Based on experiments and a theoretical analysis, we raise questions on two fundamental mechanisms of femtosecond
laser desorption/ablation of solids, namely Coulomb explosion (CE) and plasma etching. The effects of laser-induced
ionization and surface charging are analyzed which can be responsible for ultrafast ions observed in time-of-flight mass-spectra
under ultrashort laser irradiation of solids. The importance of surface charging in formation of velocity
distributions of desorbed/ablated species has been revealed for conditions when the CE mechanism is inhibited. The
influence of ambient plasma formation on the dynamics of heating of metallic targets by femtosecond laser pulses is
studied based on 2D modeling of laser-induced target heating and dynamics of the ambient plasma. The calculations
show an intriguing picture of the laser-induced ambient gas motion. We propose a model of laser-induced breakdown of
an ambient gas in a region in front of the irradiated target and analyze plasma-chemical processes which can affect laser
processing of surfaces in the presence of air or highly reactive media.
The ionization/recombination processes during the expansion of laser ablation products into a vacuum and into an ambient gas are investigated on the basis of gas dynamic model. The plasma is assumed to be heated immediately by laser irradiation up to the high temperature, and to reach the ionization equilibrium. The laser energy is estimated to be spent on the vaporization, dissociation and heating of an ablated material and on the ionization of evaporated particles. The plasma expansion is described in two-temperature approximation by the Euler equations. The model is used to analyze the laser ablation of YBaCuO superconductor in oxygen atmosphere under actual film deposition conditions. To understand the role of ionization, we compare the numerical results with one's computed ignoring the ionization processes and with the time-of-flight data. The computations show that only a small part of ionization energy goes away from the cloud with the radiation. The main part is converted into translation energy of expanding products at the initial stage of the expansion. As a result, the kinetic energy of the plasma is approximately doubled.
The expansion of laser-induced plume into an ambient gas under typical thin film deposition conditions is investigated. A simplified theoretical model has been developed to understand the dynamics of plume-ambient gas interaction under the gas pressure of typically a few tens Pa. The model is based upon the generation of a high-temperature and high-pressure plasma cloud which is initially confined to a sphere of irradiated spot radius and is then suddenly allowed to expand into a gas. The expansion is governed by the Euler system of nonstationary equations. The model has been applied to investigate the dynamics of laser ablation of YBaCuO in oxygen. Numerical results show that the series of density jumps following one after another are formed in the plume. The origin of these pulsations is attributed to the repeated reflections of the secondary shock wave due to the effect of plume overexpansion. Using the calculated data, the time-of-flight signal has been simulated to compare the numerical results with available experimental data. Surprisingly good quantitative agreement has been achieved.
The gas-phase processes relevant for oxidation during Nd:YAG laser ablation of YBaCuO superconductor in vacuum and in an oxygen environment are investigated by mass spectrometry used in conjunction with molecular beam technique. The dynamics of monoxides YO and BaO as well as small clusters formation are analyzed. To separate the processes involving ablated and ambient oxygen, argon ambient atmosphere is also used. In vacuum, the oxidation is caused predominantly by interaction of low velocity portions of ablated Y and Ba atoms with the ablated oxygen which is mostly realized, at the laser fluences used (3-5 J/cm2), in atomic form. In oxygen background, efficient formation of oxides and oxygen-contained clusters is observed at all expansions stages. The pressure dependence of the atomic and oxide fluxes for the high velocity species has been described as an attenuation of molecular beam due to reactive and elastic scattering, and thus the cross section for oxidation reactions have been estimated. Hydrodynamic effects are found to have a significant influence on the oxidation processes within the plume. In argon background, oxide fractions decrease with the pressure due to dilution of the ablated oxygen flux.
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