Microscopic mechanisms of laser ablation of organic solids in the thermal and stress confinement irradiation regimes

Abstract

The results of large-scale molecular dynamics simulations demonstrate that the mechanisms responsible for material ejection as well as most of the parameters of the ejection process have a strong dependence on the rate of the laser energy deposition. For longer laser pulses, in the regime of thermal confinement, a phase explosion of the overheated material is responsible for the collective material ejection at laser fluences above the ablation threshold. This phase explosion leads to a homogeneous decomposition of the expanding plume into a mixture of liquid droplets and gas phase molecules. The decomposition proceeds through the formation of a transient structure of interconnected liquid clusters and individual molecules and leads to the fast cooling of the ejected plume. For shorter laser pulses, in the regime of stress confinement, a lower threshold fluence for the onset of ablation is observed and attributed to photomechanical effects driven by the relaxation of the laser-induced pressure. Larger and more numerous clusters with higher ejection velocities are produced in the regime of stress confinement as compared to the regime of thermal confinement. For monomer molecules, the ejection in the stress confinement regime results in broader velocity distributions in the direction normal to the irradiated surface, higher maximum velocities, and stronger forward peaking of the angular distributions. The acoustic waves propagating from the absorption region are much stronger in the regime of stress confinement and the wave profiles can be related to the ejection mechanisms.

Measurements of mean initial velocities of analyte and matrix ions in infrared matrix-assisted laser desorption ionization mass spectrometry

“…Depending on the geometry of the ion source, refocusing of the beam by the ion lens, if present, and the detector size, only the most axially-pointed fraction of the ions will be recorded. This center portion will, on average, contain ions with higher (axial) velocities [19,23,35]. For the experimental conditions used in this study such a discrimination has indeed been substantiated by a SIMION simulation.…”

“…Recent MD simulations by Zhigilei and Garrison can also help to solve the open questions [35]. These simulations revealed comparably high center of mass velocities for both simulated UV and approximated IR MALDI conditions.…”

“…Finally, a laser induced fluorescence (LIF) technique has been employed to characterize UV MALDI plume dynamics [27]. The experimental studies have been supplemented by a number of theoretical studies [30 -32] and more recently, by molecular dynamics simulations (MD) [33][34][35][36][37].…”

The analysis of polystyrene and polystyrene aggregates into the mega dalton mass range by cryodetection MALDI TOF MS

“…Such a high rate of sample removal was not observed with the other matrix compounds in our studies. This observation suggests that an exceptionally high ablation efficiency [46] is required to observe a MALDI signal from these heavy polymers.…”

The influence of organobentonite clay on CO2 laser grooving of nylon 6 composites

“…Vibrational excitation can occur both with infrared IR as well as with ultraviolet UV irradiation. In addition, the ultraviolet can also photo-fragment the molecule when the laser energy is expended in breaking the chemical bonds [1]. In both cases, the ablation starts when the density of broken bonds at the surface reaches a certain critical value.…”