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, F-th,F-L, is 0.028 J/cm(2), and two higher emission thresholds were identified at fluences of F-th,F-M=0.195 J/cm(2) and F-th,F-H=0.470 J/cm(2). The relation between the number of emitted ions per pulse N-i and the laser fluence F was in good agreement with N-i proportional to F-4 for F-th,F-L-F-th,F-M, N-i proportional to F-3 for F-th,F-M-F-th,F-H, and N-i proportional to F-2 for >= F-th,F-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/cm(2), the emitted ions have an energy of 30 eV, and the ion energy depends on the laser fluence (790 eV at 14.4 J/cm(2)). 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.