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 o f fluorine more than 1 MeV produced via the optical field ionization (OFI) from a solid target irradiated by an intense I = (2-4) x 10(18) W/cm(2) (60 fs, lambda = 800 nm), obliquely incident p-polarized pulse laser. The meas ured blue wing of He-alpha, He-beta, and Ly(alpha) lines of fluorine shows a feature of the Doppler-shifted spectrum due to the self-similar ion expan sion dominated by superthermal electrons with the temperature T(h)similar t o 100 keV. Using a collisional particle-in-cell simulation, which incorpora tes the nonlocal-thermodynamic-equilibrium ionization including, OFI, we ha ve obtained the plasma temperature, line shape, and maximal energy of accel erated ions, which agree well with those determined from the experimental s pectra. The red wing of ion spectra gives the temperature of bulk plasma el ectrons. [S1063-651X(99)05009-6].