Strong-field ionization and Coulomb explosion of argon clusters by few-cycle laser pulses

Energy distributions are measured for ions emitted upon Coulomb explosion of Ar-n clusters (n = 400-900) upon irradiation by intense three-cycle pulses (10 fs) of 800-nm laser light of peak intensity 5 x 10(14) Wcm(-2). With few-cycle pulses, there is insufficient time for the cluster to undergo expansion; this results in overall dynamics that are significantly different from those in the many-cycle regime. The peak ion energies are much lower than those obtained when 100-fs pulses of the same intensity are used; they are almost independent of the size of the cluster (over the range 400-900 atoms). Ion yields are measured to be larger in the direction that is perpendicular to the laser-polarization vector than along it. Model molecular dynamics calculations are used to qualitatively rationalize this unexpected anisotropy in terms of shielding by a spatially asymmetric electronic-charge cloud within the cluster.