טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentChen Yuheng
SubjectDynamic and Kinetic Study of Laser-Induced Aerosol by
Spectrometric Methods
DepartmentDepartment of Aerospace Engineering
Supervisors Professor Israel Schechter
Professor Josef Stricker
Dr. Liviu Singher
Full Thesis textFull thesis text - English Version


Abstract

In laser ablation procedures a high-power laser beam is focused on a solid target, resulting in breakdown on its surface. Plasma plume is generated and a small amount of material is ejected. Generally, the plume consists of vapors and tiny particles or aerosols of the target material. This study is focused of the dynamics and kinetics of these laser induced aerosols (LIA).  It is important for optimizing laser ablation processes, such as pulse-laser deposition (PLD), direct sampling of solid state material, nanometer size particle synthesis and laser machining.

The temporal evolution and spatial distribution of LIA of a multi-component target were investigated. The combination of laser-induced breakdown spectroscopy (LIBS) and cavity ring-down spectroscopy (CRDS) was applied to study the dynamics and kinetics of LIA.

A new application of LIBS was developed for elemental analysis of LIA in real time. The system consisted of two synchronized pulsed Nd:YAG lasers¾the first laser beam was focused on the target to generate a plume and the second laser was focused on the plume and generated a second spark. The composition of LIA was determined by the emission of the second spark. The temporally and spatially resolved information were obtained by changing the time delay between two pulses and the focal point of the second beam. By this technique, the elemental mapping of LIA was obtained.

The dynamics at the early stages of the plasma plume could not be studied by LIBS, since the concentrations are too low and the particulate size is too small. Therefore, we applied the high sensitivity feature of CRDS. The laser-induced plume was located in the light path of the cavity. Extinction of the plume was determined by measuring the decay of the light in the cavity. The extinction was mainly contributed by the absorption and scattering of the LIA. The time-resolved and spatially-resolved features of LIA density were determined using this method.

In addition, CRDS technique was applied for detecting transparent and absorbing airborne particulate. The extinction of the absorbing aerosol as function of wavelength was determined. A new method, based on the fluctuation analysis of the CRDS signals, was developed and applied for evaluating the absolute number concentration and refractive index of transparent aerosol.