Electron and ion emission in high-intensity laser irradiation of aluminum

Electron and ion emission from a laser-irradiated aluminum surface have been studied using time-of-flight spectrometry for laser intensities in the ${10}^{6–}$ ${10}^9$ W ${\mathrm{cm}}^{\mathrm{-}2}$ range, at photon energies of 1.17, 2.34, or 3.51 eV and laser pulse durations varying from 29 to 150 ps. Both ion and electron emission were studied. Electron emission is found to dominate in the whole range of laser parameters explored, albeit by an amount that depends on the laser wavelength and pulse duration. Only singly charged ions are detected if one stays out of plasma conditions in front of the surface. We estimate an order of magnitude of the multiphoton-absorption probabilities using an inverse bremsstrahlung model on three different kinds of potentials (Yukawa, surface, and muffin-tin), and find that multiphoton absorption can almost certainly be understood using lowest-order perturbation theory. Fast electrons are detected, as in many other experiments of this kind, but we show that the spectrum broadening is symmetric, and probably due to transport effects. The dependence of the photocurrent on the different laser parameters is well explained using the Fowler-Dubridge theory of photoemission. Ion emission is found to have a ‘‘thermal’’ character, which is essentially governed by the laser fluence rather than the intensity.