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Dual-Pulse LIBS of Titanium

Sensitivity of laser-induced breakdown spectroscopy can be increased by using two laser pulses for ablation with controllable time delay between them. For any sample time delay between laser pulses can be estimated in such way that signal reach the maximum and typically 10 to 100-fold enhancement may be observed. Physical processes involved in dual-pulse laser ablation are not completely understood. Our experimental set-up allows both emission (LIBS) and absorption (Cavity ringdown spectroscopy - CRDS) measurements under same conditions  1, 2 . We used as a test case titanium target and performed single- and orthogonal dual-pulse laser ablation in vacuum or in background hydrogen gas. Lasers used for ablation are Q-switched Nd-YAG lasers (Brilliant, 350 mJ at 1064 nm, pulse duration 5 ns and Brio, 100 mJ at 1064 nm, 5 ns). Laser pulses were triggered and synchronized by means of a digital delay generator and were focused onto the target surface with 33.5 cm focal length lens. Target was rotated to avoid drilling and was mounted on an X-Y translation stage to allow precise positioning in respect to observation axis. Emission spectra were measured with LIBS2000+ or Avantes AvaSpec 3648 spectrometers in the range from 200 to 1000 nm. Spectra show large intensity increase in the case of dual-pulse laser ablation. In addition, emission from plasma was monitored by means of a CRDS set-up photomultiplier. Ionic, atomic, molecular or background radiation was separated using various interference filters. Measurement parameters are: time ablation conditions (single or dual mode), time delay between lasers, position of the target in respect to the observation axis (orthogonal spatial resolution was typically 1 mm), focusing conditions, integration of time-depended emission over the bandpass of interference filter, etc. Results show different emission behaviour of ionic and atomic features. In the early stage of plasma formation (up to about 300 ns) atomic emission is dominated while ionic intensity is low. We also studied how crater profile is influenced by various ablation conditions such as number of pulses, repetition rate, single or dual regime, vacuum or background gas, etc. Largest crater depth (or ejected volume) is achieved for time delay which is different from the time delay where maximum of absorption or emission is observed. We shall discuss various aspects of absorption and emission measurements.  1 N. Krstulović. I. Labazan, S. Milošević, Study of Mn laser ablation in methane atmosphere, Eur. Phys. J. D 37 (2006) 209-215 2 N. Krstulović, N. Čutić and S. Milošević, Modeling of cavity ring-down spectroscopy characterization of laser-induced plasma plume, IEEE TRANSACTIONS ON PLASMA SCIENCE, in press, 2008.

Laser-induced Breakdown Spectroscopy

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