PLASMA SHIELDING DURING PICOSECOND LASER SAMPLING OF SOLID MATERIALS BY ABLATION IN HE VERSUS AR ATMOSPHERE

The influence of plasma shielding on the coupling of laser energy to a target surface during picosecond pulsed laser-material interactions i s demonstrated using a He and Ar gas atmosphere. An inductively couple d 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 dept h of craters in the copper targets decreased as the gas pressure was i ncreased beyond 10 Torr in Ar and 100 Torr in He. Possible mechanisms of shock waves, multiphoton ionization, and plasma shielding to explai n these observations are discussed. For plasma shielding to occur in t he picosecond time regime, the existence of high-energy photoelectrons emitted from a Cu sample during the leading edge of laser pulse is po stulated. These electrons form a plasma in the gas above the target vi a an inverse bremsstrahlung process and the plasma absorbs part of las er energy. The electron density versus pressure was calculated from a simple model and found to have similar behavior as the crater-depth da ta.