Characterization of a collinear double pulse laser-induced plasma at several ambient gas pressures by spectrally- and time-resolved imaging

A collinear double pulse laser-induced plasma was characterised by means of a spectrally and time resolved imaging technique. The beams of two Q-switched Nd:YAG lasers were focused on a brass target in a vacuum chamber to form the plasma. The plume emission intensity and spatial distribution were recorded with temporal resolution using an intensified CCD. Using a set of interference filters, we collected images of the emission from the major target components as well as from oxygen. Both the laser inter-pulse separation (in the range between 0 and 10 s) and the ambient air pressure value (in the range between 10⁵ and 10 Pa) were varied during the experiment. At atmospheric pressure, an enhancement of the line emission from the target elements was observed for delayed laser pulses compared to coincident pulses. However, this enhancement effect tends to fall at low pressure values, and a decrease of the signal is observed for pressures under about 10⁴ Pa. Moreover, it was observed that the evaluation of the enhancement factor strongly depends on the detector field of view. The propagation of the emitting plume was also studied at several pressures and inter-pulse delays.