Features of ablation lasers

The performance of ablation lasers depends largely on two factors, namely the nominal laser wavelength and intensity, and whether the continuous or pulsed mode of irradiation is used. Studies on the first two factors have yielded controversial results that can be ascribed to differences in sample nature or grain size. Thus, ruby (694 nm), Nd YAG (1064, 532, 266 and, recently, 213 nm) and excimer (308, 248, 222, 193 and, recently, 157 nm) lasers have been used for laser ablation (LA). A comparison of the Nd YAG laser operating at 1064 nm and 266 nm revealed better absorption of UV wavelength by most types of samples and also decreased fractionation. The particle sizes produced during ablation were smaller with 266 than with 1064 nm, which was possibly the reason for the reduced 

Lasers for solid sampling should always be operated in the pulsed mode. The continuous, CW mode, as often used for drilling and welding in engineering, would have serious drawbacks. The application of a CW laser provides much lower intensities than the peak intensities of pulsed laser. There would be continuous removal from the sample, which is closer to ordinary evaporation than the explosion-like ablation process of pulsed lasers. 

The small area irradiated by a CW laser would continuously melt and the spatial composition in the liquid would change with time because of fractional evaporation and mixing in the liquid phase. Furthermore, the plasma above the sample would be continuously heated and, if the intensity is not low enough, it would shield the sample from the laser beam because of absorption by the plasma can be reduced by the choice of a laser with a shorter wavelength, a different kind of gas or a lower gas pressure. 

The most popular lasers for ablation have nanosecond pulse lengths. Sufficiently high intensities for explosion-like ablation can be used and there is additional heating of the plasma gas. The intensities, the wavelength, the kind of gas and its pressure, must, however, also be chosen carefully in order to avoid plasma shielding of the sample. 

The application of lasers with shorter pulses (e.g. in the picosecond or even femtosecond range) reduces heat dissipation by collisions of the high-energy electrons with the atoms in the solid, thereby minimizing melting of the laser crater region, as shown in Fig. 9.3, and fractional evaporation from the liquid phase. On the other hand, there is also less energy deposited in the gas plasma this usually facilitates atomization of the material ablated from the sample. 

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