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Spectroscopy on laser induced plasma in cavitation bubbles

Spectroscopic results with laser induced breakdown spectroscopy (LIBS) in liquids have shown a signal improvement using double pulse experiments compared to single pulse measurements. A setup of two Q-switched Nd:YAG lasers in an orthogonal arrangement have led to bright illuminated plasma inside cavitation bubbles. A Nd:YAG laser pulse focussed by a lens (6 ns@532 nm, 2.9*1019 W/m2) into the center of a water filled cuvette gives rise to a cavitation bubble. A second tightly focused Nd:YAG laser pulse (5 ns@1064 nm, 3.1*1019 W/m2 ) induces a plasma at approximately 70 μs delay within the cavitation bubble. These absolutely reliable processes allow to capture image sequences of plasma filled cavitation bubbles with an ultrafast camera. The optical emission is guided via a quartz fibre to a Czerny-Turner spectrograph and recorded by an intensified CCD camera. A delay time of more than 1.7 μs between plasma ignition and spectroscopic data taking is necessary to avoid inverse bremsstrahlung and residual lines arising from bright plasma emission. Characteristic spectral line intensities of manganese (Mn) and Palladium (Pd) are recorded and correlated with an internal calibration standard using strontium (Sr) and chromium (Cr) respectively. The LOD of manganese is 0.3 mg/L.