_{2}$ molecules is redirected by sending it through the intensity gradient near the focus of a laser beam. We trace the direction of the molecules in the molecular beam, showing that the molecules that pass near the center of the high intensity laser beam will focus.

DOI
10.1103/physrevlett.79.2787

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The ac Stark shift produced by nonresonant radiation creates a potential minimum for a ground state molecule at the position where the laser intensity is maximum. The gradient of this potential exerts a force on the molecule. We experimentally observe this force when a beam of CS2{\mathrm{CS}}_{2}CS2​ molecules is redirected by sending it through the intensity gradient near the focus of a laser beam. We trace the direction of the molecules in the molecular beam, showing that the molecules that pass near the center of the high intensity laser beam will focus.
When a molecule is placed in a nonresonant laser field, the Stark interactions between the laser field and the induced molecular dipole result in a mechanical force on the molecule. This nonresonant dipole force is proportional to the intensity gradient of the laser, thus requiring a strong and focused pulsed laser for a sizable impact on the molecule. 36.4 mJ pulses of a 1064 nm Nd:YAG laser focused with a 17.5 cm focal length convex lens produced a 6.4 m/s change in the transverse velocity of a CS2 molecular beam. Using spherical mirrors with shorter focal lengths such as 10.0, 7.5, and 5.0 cm, dipole forces of similar magnitude were obtained with laser pulses of much lower energies. In particular the 5.0-cm focal length spherical mirror provided an 11.3 m/s change in the transverse velocity using 3.6 mJ laser pulses. This corresponds to 18-fold increase in the deflection efficiency, the ratio between the maximum velocity change and the pulse energy. From the improved efficiency, the nonresonant dipole force can be exerted with ease.
We demonstrate that a bichromatic standing-wave laser field can exert a significantly larger force on a molecule than ordinary radiation pressure. Our experiment measures the deflection of a pulsed supersonic beam of CaF molecules by a two-frequency laser field detuned symmetrically about resonance with the nearly closed $X(v=0)\ensuremath{\rightarrow}B({v}^{\ensuremath{'}}=0)$ transition. The inferred force as a function of relative phase between the two counterpropagating beams is in reasonable agreement with numerical simulations of the bichromatic force in this multilevel system. The large magnitude of the force, coupled with the reduced rate of spontaneous emission, indicates its potential utility in the production and manipulation of ultracold molecules.
The ability to deflect neutral polar molecules present in supersonic molecular beams using inhomogeneous electric fields is a consequence of these molecules possessing a dipole moment. As a dipole moment is inherently a quantum state-dependent feature, it can thus be exploited to allow spatial separation of neutral molecules in different quantum states. In this report, the Stark interaction was exploited to provide deflection of a gaseous sample of indole (C8H7N). Spatial profiles of the molecular beam at various electric field strengths were measured, showing an observed deviation in the beam path attributed to the indole molecules residing in different quantum states. These state-selected samples produced act as ideal samples for laser alignment techniques, in preparation for further experiments such as diffraction on gaseous molecular samples.
Abstract : An experiment that tests a novel method for deflecting molecular beams using inhomogeneous resonance fields is described. If an electric field rotates at the same frequency as a molecule placed in it, the field and molecule remain aligned and the dipole moment-electric field interaction does not average to zero. If the field is spatially inhomogeneous then there is a net force on the molecule. This force can be used in making a molecular beam deflector. Resonance deflection of molecules in the J = 0 and 1 rotational states of a molecular beam of CsF has been observed. The deflection is produced by passing a collimated molecular beam of CsF through the center of a TE(111) microwave cavity which is oscillating at the J = 0 1 transition frequency, 11.019 GHz. The theory of the effect as well as potential applications are also discussed. (Modified author abstract)
We report a different type of optical Stern-Gerlach effect, where a magnetic-field gradient is replaced with a light-intensity gradient and a paramagnetic atom is deflected according to its magnetic quantum number. The laser light is detuned between the D1D1D1 and D2D2D2 frequencies, with the size of the detuning from the D2D2D2 resonance being twice that from the D1D1D1 resonance, and it is circularly polarized to produce an ac Stark shift that takes the form of a pure Zeeman shift. Slow rubidium atoms are extracted from a magneto-optical trap and then spin polarized. The atoms traversing the laser-intensity gradient on one side of the Gaussian beam profile show deflections that depend on the atomic spin state and the laser polarization. When the laser-beam axis is aligned with the slit that defines the atomic beam, we observe focusing and defocusing of the atomic beam.