Ablation efficiency of gold at fs/ps laser treatment in water and air

A comparison of single-pulse laser ablation of gold target by pulses with a 0.3–10 ps duration and a wavelength of 515 nm in air and in water was performed. The radiation was focused on the sample surface through the objectives with numerical apertures NA = 0.65 and 0.25. The influence of the medium, pulse duration, and spot size on the crater morphology was studied. A significant difference in crater morphology was found for different lenses. The ablation efficiency was studied by measuring the profiles of single-shot pulse craters using scanning force microscopy. The contribution of filamentation to the ablation process is shown quantitatively.

Morphometric Study of Craters on Saturn’s Moon Rhea

Morphometric studies of impact craters on icy moons can be used to understand modification of crater topography. Several processes (e.g., viscous relaxation, ejecta deposition, repeated and overlapping impacts) act to shallow crater depth and relax the crater wall slope to similar or varying extents. Resolving these processes can help constrain the interior structure and surface properties of icy moons. Here, using morphometric measurements of craters on Rhea, we aim to constrain the processes that led to the observed crater population. We measured crater diameter, depth, and wall slope, as well as overall crater morphology (e.g., simple versus complex craters). Our results indicate that there exists a linear correlation between impact crater depth-to-diameter ratio and crater wall slope. This may suggest that the dominant modification process on Rhea is one that affects both properties simultaneously, which supports past heating events as the primary post-impact modification process. Additionally, the simple-to-complex crater transition for Rhea was found to be 12 ± 2 km, which is consistent with reported transition diameters for comparably sized icy bodies, indicating similar surface properties. A transition to shallower crater depths for large complex craters was not documented, indicating the absence of a rheological transition at depth in Rhea’s icy lithosphere, which may support the interpretation that Rhea is not fully differentiated.

Near-Infrared Femtosecond Laser Ablation of Au-Coated Ni: Effect of Organic Fluids and Water on Crater Morphology, Ablation Efficiency and Hydrodynamic Properties of NiAu Nanoparticles

Scanning electron microscopy (SEM) and profilometry of the crater morphology and ablation efficiency upon femtosecond laser ablation of Au-coated Ni targets in various fluids revealed a pronounced dependence on the ablation medium. For ethanol, a sufficient ablation efficiency was obtained, whereas for 2-butanol a higher efficiency indicated stronger laser–target interaction. Hierarchical features in the crater periphery pointed to asymmetrical energy deposition or a residual effect of the Coulomb-explosion-initiating ablation. Significant beam deviation in 2-butanol caused maximum multiple scattering at the crater bottom. The highest values of microstrain and increased grain size, obtained from Williamson–Hall plots, indicated the superposition of mechanical stress, defect formation and propagation of fatigue cracks in the crater circumference. For n-hexane, deposition of frozen droplets in the outer crater region suggested a femtosecond-laser-induced phase explosion. A maximum ablation depth occurred in water, likely due to its high cooling efficiency. Grazing incidence micro X-ray diffraction (GIXRD) of the used target showed residual carbon and partial surface oxidation. The produced nanoparticle colloids were examined by multiangle dynamic light scattering (DLS), employing larger scattering angles for higher sensitivity toward smaller nanoparticles. The smallest nanoparticles were obtained in 2-butanol and ethanol. In n-hexane, floating carbon flakes originated from femtosecond-laser-induced solvent decomposition.

Anomalous Dielectric Variability in the central peak of Tycho crater on the Moon: New insights from Mini-RF S-band Observations

<p>One of the unique candidates to explore the evolution of physical surface processes on the Moon is Tycho, a dark haloed impact crater representing well-preserved bright ray pattern and intact crater morphology. Sampling of the central peak in such complex crater formation proves significant in terms of unraveling intriguing science of the lunar interior. With the current state-of-the-art radar technology, it is possible to evaluate the response of the geologic features constrained in the near surface and subsurface regolith environments. This can be achieved by modelling the dielectric constant of media, which is a physical parameter crucial for furthering our knowledge about the distribution of materials within different stratigraphic layers at multiple depths. Here, we used the applicability of Mini-RF S-band data augmented with a deep learning based inversion model to retrieve the dielectric variations over the central peak of the Tycho crater. A striking observation is made in certain regions of the central peak, wherein we observe anomalously high dielectric constant, not at all differentiated in the hyperspectral image and first Stokes parameter image, which usually is a representation of retrieved backscatter of the target. The results are also supported by comparing the variations in the scattering mechanisms. We found those particular regions to be associated with high degree of depolarization, thereby attributing to the presence of cm- to m- scale scatterers buried within a low dielectric layer that are not big enough to produce even-bounce geometry for the radar wave. Moreover, we also observe high rock concentration in the central peak slopes from DIVINER data and NAC images, indicating the exposure of clasts ranging in size from 10 meter to 100s of meter. Furthermore, from surface temperature data, these distinctive outcrops sense warmer temperature at night than the surrounding, which suggests the existence of thermal skin depth in such vicinities. Interestingly, we are able to quantify the pessimistic dielectric constant limit of the large boulder in the middle of the central peak, observable at the Mini-RF radar wavelength, as 4.54 + j0.077. Compared to the expected dielectric constant of rocks, this value is lowered significantly. One probable reason could be the emergence of small radar shadows due to the rugged surface of the boulder on the radar illuminated portion. From our analysis, we showcase the anomalous dielectric variability of Tycho central peak, thereby providing new insights into the evolution of the impact cratering process that could be important for both science and necessary for framing human or robotic exploration strategies.  </p>