A microsecond time-resolved spectroscopic study of laser induced plasmas and their interaction with solid materials

Abstract

Photon-material interactions are driven by fast chemical and physical phenomena. With enough incident energy, photons are capable of inducing photoionization processes converting the solid material to a plasma. The highly energetic resulting electrons and ionic species undergo rapid transformation with the underlying material and themselves in the nano-second to micro-second time scale before the decay back to a gaseous state occurs. These actions drive the many interesting interactions that take place in a laser induced plasma event. Study of the plasma emission can give insight into the mechanisms of interaction, and the resulting samples after irradiation can provide information on how the plasma formed. Chapter 1 gives an overview of the current state of energetics materials with a history of the current methods and techniques of initiation and testing. The fundamental operation of lasers is discussed, along with their interaction mechanisms with materials. An overview of the resulting analytical techniques to study laser induced plasmas is given. Chapter 2 discusses the experimental processes employed during each experiment. A description of sample preparation is given. The design and construction of the driving electronics, optical set-up, and spectroscopy are detailed. The custom programming for data collection and analysis is discussed. Chapter 3 discusses the time-resolved spectroscopic study of the laser induced plasma and the decay process that was evident from the resulting spectra. The plasma decay process as observed is believed to be that of a phase transition from a plasma to a gaseous state. Chapter 4 details the mechanisms of material removal from the molecular solid RDX. Experiments were done at various lens to sample distances to understand how the fluence influenced the plasma plume formation. The results are interpreted in terms of four different mechanisms of material removal dependent upon the laser fluence. Chapter 5 is a study of the laser ablation of polycarbonate covered RDX samples. The mechanisms and amounts of material removal in PC and RDX differ at constant laser fluence. However, when coating the RDX samples in PC, the increase in depth of material removal is 8x that of RDX alone. These results suggested that the mechanism of removal involves a chemical process driven by the PC laser induced plasma interaction with the underlying RDX material. Chapter 6 gives an overview of the cumulative understanding of the laser induced plasma plume formation process as related to these studies. There are fundamental processes that are unique to these laser material interactions and they are discussed. Suggested future work is outlined.