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DTSTAMP:20260414T164138
DTSTART:20150508T100000
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URL:https://murmitoyen.com/events/vanille/udem/detail/601969
LOCATION:Université de Montréal - Pavillon Roger-Gaudry\, 2900\, chemin d
 e la Tour\, Montréal\, QC\, Canada\, H3T 1J6
SUMMARY:Soutenance de doctorat de Mahmoud Dawood
DESCRIPTION:Caractérisation spatio- temporelle de plasmas induits par lase
 r pour des applications à la chimie analytique et au dépôt de couches m
 inces Space and time characterization of laser-induced plasmas for applic
 ations in chemical analysis and thin film deposition After decades of dev
 elopment\, laser ablation has become an important technique for a large nu
 mber of applications such as thin film deposition\, nanoparticle synthesis
 \, micromachining\, chemical analysis\, etc. Experimental and theoretical 
 studies have been conducted to understand the physical mechanisms of the l
 aser ablation processes and their dependence on the laser wavelength\, pul
 se duration\, ambient gas and target material. The present dissertation de
 scribes and investigates the relative importance of the physical mechanism
 s influencing the characteristics of aluminum laser-induced plasmas. The g
 eneral scope of this research encompasses a thorough study of the interpla
 y between the plasma plume dynamics and the ambient gas in which they expa
 nd. This is achieved by imaging and analyzing the temporal and spatial evo
 lution the plume in terms of spectral intensity\, electron density and exc
 itation temperature within various environments extending from vacuum (10
 ‾7 Torr) to atmospheric pressure (760 Torr)\, in an inert gas like Ar an
 d He\, as well as in a chemically active gas like N2. Our results show tha
 t the plasma emission intensity generally differs with the nature of the a
 mbient gas and it is strongly affected by its pressure. In addition\, for 
 a given time delay after the laser pulse\, both electron density and plasm
 a temperature increase with the ambient gas pressure\, which is attributed
  to plasma confinement. Moreover\, the highest electron density is observe
 d close to the target surface\, where the laser is focused and it decrease
 s by moving away (radially and axially) from this position. In contrast wi
 th the significant axial variation of plasma temperature\, there is no lar
 ge variation in the radial direction. Furthermore\, argon was found to pro
 duce the highest plasma density and temperature\, and helium the lowest\, 
 while nitrogen yields intermediate values. This is mainly due to their phy
 sical and chemical properties such as the mass\, the excitation and ioniza
 tion levels\, the thermal conductivity and the chemical reactivity. The ex
 pansion of the plasma plume is studied by time- and space-resolved imaging
 . The results show that the ambient gas does not appreciably affect plume 
 dynamics as long as the gas pressure remains below 20 Torr and the time de
 lay below 200 ns. However\, for pressures higher than 20 Torr\, the effect
  of the ambient gas becomes important and the shorter plasma plume length 
 corresponds to the highest gas mass species and the lowest thermal conduct
 ivity. These results are confirmed by Time-Of-Flight (TOF) measurements of
  Al+ line emitted at 281.6 nm. Moreover\, the velocity of aluminum ions is
  well defined at the earliest time and close to the target surface. Howeve
 r\, at later times\, the ions travel through the plume and become thermali
 zed through collisions with plasma species and with surrounding ambient ga
 s.
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