Electric signal detection at the early stage of laser ablation in air

A tiny metal probe was used to detect electric signals induced at the early stage of laser ablation in air. It is found that the electric signals result from probe ablation, plasma-probe interaction, and plasma-induced electric field. The recorded signals strongly depend on the probe positions. For a probe placed out of the plasma-probe interaction region, the detected electric signal is a negative peak in the nanosecond range, due to the plasma-induced electric field. The peak arrival time corresponds to the total amount of ion emission from the substrate surface, and therefore, does not vary with the probe position. The signal amplitude is inversely proportional to the square of the probe distance, consistent with the distance dependence of the field intensity from an electric dipole. The signal amplitude increases with the laser fluence while the peak arrival time reduces, reflecting the earlier plasma generation at a higher laser fluence. Both peak width and its arrival time of the electric signals increase with laser fluence and tend to saturate above 6.4 J/cm(2). The electric signals were analyzed for laser ablation of different substrate materials. The electric signal detection was also applied to monitor the laser cleaning of organic contamination in real time. The mechanism of the electric signal generation and the process of electron and ion emission are briefly discussed. (C) 1999 American Institute of Physics. [S0021-8979(99)02717-6].