Influence of UO₂ crystal orientation on laser ablation performance

Crystallographic orientation dependence deteriorates the performance of surface analysis methods such as secondary ion mass spectrometry (SIMS) and focused ion beam (FIB). This study explores the corresponding potential challenges of laser ablation (LA) as a powerful sampling tool for inductively coupled plasma-mass spectrometry (ICP-MS). To this end, three UO₂ single crystals of different orientation as well as polycrystalline UO₂ were produced and characterized. Subsequently, a ns-laser ablation system was employed to study laser-matter interaction in detail. Firing the laser continuously at 1 Hz with various single shot fluence (2, 4, 6, 8, 12 J cm^-2) for diverse periods created LA craters impacted by cumulative fluence between 50 and 650 J cm^-2. Repeated LA experiments on the (100) plane of a UO₂ single crystal at the beginning and end of the entire study revealed highly reproducible (<3%) LA rates, only limited by the fluctuation of the laser energy output of the ns-LA system. After thorough cleaning of the ablated samples, surface roughness and average depth of LA craters were determined using confocal laser scanning profilometry. Both LA rate and average depth of craters decreased exponentially with increasing single shot fluence independently of the crystal orientation. Surface roughness increased linearly with increasing cumulative fluence having largest intensification for lowest single shot fluence. Scanning electron microscope (SEM) images not only revealed the conical silhouette of LA craters, but also identified a convex meniscus at its bottom. This particular shape of the crater bottom with a deeper ring surrounding the central region is a result of melted and re-solidified UO₂ generated during the LA process and the main limiting factor for the achievable depth resolution. The rapid re-solidification of the liquid phase after each single laser shot created tiles of different shape and orientation, depending on UO₂ crystal orientation. Three different types of ejected particles radially distributed around the LA craters were identified by SEM, providing profound insights into laser-UO₂ interaction.