Abstract
A cloud of ultracold atoms confined in a magneto-optical trap (MOT) has 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, 59 (1994) 283) that this temperature excess is proportional to the cube root of the total number of trapped atoms (N1/3). We present an explanation for this effect in terms of the scattering which arises when photons spontaneously emitted by atoms have an appreciable probability of being reabsorbed within the cloud. This hypothesis has been tested by time-of-flight measurements of the temperature of clouds of atoms released from a MOT into 1-dimensional and 3-dimensional optical molasses in the σ+-σ- and lin ⊥ lin configuration.