Energy distributions of ions emitted from argon clusters Coulomb-exploded by intense femtosecond laser pulses

Energy distributions of ions emitted from argon clusters (up to 12 800 atoms/cluster) Coulomb-exploded with an intense femtosecond laser have been experimentally studied. Under the laser intensity of 2x10(16) W/cm(2), the ions of >15 keV are anisotropically emitted, while those lower than 15 keV tend to be isotropic. The argon ions of charge state up to Ar14+ were generated, and their high charge state is more than expected by the barrier suppression ionization model. The yield and maximum energy of Ar8+-Ar14+ ions depend on laser polarization direction, while those of Ar+-Ar7+ are much less sensitive to the laser polarization. The present results suggests that high-charge-state and anisotropically emitted ions are produced by impact ionization of electrons that are produced by optical field ionization with the intense laser.