Laser produced plasma in crystalline α-Al₂O₃ and aluminum metal

A comparative study of the laser produced plasma (LPP) in vacuum at the surface of transparent crystalline α-Al2O3 (sapphire) and Al metal generated by 248-nm excimer laser pulses of 15-ns duration is reported. It was found that the threshold fluence for strong LPP emission in sapphire is 0.55 J/cm2, which is significantly lower than that needed for the same process in clean Al (1.7 J/cm2). At fluences higher than 4 J/cm2 the LPP emission from both targets is similar. The results obtained in sapphire require that its absorption coefficient increase by many orders of magnitude during the laser pulse. A possible explanation for the observed threshold difference is that the greater thermal diffusivity of Al more than compensates for its greater volatility; hence, a greater fluence is necessary to create the LPP in Al.