Ion emission from a metal surface through a multiphoton process and optical field ionization

In order to investigate the physics of ion emission under an intense optical field, the ions emitted from a laser-irradiated copper surface were studied by time-of-flight energy spectroscopy. The lowest laser fluence at which ions are emitted, Fth,L, is 0.028 J/cm², and two higher emission thresholds were identified at fluences of Fth,M=0.195 J/cm² and Fth,H=0.470 J/cm². The relation between the number of emitted ions per pulse Nᵢ and the laser fluence F was in good agreement with Nᵢ∝F⁴ for Fth,L-Fth,M, Nᵢ∝F³ for Fth,M-Fth,H, and Nᵢ∝F² for ≥Fth,H. The dependence of ion production on laser energy fluence is explained well by multiphoton absorption and optical field ionization. Even at a low laser fluence such as 0.136 J/cm², the emitted ions have an energy of 30 eV, and the ion energy depends on the laser fluence (790 eV at 14.4 J/cm²). The laser fluence dependence of ion energy is reasonably well related to those of the interspaces of gratings that are self-organized on a metal surface by femtosecond laser pulses.