**5. Laser ablation in background gases**

Laser ablation in different background gases such as He, Ne, Ar, Kr, Xe and N2 has been used to improve the cutting-edge quality of laser ablation [24]. Heavier background gases produce a slower expansion, or more confinement of the vapour plume [3]. The gas environment has an effect on ablation efficiency. The improvement in laser-machining in different gases has been found to correlate to their potential ionisation [25].

As shown in **Figure 6**, the surface temperature increases due to the laser (photon) influence to a maximum of about 7000 K at 8 ns. This corresponds to the maximum laser irradiance time profile. When the laser pulse is finished, the surface temperature reduces sharply to about 3000 K after 20 ns, after which point, it gradually drops to about 1100 K at 100 ns. It has been shown that the maximum surface temperatures for He, Ne, Ar, Kr and N2 are 7088, 7062, 7036, 7025 and 7037 K, respectively. It can be concluded that the surface temperature increases slightly with decreasing mass and increasing ionisation potential of the background gas [3].

Concerning the generation of nanoparticles in a background gas, Nichols et al. [26] produced Ag nanoparticles by laser ablation in argon, nitrogen and helium at a variety of gas pressures. It was concluded that by selecting an appropriate gas type and pressure, Ag nanoparticles can be produced and controlled in the range of 4–20 nm. In addition, the smallest Ag nanoparticles (with a mean diameter of 5 nm) were produced in helium gas at 1 atm and below, and the

**Figure 6.** Calculated surface temperature (*T*surf) and laser irradiance-time profile as a function of time (*t*) for a laser of 266 nm, with 5 ns, and an irradiance of 1 GW/cm2 .

largest nanoparticles (with a mean diameter of 19 nm) were produced in nitrogen gas. Furthermore, the average size of the nanoparticles increases with increasing molecular weight of the gas. The kinetic energy of the ablated atoms in a gas background and the width of their angular distribution has also been found to decrease with increasing ambient pressure [27].
