An Insight Into the Expansion of Laser Induced Plasmas

Abstract

This thesis explores the expansion dynamics of multicomponent laser-induced plasmas as a study of the ‘stoichiometry issues’ of pulsed laser deposition. Two techniques have been applied for the characterization of the plume expansion: energy resolved mass spectrometry and spectrally resolved plasma imaging. Both techniques provide element-specific information and in combination give an overall picture of the plume expansion. Energy resolved mass spectrometry characterizes the plasma plume with respect to the ion energy distributions. Two types of energy distributions have been found from the ablation of different targets: Type I, a low energy peak plus an energetic tail, and Type II, a low energy peak plus a high energy peak. Additionally, a correlation between the maximum kinetic energy and the ion mass has been found: in a multicomponent plume, the larger the mass of the ion, the higher its maximum kinetic energy. Alternatively, when the maximum kinetic energy is calculated to velocity, the correlation to the mass is reversed: the larger the mass, the slower the velocity. Even though the ion energy varies for all angular directions with respect to the target surface normal, the above correlations are valid. A dynamic double layer model is proposed to describe the expansion of ions in the plume. The double layer is formed at the front of the plasma due to the absorption of the laser light in the plasma vapor. Ions in the double layer are accelerated by the electric field generated by the charge separation. The light and heavy ions, due to the different masses and thus the different acceleration rates, separate during the acceleration / expansion. The light ions with a large acceleration rate move to the front of the double layer or cross the double layer, where the electric field is low or zero, and thus the acceleration stops. Contrarily, the heavy ions stay in the double layer for a longer time and receive more energy from the double layer acceleration. This gives a physical explanation for the observed correlation of the ion mass to its maximum energy as well as its maximum velocity. This model is further developed in three dimensions with the assumption of a hemispherical ‘electron front’ and a planar plasma core. With this geometry, the double layer is inhomogeneous, which is ‘thicker’ in the target surface normal direction, and ‘thinner’ in the target surface parallel direction. Therefore, in target surface normal direction, the ion acceleration takes a longer time and the ions gain more energy, while in the direction off the target surface normal, the ions can cross the double more easily and the acceleration stops earlier. This results in an anisotropic acceleration in the target surface normal direction. The proposed double layer model can also provide explanations for the influences of the laser wavelength and fluence on the ion energy distributions. When the laser wavelength is changed from 248 nm to 308 nm, the ion energy is reduced almost by half and the angular energy distribution becomes more isotropic. With increasing fluence, the ion energy increases in a logarithmic manner and the angular energy distribution gets broader. These changes can be explained via evaluating the dimensional changes of the electron layer and the plasma core caused by the changes in the laser light absorption in the plume as well as in the ablation rate. Complementarily, the plume is characterized by spectrally resolved plasma imaging for the expansion of excited neutrals. The characterization shows that the expansion of neutrals resembles a gas cloud with a flow velocity normal to the target surface. This feature fits to the description of the plume expansion with a Knudsen layer formation. Additionally, different species have different expansion velocities which are constant in the measurement time regime. An estimation of the plume mean free path with the plume dimension from imaging reveals that the expansion is collision free. Combining the two techniques, an overall picture of the plume expansion can be obtained. The ions are accelerated by the double layer and expand radially from the laser spot, while the neutrals are accelerated by the gas dynamics and expand as a gas cloud with a flow velocity. Additionally, the expansion of the plume in O2 background gas environment is also characterized. The energy analysis of metal-oxide ions (MO+) reveals that the MO+ ions only have very low energies, regardless of the O2 pressure and metallic element. It seems that the MO species are formed preferentially by the low energy metallic species with the O2 background molecules. Nevertheless, the kinetics of the reaction needs further investigations.