  It is known that rare-gas or metal clusters absorb incident laser energy very
efficiently. However, due to the intricate dependencies on all the laser and
cluster parameters it is difficult to predict under which circumstances
ionization and energy absorption is optimal.
  With the help of three-dimensional particle-in-cell simulations of xenon
clusters (up to 17256 atoms) we find that for a given laser pulse energy and
cluster an optimum wavelength exists which corresponds to the approximate
wavelength of the transient, linear Mie-resonance of the ionizing cluster at an
early stage of negligible expansion. In a single ultrashort laser pulse, the
linear resonance at this optimum wavelength yields much higher absorption
efficiency than in the conventional, dual-pulse pump-probe set-up of linear
resonance during cluster expansion.
