High charge state carbon ions are observed from the rear surface of thin foil irradiated by intense femtosecond laser pulse at intensities up to 6.4×1018 W/cm2. The origin of the ions is studied by analyzing the basic ionization process occurring at the rear surface. It is shown that the normally dominant ionization process is field ionization by barrier suppression for charge states less than He-like (C4+), while collisional ionization is significant for C5+ and C6+.
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