Expansion of the laser ablation vapor plume in to a background gas: Part I: Analysis

A study of the gas dynamics of the vapor plume generated during laser ablation was conducted including a counterpropagating internal shock wave. The density, pressure, and temperature distributions between the external shock wave front and the sample surface were determined by solving the integrated conservation equations of mass, momentum, and energy. The positions of the shock waves and the contact surface (boundary that separates the compressed ambient gas and the vapor plume) were obtained when the incident laser energy that is transferred to the vapor plume and to the background gas, E, and the vaporized sample mass, M, are specified. The values for E and M were obtained from a comparison of the calculated trajectories of the external shock wave and the contact surface with experimental results for a copper sample under different laser fluences. Thus E and M, which are the two dominant parameters for laser ablation and which cannot be measured directly, can be determined. In addition, the internal shock wave propagation within the vapor plume was determined; the interaction of the internal shock wave with the sample may be one of the mechanisms inducing liquid sample ejection during laser ablation.