Collisions of trapped molecules with slow beams

T V Tscherbul,A Dalgarno,Z Pavlovic,R Côté,H R Sadeghpour

doi:10.1103/physreva.82.022704

Abstract

We present a theoretical study of molecular-trap loss induced by collisions with slow atomic beams based on an explicit analysis of collision kinematics in the laboratory frame and a rigorous quantum description of atom-molecule scattering in external fields. The theory is applied to elucidate the effects of nonuniform magnetic and optical trapping fields on low-temperature collisions of OH ($J=3/2,{M}_{J}=3/2,f$) molecules with $^{4}\mathrm{He}$ atoms. Our calculations quantify the extent to which both elastic and inelastic cross sections are suppressed by external trapping fields, clarify the role of small-angle scattering in trap loss, and may benefit future experiments on collisional cooling of molecules in electromagnetic traps. The calculated cross sections for trap loss in $^{4}\mathrm{He}$ $+$ OH collisions are consistent with recent experimental observations at low beam energies [B. C. Sawyer et al., Phys. Rev. Lett. 101, 203203 (2008)], demonstrating the importance of including the effects of nonuniform trapping fields in theoretical simulations of cold collision experiments with trapped molecules and slow atomic beams.

Highlights

The creation of dense ensembles of cold polar molecules has prompted avid interest in the study of field-controlled molecular collisions at temperatures below 1 K [1,2,3]
Since Uγ < 0 for the antitrapped states, trap loss via inelastic transitions to these states is independent of η, which has the effect of weakening the dependence of the total inelastic cross sections (25) on trap depth
Our study shows that collisions of trapped OH molecules with slow beams of He atoms are sensitive to trap depth, the kinetic energy of the incident beam, and its orientation with respect to the quantization axis defined by the external field

Summary

INTRODUCTION

The creation of dense ensembles of cold polar molecules has prompted avid interest in the study of field-controlled molecular collisions at temperatures below 1 K [1,2,3]. The interactions of 2 molecules such as OH with He atoms [10,11], Rb atoms [12], and each other [13] are strongly anisotropic, which leads to rapid collisional relaxation of low-field-seeking Zeeman Another powerful technique for studying molecular collisions is based on Stark deceleration of molecular beams [14,15,16,17]. Measurements of trap loss induced by collisions of Rb atoms with room-temperature Ar gas have shown that the recapture effect can be pronounced even at very small trap depths [21] These observations suggest that the proper account of nonuniform trapping potentials might be essential for quantitative interpretation of collision experiments with trapped atoms or molecules.

Beam-gas collision cross sections

Cross sections for trap loss

Scattering calculations

Cross sections for beam-gas collisions versus collision energy and trap depth

Comparison with experiment

CONCLUSIONS