THE TEMPERATURE OF ATOMS IN A MAGNETOOPTICAL TRAP

A cloud of ultracold atoms confined in a magneto-optical trap (MOT) ha s a temperature greater than that of a low-density vapour in the same light field. It has been observed (Drewsen M. et al., Appl. Phys. B, 5 9 (1994) 283) that this temperature excess is proportional to the cube root of the total number of trapped atoms (N1/3). We present an expla nation for this effect in terms of the scattering which arises when ph otons spontaneously emitted by atoms have an appreciable probability o f being reabsorbed within the cloud. This hypothesis has been tested b y time-of-flight measurements of the temperature of clouds of atoms re leased from a MOT into 1-dimensional and 3-dimensional optical molasse s in the sigma+-sigma- and lin perpendicular-to lin configuration.