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, 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 i ∝ F2 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/ 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. © 2010 The American Physical Society.