  Calculating the energy absorption of atomic clusters as a function of the
laser pulse length $T$ we find a maximum for a critical $T^*$. We show that
$T^*$ can be linked to an optimal cluster radius $R^*$. The existence of this
radius can be attributed to the enhanced ionization mechanism originally
discovered for diatomic molecules. Our findings indicate that enhanced
ionization should be operative for a wide class of rare gas clusters. From a
simple Coulomb explosion ansatz, we derive an analytical expression relating
the maximum energy release to a suitably scaled expansion time which can be
expressed with the pulse length $T^*$.
