Laser-induced nonthermal surface modification of aluminum

摘要

Low fluence UV laser stimulate desorption of ions form metal surfaces leads to the production of high kinetic energy species and also modifies the surface left behind. Both of these effects are small for an individual laser shot. The number density of the desorbed ions by the laser is very low - much less than a monolayer per pulse. The changes in the crystal surface are also subtle. However, in both cases there are cumulative effects after many laser shots which cannot be attributed to thermal or plasma pulse laser interaction processes. There is no evidence for thermal ions in the TOF mass spectrum. A laser ionization experiment was performed to measure the kinetic energy distribution of the desorbed neutral aluminum species. The high kinetic energy Al$+$PLU$/ ions were used as monitor for insuring that the laser induced desorption process was always in the non- thermal regime during the surface modification experiments. We observe, via SEM, surface modification only after exposure to a large number of laser shots. Examination of the laser exposed surface with electron microscopy reveals quasiperiodic patterns made up of sub-micron scale ridges. After exposures to more than 100K laser shots, the quasiperiodic patterns break-up into irregular nodule-like shapes. Our data indicates the surface modification occurs without desorbing significant amounts of material.