High-Speed Camera Imaging for Laser Ablation Process: For Further Reliable Elemental Analysis Using Inductively Coupled Plasma-Mass Spectrometry

Abstract

Production of laser ablation-induced sample aerosols has been visualized using a high-speed camera device coupled with shadowgraphy technique. The time resolution of the method is 1 micros, and production of the sample grains was successfully defined by the imaging system. An argon-fluoride excimer laser operated at 193-nm wavelength was used to ablate the solid samples. When the laser was shot onto the sample (Si wafer), a dome-shaped dark area appeared at the ablation pit. This dark area reflects changes in refractive index of ambient He probably due to emission of electrons or ions from the ablation pit. The dark area expanded hemispherically from the ablation pit with a velocity close to the speed of sound (approximately 1000 m/s for He at 300 K). This was followed by the excitation or ionization of the vaporized sample, known as the plasma plume. Immediately after the formation of the plasma plume, sample aerosols were produced and released from the ablation pit along the propagation of the laser-induced shockwave. Production of the sample aerosols was significantly delayed (approximately 4 micros) from the onset of the laser shot. The typical speed of particles released from the ablation pit was 100-200 m/s, which was significantly slower than the reported velocity of the plasma plume expansion (104 m/s). Since the initial measured speed of the sample particles was rather close to the speed of sound, the sample aerosols could be rapidly decelerated to the terminal velocity by a gas drag force with ambient He. The release angle of the sample aerosols from the ablation pit was very shallow (<10 degrees ), which may be due to the downforce produced by the thermal expansion of the ambient gas above the ablation pit. The shallower release angle and the contribution of the downforce probably results in the redeposition of sample aerosols or vapor around the ablation pit. In fact, the degree of sample redeposition around the ablation pit can be effectively minimized by evacuation the sample cell down to 20 kPa. In the case of glass samples, almost no visible laser-induced sample particles could be found using the spatial resolution of our device. This was mainly due to the smaller size distribution of the sample aerosols from the glass materials. Differences in size distribution of the sample aerosols between Si wafers and glass materials can affect analytical sensitivity and precision of elemental analyses using the laser ablation-ICP-mass spectrometry (LA-ICPMS). Although details of the mechanism of production and release of the sample aerosols from the ablation pit are not fully understood, the present imaging device for laser ablation has various implications for further precise elemental and isotopic analyses using LA-ICPMS.