Laser ablation of copper in different background gases: comparative study by numerical modeling and experiments

A comprehensive model was developed to describe laser ablation of metals (Cu) with expansion in different background gases, i.e., He, Ne, Ar, Kr and N2. The effect of the background gas on the target heating, melting and vaporization, and on the plume expansion dynamics and plasma formation and plasma shielding, is investigated. It is found that the heavier background gases result in a slower expansion, or more confinement, of the vapour plume. Although background gases with a lower ionization potential exhibit a higher ionization degree in the plasma, this effect is of minor importance compared with the ionization of the vapour plume. Hence, the model predicts that the background gas has no significant influence on the plume temperature and electron density in the early stage of laser ablation (100 ns). Moreover, because laser absorption in the plasma mainly takes place in the vapour plume, plasma shielding is only slightly affected by the background gas. Nevertheless, the effect is large enough to yield some differences in the target evaporation depths, which are calculated to be slightly deeper for He than for Ne, Ar and Kr. These calculation results are in reasonable agreement with crater depths measured in the case of the different background gases.