Photochemical Processes in Laser Ablation of Organic Solids: Molecular Dynamics Study.

Open Access
Author:
Yingling, Yaroslava G.
Graduate Program:
Materials
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
October 22, 2002
Committee Members:
  • Barbara Shaw, Committee Member
  • Mark Maroncelli, Committee Member
  • Kristen Ann Fichthorn, Committee Member
  • Jayanth R Banavar, Committee Chair
  • Barbara Jane Garrison, Committee Chair
Keywords:
  • molecular dynamics
  • processes
  • laser ablation
  • simulations
  • photodecomposition
  • mechanisms
Abstract:
Ultraviolet (UV) laser ablation has been investigated because of the significance of its current and potential applications. Interaction of UV laser light with material can induce vibrational excitation or/and photochemical decomposition of molecules. These two processes are interrelated with each other in UV ablation, therefore, their relative contribution has been a subject of numerous studies. Despite numerous studies, the mechanistic understanding of the role of the photochemical processes and their coupling with the photothermal processes in UV laser ablation is still far from being complete. In this thesis, a comprehensive study of the effect of the photochemical processes on laser ablation mechanisms has been conducted using molecular dynamics (MD) simulations. In order to provide an adequate description of the diverse processes induced by laser irradiation, a computational model has been developed. In particular, we developed a new concept for modeling photochemical processes in laser ablation of organic films using a mesoscopic coarse-grain breathing sphere model for MD simulations. The main advantage of our model is the ability to study the dynamics of the system at the mesoscopic length scale, a regime that is not accessible either with atomistic or continuum computational methods. The photodecomposition of the excited molecules and the chemical reaction patterns in our simulations are based on the photochemistry of chlorobenzene due to ease of its fragmentation and available experimental data. Interpretation of the experimental data is the main objective of our theoretical efforts. MD simulations technique has been applied to investigate the fundamental aspects of laser ablation and the effect of photochemical processes on material ejection mechanisms in 248-nm laser irradiation of organic solids. Among the characteristics that can be related to the experimental results, we observed that the presence of photochemical decomposition processes and subsequent chemical reactions lowers the ablation threshold, intensifies the pressure wave propagation and temperature in the absorbing region, causes the ejection of massive molecular clusters and thus escalates the molecular yield, and increases the density of the plume over pure photothermal ablation. Our MD simulations results are in good agreement with experiment and provided microscopic perspective of photochemical processes in laser ablation to experimental investigations.