Poly͑methyl methacrylate͒ ͑PMMA͒ is highly resistant to laser ablation at 308 nm. Either very high fluences or absorbing dopants must be used to ablate PMMA efficiently at this wavelength. We investigate two dopants, pyrene and a common solvent, chlorobenzene, using time-of-flight mass spectroscopy. Both compounds improve the ablation characteristics of PMMA. For both dopants, the first step in ablation is an incubation process, in which absorption at 308 nm increases due to the production of CvC bonds along the polymer backbone. Incubation at 308 nm is similar to that observed for shorter ultraviolet wavelengths in previous studies. The principal ablation products and their corresponding temperatures are consistent with a photothermal ablation mechanism.
We present observations of submicrometer- to micrometer-sized particles generated by high-fluence (≥10 J/cm2) 248-nm laser ablation of single-crystal NaNO3 in vacuum and at atmospheric pressure. Small particles (50–200 nm in diameter) are ejected by hydrodynamic sputtering. Larger particles (1–20 μm in diameter) are produced by cavitation and spallation in the melt. Many particles formed in air carry electric charge, with roughly equal numbers of positively and negatively charged particles. The particle composition is consistent with substantial nitrate decomposition. The implications of these observations with respect to laser-based chemical analysis are discussed.
The emission of charged and neutral particles from single-crystal MgO irradiated with pulsed 248 nm excimer laser light is studied by means of quadrupole mass spectrometry, time-resolved emission spectroscopy, luminescence spectroscopy, and scanning electron microscopy (SEM) observations. The role of the initial distribution of near-surface defects plus defects which result from repeated application of laser pulses is explored. This increase in defect density eventually leads to formation of a visible plume and rapid material vaporization. SEM observations after irradiation indicate that substantial surface fracture is present prior to the onset of rapid vaporization. Defect production during irradiation is attributed to mechanical processes involving deformation and fracture with accompanying dislocation motion. The accumulation of these defects increases laser absorption in the near-surface region resulting in rapid thermal etching and cluster emission.
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