Comparison of the Ablation Behavior of Polymer Films in the IR and UV with Nanosecond and Picosecond Pulses

Ultrafast photofragment ion spectroscopy of the Wolff rearrangement in 5-diazo Meldrum's acid

We investigate the gas-phase photochemistry of 5-diazo Meldrum's acid (DMA), a photoactive compound used in lithography, by femtosecond photofragment ion spectroscopy. Transient-absorption studies in solution had revealed an ultrafast intramolecular Wolff rearrangement to a ketene after UV excitation, followed by reactions which also involve the solvent. Due to the absence of solvent molecules in this gas-phase study, we are able to focus purely on the photochemistry of the Wolff rearrangement and subsequent reaction steps. The observation of the time-resolved photofragment ion signals allows us to discriminate the dynamics of ketene and carbene products. By identification of the different possible molecular origins for a certain fragment ion signal, the time scale of the Wolff rearrangement and the lifetime of the ketene product are inferred. We further identified experimental signatures of a second Wolff rearrangement emanating from the carbene product, as had been conjectured indirectly for this molecule from pyrolysis studies.

Coarse-Grained Model of the Interaction of Light with Polymeric Material: Onset of Ablation

A coarse-grained model has been developed for molecular dynamics simulations of the interaction of light with polymeric materials. The photon energy can result in a vibrational excitation (photothermal process) or disruption of a chemical bond (photochemical process) in a polymer. In the latter case, the formation of active radial sites and the occurrence of chemical reactions have to be taken into consideration. The novel feature of this model is the incorporation of chemical reactions into the united atom approximate representation of the polymer structure, which permits the study of laser ablation, degradation, or the effect of various chemical reactions on large time and length scales. The chemical reactions are included in the model in a probabilistic manner as in the kinetic Monte Carlo method. This model adopts physically and experimentally known quantities such as enthalpies and probabilities of reactions. Properties such as laser irradiation time, laser fluence, and wavelength are explicitly included. Moreover, no chemically correct interaction potential is required to incorporate the effects of chemical reactions on the dynamics of the system after energy deposition. We find that the model provides a plausible description of the essential processes. The laser-induced pressure relaxation is the main mechanism responsible for the onset of polymer ablation. Since the pressure relaxation processes are slow, there is a delay in the onset of ablation after the end of the laser pulse as is observed experimentally. The vaporization processes are not efficient for material removal, and the effect is minimal for both photochemical and photothermal processes. A lower fluence is needed for the onset of ablation with photochemical processes than photothermal processes.