engleski

Characterization of molecules within laser induced plasma plume by cavity ring-down spectroscopy

Single and dual-pulse laser ablation of lithium metal target in vacuum were produced by Nd-YAG lasers. Target was mounted on an X-Y translation stage to allow precise positioning in respect to the cavity formed by two high reflectivity mirrors. It was also rotated to avoid drilling. Both target and the focusing lens could be moved by stepping motors. Lasers, digital delay generator and step motors were controlled by a PC computer using a homemade LabVIEW program. For cavity ring-down spectroscopy (CRDS) an excimer pumped dye laser working with Rhodamine 110 dye was used and detection of the ringdown signals was performed by a photomultiplier. An appropriate interference filter was placed between the end-cavity mirror and the photomultiplier to stop the light from laser-induced plasma and scattered 1064 nm laser light. A digital oscilloscope was used to measure the CRD decay curves, which were stored in a PC for further analysis. Typical emission spectrum from such plasma shows many lithium atomic lines broadened by Stark effect. Molecular spectrum could not be observed in emission from the plume. Applying the absorption CRDS technique lithium dimer molecules have been observed in the spectral region of A-X band around 650 nm. Rotationally resolved spectra were studied versus laser energy, spot size, time delay between ablation lasers etc. Comparison with spectral simulations allows determination of rotational temperature under different laser ablation conditions

laser induced plume; molecues; cavity ringdown spectroscopy

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