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 120.2em0ex8000.3em0exatoms∕cluster) Coulomb-exploded with an intense femtosecond laser have been experimentally studied. Under the laser intensity of 2×10¹⁶0.3em0exW∕cm², the ions of >150.3em0exkeV are anisotropically emitted, while those lower than 150.3em0exkeV 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.