We present results of experiments and simulations of the interaction between a high intensity, femtosecond laser pulse and an atomic cluster that show the temporal profile of the laser pulse plays a crucial role in the expansion dynamics of the cluster. Experiments were conducted in rare gas clusters of Xe, Kr and Ar of radius 30 - 80 A with a laser pulse of duration 70 - 240 fs and a peak intensity of ~1016 Wcm-2. The shape of the laser pulse was a Gaussian with a shoulder of intensity 0.02 times the peak pulse intensity appearing on either the rising or falling edge of the main pulse depending on the sign of chirp applied to the laser pulse. Significant differences (up to a factor of 2) in the energies of the ions obtained from the laser-cluster interaction were measured when the shape of the laser pulse was varied.