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.
Highlights
The physics of the interaction between a solid surface and an intense optical field has been studied since the appearance of intense femtosecond lasers
In order to elucidate the dynamics of the ejected particles, the velocity distribution of ions emitted from the metal by femtosecond laser ablation was measured by time-of-flightTOFmass spectrometry
The process of ion production is well explained by multiphoton absorption and optical field ionization
Summary
The physics of the interaction between a solid surface and an intense optical field has been studied since the appearance of intense femtosecond lasers. In order to elucidate the dynamics of the ejected particles, the velocity distribution of ions emitted from the metal by femtosecond laser ablation was measured by time-of-flightTOFmass spectrometry. The low-energy componenttens of eVwas due to thermal ions, and the high-energy componentup to several keVwas produced by nonthermal processes Such observations are quite general to metals[16–19] in this high laser intensity range. The interspaces of the grating structures depended on the laser fluence, and this phenomenon was well explained by the parametric decay model[20] proposed by Sakabe et al In this model, a femtosecond laser pulse interacts with the metal and a photon in the IR region, and a plasma wave decays along the surface. The velocity distributions and the species of ions emitted from the metal surface were measured near the low ablation threshold. The relation between the dynamics of ion emission and grating structure formation is discussed
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