Enhanced excitation and ionization of neonlike silver in laser-produced plasmas simultaneously irradiated by two wavelengths

In a series of experiments Formvar-supported circular silver-dot targets were simultaneously irradiated with 1.06-μm and 0.35-μm laser light in the mid-${10}^{14}$-W ${\mathrm{cm}}^{\mathrm{-}2}$ range. In some of the shots, only 0.35-μm light was used. The 3000-Å thickness of the silver was chosen to assure an electron density in excess of the critical ${10}^{21}$ ${\mathrm{cm}}^{\mathrm{-}3}$ at the time of superposition of the 1.06-μm beam, thereby facilitating the generation of suprathermal electrons by resonance absorption. Both high-resolution and broadband spectral data reveal that higher states of excitation and ionization of neonlike and fluorinelike silver were achieved in the presence of the 1.06-μm beam. Evidence that this enhanced excitation and ionization is due to suprathermal-electron pumping is provided by the broadband-continuum data which show little or no increase in the thermal-electron temperature, when the 1.06-μm light is superimposed. Analysis of the data indicates that, in a similarly structured linear target, the gain of the 3p-3s J=2 to 1 neonlike line at 99.6 Å would increase from 0.4 to ∼1 ${\mathrm{cm}}^{\mathrm{-}1}$ when using both laser wavelengths. This conclusion is independent of the mechanism producing the increased excitation and ionization.