Formation of van der Waals Molecules in Buffer-Gas-Cooled Magnetic Traps

Abstract

We predict that a large class of helium-containing cold polar molecules form readily in a cryogenic buffer gas, achieving densities as high as 10(^{12}) cm(^{-3}). We explore the spin relaxation of these molecules in buffer-gas-loaded magnetic traps and identify a loss mechanism based on Landau-Zener transitions arising from the anisotropic hyperfine interaction. Our results show that the recently observed strong (T^{-6}) thermal dependence of the spin-change rate of silver (Ag) trapped in dense (^{3})He is accounted for by the formation and spin change of Ag(^{3})He van der Waals molecules, thus providing indirect evidence for molecular formation in a buffer-gas trap.