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Title: Pulsed laser ablation and laser-induced plasma diagnosis by optical emission spectroscopy
Author: Hao, Liu
ISNI:       0000 0004 5994 8832
Awarding Body: University of Bristol
Current Institution: University of Bristol
Date of Award: 2016
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Fundamental aspects of the laser ablation of solid targets using nanosecond pulses with irradiances in the range ≈ 1-30 OW /cm2 have been investigated theoretically, modelled using a simulation code, and explored experimentally by time-gated, spatially and spectrally resolved imaging of the optical emission that accompanies the ablation plume. During the earliest stages of the laser-target interaction, the target surface temperature rises rapidly and electrons are ejected by both thermionic and photo-emission. These electrons absorb energy from the · laser pulse by strong electron-photon coupling, leading to formation of an embryonic plasma which continues to gain energy via photoionization and, particularly, inverse bremsstrahlung (IB) absorption. The target is further heated by the hot plasma and may reach temperatures of several thousand Kelvin, approaching its critical point, and undergo an explosive phase transition to the supercritical fluid. Such a super-hot target surface emits an abundance of electrons, neutrals and ions. The present work shows that the electron yield following 532 nm PLA of a Si target is significantly higher than that produced using 1064 nm pulses of same irradiance. The fastest electrons emerge at the leading edge of the plasma plume, creating an electric field gradient with respect to the net positively charged body of the plume. This has the effect of accelerating ions in the plume, to extents that ·depend on their charge state. Emissions from any given charge state show identical spatial distributions in the time-gated images. The most highly charged ions (e.g. SiIV ions in the case of PLA of Si in vacuum) exhibit velocities of ~ 1 00 km/s. Experiment and simulation show that plasma produced by 1064 nm excitation is hotter and expands faster than that formed by 532 nm PLA - a result that can be attributed to the stronger IB absorption of the longer wavelength radiation. A new and improved method for determining local electron densities (Ne) and temperatures (Te) in laser induced plasmas is introduced. The model relies on fitting Stark broadened line shapes but, in contrast to most rival approaches, makes no preassumption regarding local thermodynamic equilibrium. The method is used to determine temporally and spatially evolving Ne and Te distributions in plumes arising from PLA of Si and SiC targets, as functions of irradiance, excitation wavelength and ambient pressure. We also demonstrate how Ne and Te values determined by fitting lines associated with one spectral carrier allow derivation of (hitherto unknown) Stark parameters for transitions involving other carriers. A clear shock front is observed following PLA of Si in background Ar pressures ~0.2 Torr, even as early as 40 ns. Strong collisions between highly charged ions in the plume and the surround gas are seen to introduce another level of complexity (e.g. ionization, recombination, charge transfer, etc.) within and beyond the shock front in the plume.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available