Atomic alkali-metal gas cells at liquid-helium temperatures: Loading by light-induced atom desorption

We have studied the loading of gaseous alkali-metal atoms into glass cells held at about 2 K by light-induced atom desorption (LIAD). In LIAD loading, the cell containing dense helium gas and a small amount of alkali metal is irradiated with cw laser light. This technique has two unique features to be investigated: alkali-metal atoms are desorbed, even by weak laser irradiation, from the cell walls not covered visibly with alkali metal, and loading efficiency is much higher below the superfluid transition temperature for the helium inside the cell than above this temperature. On the basis of extensive experimental studies for Rb presented in this paper, we give a consistent picture of the loading mechanism: alkali-metal atoms are optically desorbed from alkali-metal particles on the walls, then transported effectively into the cell volume by helium-gas flow produced by the continuous evaporation of the helium film, which is supplied by the superfluid-film flow on the walls from the bulk liquid helium at the bottom of the cell.