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

Abstract

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,L}, is 0.028 J/cm[2], and two higher emission thresholds were identified at fluences of F_{th,M}=0.195 J/cm[2] and F_{th,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}∝F[4] for F_{th,L}−F_{th,M}, N_{i}∝F[3] for F_{th,M}−F_{th,H}, and N_{i}∝F[2] for ≥F_{th,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.