**3. Laser ablation in a vacuum**

Laser ablation has been used in a vacuum using a vacuum chamber. A vacuum chamber is an empty rigid enclosure free from air and any gases, which are removed using a vacuum pump. As a result, a low-pressure environment is produced in the chamber. Laser ablation in a vacuum has been used to avoid any contamination during laser-material processing.

Laser-material interaction in a vacuum produces plasma at the surface of the target material, which expands considerably in the vacuum chamber [18]. An important factor in pulsed-laser ablation in different environments is the surface temperature of the target material. **Figure 4** shows the temporal variations of the surface temperature (*T*w) and equilibrium vapour density (*n*s) above the surface of a bulk flat material of pure niobium (Nb) in a vacuum chamber at a base pressure of 10−5 Pa. The target material was irradiated using a Nd:YAG laser pulse at the 1064 nm wavelength. It can be noted that the maximum values of both *T*w and *n*<sup>s</sup> were reached at 0*.*3 *τL* (4 ns) at the maximum laser pulse [19]. It is worth mentioning that the temperature profile is not only a function of the laser-beam parameters but is also a function of the type of target material.

**Figure 4.** (a) The surface temperature, *T*w, (b) equilibrium vapour density, *n*s, as a function of time and (c) irradiated pulsed laser.

The temperature profile of laser-material interaction in the vacuum can also be shown as a function of plume density. **Figure 5** shows the temperature-density phase diagram of an aluminium (Al) foil target material irradiated by a femtosecond laser (wavelength: 800 nm and pulse duration: 100 fs) in a vacuum chamber at a backing pressure of 10−7 mbar [20]. The corresponding temperature (*T*) of the plasma from which the ions originated, which were estimated for the IR and UV laser wavelengths, is about 106 and 105 K, respectively [18].

**3. Laser ablation in a vacuum**

of the type of target material.

pulsed laser.

Laser ablation has been used in a vacuum using a vacuum chamber. A vacuum chamber is an empty rigid enclosure free from air and any gases, which are removed using a vacuum pump. As a result, a low-pressure environment is produced in the chamber. Laser ablation in a

Laser-material interaction in a vacuum produces plasma at the surface of the target material, which expands considerably in the vacuum chamber [18]. An important factor in pulsed-laser ablation in different environments is the surface temperature of the target material. **Figure 4** shows the temporal variations of the surface temperature (*T*w) and equilibrium vapour density (*n*s) above the surface of a bulk flat material of pure niobium (Nb) in a vacuum chamber at a base pressure of 10−5 Pa. The target material was irradiated using a Nd:YAG laser pulse at the 1064 nm wavelength. It can be noted that the maximum values of both *T*w and *n*<sup>s</sup> were reached at 0*.*3 *τL* (4 ns) at the maximum laser pulse [19]. It is worth mentioning that the temperature profile is not only a function of the laser-beam parameters but is also a function

**Figure 4.** (a) The surface temperature, *T*w, (b) equilibrium vapour density, *n*s, as a function of time and (c) irradiated

The temperature profile of laser-material interaction in the vacuum can also be shown as a function of plume density. **Figure 5** shows the temperature-density phase diagram of an aluminium (Al) foil target material irradiated by a femtosecond laser (wavelength: 800 nm and pulse duration: 100 fs) in a vacuum chamber at a backing pressure of 10−7 mbar [20]. The

vacuum has been used to avoid any contamination during laser-material processing.

182 Applications of Laser Ablation - Thin Film Deposition, Nanomaterial Synthesis and Surface Modification

**Figure 5.** Diagram showing temperature as a function of plume density of Al foil target material irradiated by a femtosecond laser in a vacuum. The solid line represents the thermodynamic path of the Al absorbing volume (at a laser fluence of 400 mJ/cm2 ). The dashed line (binodal) is the location of liquid-gas equilibrium states and the dotted line (spinodal) curve defines the limit of the homogenous phase. (a) and (b) are a particular case representing the physical conditions reached by the Al plume about 5 and 7 ps after the 100-fs beings laser pumping, respectively. (c) indicates the critical point.

After laser-material interaction in the vacuum, the debris materials from the expanding plume have different speeds. The temporal profile of the emission intensity for nanoparticles and atoms, measured 5 mm from the surface of the target material at a laser fluence of 0.8 J/cm2 , shows that the light atoms fly faster than the nanoclusters or nanoparticles. It is shown that the average velocities of the nanoparticles and atoms are about 12 × 103 and 1 × 103 m/s, respectively [20]. Time-of-flight (TOF) measurements showed that the average velocity of the ion emission from a Cu material target in a vacuum by infrared (IR) (1064 nm wavelength) and ultraviolet (UV) (308 nm) are different and about 4.7 × 104 and 2.3 × 104 m/s, respectively [18].

The ablation depth (ablation depth per pulse) and emission yield as a function of the laser fluence at low and moderate laser fluence (about 150–500 mJ/cm2 ) show logarithmic dependence. On the other hand, at higher laser fluence (>500 mJ/cm2 ) the laser ablation sharply increased [20]. The laser ablation threshold in terms of the laser power density at 1064 and 308 nm laser wavelengths are about 7 and 3 J/cm2 , respectively [18].

In the vacuum, it was shown that the nanoparticles are directly generated from the target material by phase explosion. The condensation processes in the gas phase in the first stages of the plume expansion is not a means of producing nanoparticles in the vacuum [20]. It has been shown that during femtosecond laser interaction with tungsten to produce nanoparticles in a vacuum, the atomic plume emission acquired about 400 ns, but the nanoparticle plume acquired about 100 μs after the ablating laser pulse [21].
