Direct spectroscopic observation of multiple-charged-ion acceleration by an intense femtosecond-pulse laser

We have observed evidence of the emission of energetic He-and H-like ions of fluorine more than 1 MeV produced via the optical field ionization (OFI) from a solid target irradiated by an intense I=(2--4)×10¹⁸W/cm² (60 fs, λ=800nm), obliquely incident p-polarized pulse laser. The measured blue wing of He_α, He_β, and Ly_α lines of fluorine shows a feature of the Doppler-shifted spectrum due to the self-similar ion expansion dominated by superthermal electrons with the temperature Tₕ~100keV. Using a collisional particle-in-cell simulation, which incorporates the nonlocal-thermodynamic-equilibrium ionization including OFI, we have obtained the plasma temperature, line shape, and maximal energy of accelerated ions, which agree well with those determined from the experimental spectra. The red wing of ion spectra gives the temperature of bulk plasma electrons.