Time-resolved imaging and electron diffraction of laser-driven plumes

Abstract

Pulsed laser ablation of liquids, in which laser pulses are used to remove material, has applications in a wide range of fields, including lithography, printing, surgery, and chemical analysis. The dynamics that unfold after the interaction between the ablation laser and the liquid are complex and the subject of extensive research. This thesis first describes the design and use of a brightfield imaging setup to study pulsed laser ablation of vacuum stable liquids at different fluence and external pressure ranges. The plumes generated after laser ablation in a vacuum exhibit a novel bubble formation. The behavior of these bubbles is studied and analyzed. Equilibrium viscosity and surface tension fail to explain the bubble growth and rupture dynamics. Our analysis identifies shear elasticity, not typically associated with liquids, as the driving force for this surface flow in liquid glycerol under laser ablation. We determine Young's modulus of about 0.2 MPa for the laser-driven bubbles, which is a large rubber-like value. In addition, by analyzing the rupture of the bubble and the speed of the retracting film, we again find a similarly large shear modulus of about 0.48 MPa. These values provide evidence that the glycerol shell exhibits solid-like behavior with similar elasticity values and tolerance to large deformation strains that are typically only observed in materials such as rubber. Glycerol is a well-known Newtonian fluid. When subjected to deformation, Newtonian fluids are expected to exhibit viscous behavior. Typically, a solid-like elastic response is only observed when Newtonian fluids are deformed and probed on very short time scales, shorter than the nanosecond molecular diffusion time of a single molecule. However, our results show that the elasticity of this fluid persists for much longer, extending four orders of magnitude beyond the molecular diffusion time. Our results imply the presence of a metastable state characterized by solid-like long-range correlations in the liquid. This invites a re-evaluation of our current understanding of the liquid state. The laser-ablated plumes are then studied using time-resolved electron diffraction. Laser ablation of liquids coupled with electron diffraction allowed the observation of radial distribution functions of two distinct phases of glycerol: gas and liquid. To our knowledge, this is the first time that a single sample source has successfully produced isolated and condensed phases for study by electron diffraction.