Plasma shielding during picosecond laser sampling of solid materials by ablation in He versus Ar atmosphere

Abstract

The influence of plasma shielding on the coupling of laser energy to a target surface during picosecond pulsed laser-material interactions is demonstrated using a He and Ar gas atmosphere. An inductively coupled plasma-atomic emission spectrometer (ICP-AES) is used to monitor the quantity of copper material removed during picosecond and nanosecond pulsed-laser sampling. The intensity of Cu i emission from the ICP-AES was found to be 16.4 times larger with He as the gas medium compared to Ar during picosecond laser sampling. It was also observed that depth of craters in the copper targets decreased as the gas pressure was increased beyond 10 Torr in Ar and 100 Torr in He. Possible mechanisms of shock waves, multiphoton ionization, and plasma shielding to explain these observations are discussed. For plasma shielding to occur in the picosecond time regime, the existence of high-energy photoelectrons emitted from a Cu sample during the leading edge of laser pulse is postulated. These electrons form a plasma in the gas above the target via an inverse bremsstrahlung process and the plasma absorbs part of laser energy. The electron density versus pressure was calculated from a simple model and found to have similar behavior as the crater-depth data.