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

Abstract

Energy distributions of ions emitted from argon clusters (up to $12\phantom{\rule{0.2em}{0ex}}800\phantom{\rule{0.3em}{0ex}}\text{atoms}∕\text{cluster}$) Coulomb-exploded with an intense femtosecond laser have been experimentally studied. Under the laser intensity of $2\ifmmode\times\else\texttimes\fi{}{10}^{16}\phantom{\rule{0.3em}{0ex}}\mathrm{\text{W}}∕{\text{cm}}^{2}$, the ions of $>15\phantom{\rule{0.3em}{0ex}}\text{keV}$ are anisotropically emitted, while those lower than $15\phantom{\rule{0.3em}{0ex}}\text{keV}$ tend to be isotropic. The argon ions of charge state up to ${\mathrm{Ar}}^{14+}$ were generated, and their high charge state is more than expected by the barrier suppression ionization model. The yield and maximum energy of ${\mathrm{Ar}}^{8+}\text{--}{\mathrm{Ar}}^{14+}$ ions depend on laser polarization direction, while those of ${\mathrm{Ar}}^{+}\text{--}{\mathrm{Ar}}^{7+}$ 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.