Buffer-gas loaded magnetic traps for atoms and molecules: A primer

Over the past three years we have developed the technique of buffer-gas cooling and loading of atoms and molecules into magnetic traps. Buffer-gas cooling relies solely on elastic collisions (thermalization) of the species-to-be-trapped with a cryogenically cooled helium gas and so is independent of any particular energy level pattern. This makes the cooling technique general and potentially applicable to any species trappable at the temperature of the buffer gas (as low as 240 mK). Using buffer-gas loading, paramagnetic atoms (europium and chromium) as well as a molecule (calcium monohydride) were trapped at temperatures around 300 mK. The numbers of the trapped atoms and molecules were respectively about 10¹² and 10⁸. The atoms and molecules were produced by laser ablation of suitable solid precursors. In conjunction with evaporative cooling, buffer-gas loaded magnetic traps offer the means to further lower the temperature and increase the density of the trapped ensemble to study a large variety of both static (spectra) and dynamic (collisional cross-sections) properties of many atoms and molecules at ultra-low temperatures. In this article we survey our main results obtained on Cr, Eu, and CaH and outline prospects for future work. Received 2 November 1998 and Received in final form 19 February 1999