Investigation by laser-induced fluorescence of surface vaporization during the pulsed CO2 laser irradiation of a titanium sample in an ambient gas

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Abstract
This work deals with the study of laser‐induced surface vaporization in the presence of an ambient gas, in the conditions where a plasma develops at the gas‐material interface. In the experiment, a pulsed CO2 laser is focused onto a titanium target in a cell containing He, Ar, or N2 from a few tenths of Torr up to atmospheric pressure. The temporal and spatial distributions of Ti atoms vaporized from the surface are measured by laser‐induced fluorescence spectroscopy. Strong evaporation is observed in helium up to several hundred Torr. In this gas, the Ti atom plume expands according to the propagation of a blast‐wave. On the contrary, a very small quantity of material is vaporized in the presence of Ar or N2, because a highly absorbing breakdown plasma develops in this case, as soon as the surface vaporization threshold is reached, shielding the target from subsequent laser heating. These results corroborate our previous analysis based on the spectroscopic observation of the plasma.