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 $0.028\text{ }\text{J}/{\text{cm}}^{2}$, and two higher emission thresholds were identified at fluences of ${F}_{th,M}=0.195\text{ }\text{J}/{\text{cm}}^{2}$ and ${F}_{th,H}=0.470\text{ }\text{J}/{\text{cm}}^{2}$. The relation between the number of emitted ions per pulse ${N}_{i}$ and the laser fluence $F$ was in good agreement with ${N}_{i}\ensuremath{\propto}{F}^{4}$ for ${F}_{th,L}\ensuremath{-}{F}_{th,M}$, ${N}_{i}\ensuremath{\propto}{F}^{3}$ for ${F}_{th,M}\ensuremath{-}{F}_{th,H}$, and ${N}_{i}\ensuremath{\propto}{F}^{2}$ for $\ensuremath{\ge}{F}_{th,H}$. 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 $0.136\text{ }\text{J}/{\text{cm}}^{2}$, the emitted ions have an energy of 30 eV, and the ion energy depends on the laser fluence (790 eV at $14.4\text{ }\text{J}/{\text{cm}}^{2}$). 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.