Photochemical Ablation Modeling

Photochemical Ablation

This study is one of our very active projects and one of our long standing interests dating back to 1983 and our collaboration with R. Srinivasan...Barbara

Coarse Grained Chemical Reaction Model (CGCRM)

We have developed a methodology for including effects of chemical reactions in coarse-grained computer simulations such as those that use the united atom approximation. The new coarse-grained chemical reaction model (CGCRM) adopts the philosophy of kinetic Monte Carlos approaches and includes a probabilistic element to predicting when reactions occur, thus obviating the need for a chemically correct interaction potential. The CGCRM uses known chemical reactions along with their probabilities and exothermicities for a specific material in order to assess the effect of chemical reactions on a physical process of interest. The reaction event in the simulation is implemented by removing the reactant molecules from the simulation and replacing them with product molecules. The position of the product molecules is carefully adjusted to make sure that the total energy change of the system corresponds to the reaction exothermicity. The CGCR model has been applied to simulations of laser irradiation of chlorobenzene at fluences such that there is ablation or massive removal of material. In the photochemical simulation, the photon cleaves the C-Cl bond creating two radicals that can undergo subsequent abstraction and radical-radical recombination reactions. An overview of the CGCRM is in Ref. 249.

Photochemical fragmentation mechanism

In this simulation the breathing sphere model is enhanced allowing the photon absorption event to break a bond in the molecule and then have subsequent abstraction and recombination reactions. The initial system to model is chlorobenzene which is chosen because of simplicity of its fragmentation. Results of the simulations are in a good qualitative agreement with experimental data on the ejection of photoproducts in the laser ablation of chlorobenzene. The difference in the temporal and spatial deposition of energy available for the ablation physics in photochemical vs photothermal ablation is the new concept to arise from these calculations. This concept provides the foundation to make specific comparisons with experiment and to explain experimental results as summarized below and as explained previously in more detail. Refs. 205, 213, 234 & 238. Some results are given in the reviews Refs. 218 & 235.

Photochemical reactions are found to release additional energy into the irradiated sample and decrease the average cohesive energy, therefore decreasing the value of the ablation threshold. The yield of emitted fragments becomes significant only above the ablation threshold.
The fast rise of energy deposition in time from 20-150 ps explains the presence of a shockwave with a high initial velocity, large clusters in the plume, and high velocities of particles in the plume.
The on-going chemical reactions above the surface after the laser pulse explain the higher background density in the plume with photochemical ablation than with photothermal ablation.
The association of a thermal mechanism of material removal below the ablation threshold and a volume ejection mechanism above threshold explains why non-volatile products like HCl and the matrix are only observed below threshold and all products are observed above threshold.
The lack of heat deposited below ~ 1.5 times the penetration depth may help explain the cold etching process in far UV photoablation as is used commercially in the corrective eye surgery, LASIK.

Publications


249.	Coarse Grained Chemical Reaction Model, Y. G. Yingling and B.J. Garrison, J. Phys. Chem. B, Feature Article, 108, 1815-1821 (2004).[full article - PDF]
238.	Photochemical ablation of organic solids, Y. G. Yingling and B.J. Garrison, Nucl. Instrum. Methods B, 203, 188-194 (2003).[full article - PDF]
235.	Computer Simulation of Laser Ablation of Molecular Substrates, L. V. Zhigilei, E. Leveugle, B. J. Garrison, Y. G. Yingling, and M. I. Zeifman, Chemical Reviews, 103, 321-348 (2003).[full article - PDF]
234.	Photochemical induced effects in material ejection in laser ablation, Y. G. Yingling and B. J. Garrison, Chem. Phys. Lett., 364, 237-243 (2002).[full article - PDF]
218.	MD Simulations of MALDesorption - Connections to Experiment, L. V. Zhigilei, Y. G. Yingling, T. E. Itina, T. A. Schoolcraft and B. J. Garrison, Int. J. Mass Spectrom. Ion Processes, Franz Hillenkamp, special issue, 226, 85-106 (2003).[full article - PDF]
213.	The Role of the Photochemical Fragmentation in Laser Ablation, Y. G. Yingling, L. V. Zhigilei and B. J. Garrison, J. Photochemistry and Photobiology A: Chemistry, 145, 173-181 (2001).[full article - PDF]
205.	Photochemical Fragmentation Processes in Laser Ablation of Organic Solids, Y. G. Yingling, L. V. Zhigilei, and B. J. Garrison, Nucl. Instrum. Methods Phys. Research B, 180, 171-175 (2001).[full article - PDF]
62.	Ablative Photodecomposition of Polymers, B. J. Garrison and R. Srinivasan, J. Vac. Sci. Technol. A3, 746-748 (1985).[full article - PDF]
60.	Laser Ablation of Organic Polymers: Microscopic Models for Photochemical and Thermal Processes, B. J. Garrison and R. Srinivasan, J. Appl. Phys. 57, 2909-2914 (1985).[full article - PDF]
52.	A Microscopic Model for the Ablative Photodecomposition of Polymers by Far-Ultraviolet Radiation (193 nm), B. J. Garrison and R. Srinivasan, Appl. Phys. Lett. 44, 849-851 (1984).[full article - PDF]

Barbara J. Garrison - December 23, 2003