## 4 OES for the Laser Induced Plasma Investigation
As an example of the result obtained with temporally resolved OES, Figure 3c shows the temporal temperature evolution determined from the continuum spectra of Figure 3a. Another interesting measurement of the plasma evolution can be performed by the fast imaging technique. The imaging of the total emission of plasma can also be simply detected with an ICCD coupled to an objective for studying the dynamics of the plasma [11]. Then, plasma emission imaging and OES can be matched for retrieving temporally resolved spatial maps of temperature. This can be obtained as sketched in Figure 4: the plasma  
Figure 3: A typical example of (**a**) temporal evolution of the continuum radiation spectra of LIP induced on target underwater at various delay times. In Figure 3a, the dotted lines represent the interval where the spectra hold the Planck distribution; (**b**) an example of the corresponding Plank plot analysis during PLAL. Note that the fitting red line corresponds to the linearized Planck spectrum according to Equation (2) at the given T; (**c**) corresponding temporal temperature evolution. The PLAL was performed on Ag target immersed in a flowing water layer with thickness of 2.5 mm (laser pulse width = 6 ns, laser energy = 90 mJ, laser wavelength = 1064 nm, laser frequency = 10 Hz, crater size = 1 \(\pm\) 0.2 mm, ICCD gate width = 10 ns, delay times ranging between 10 ns and 80 ns, as reported for each spectrum in (**a**)). The experimental set-up is described in detail in Ref. [16]. emission image is split and focused on different areas of the ICCD. Along the optical paths, two narrowband filters at two different wavelengths are located in the optical paths of the split image, therefore the simultaneous acquisition of two plasma images at different wavelengths is collected. In this way, it is possible to employ Equation (2) in the form of an intensity ratio for the estimation of the temperature trend across the plasma. Although this approach is not so accurate as the one using a Planck-like distribution obtained with the wavelength range of the whole visible spectrum, if the two filter wavelengths are optically far enough (>100 nm) from each other, the investigation of the temperature of the plasma with a micrometric spatial resolution can be done as shown in Figure 5. Ref. [7] describes in detail both the experimental procedures and the used equations for retrieving the plasma temperature maps temporally and spectrally resolved. As an example, Figure 5 shows the temperature maps obtained with laser ablation of a titanium target immersed in water. In particular, Figure 5 reports the results at three delay times from the laser pulse and the corresponding axial temperature profile along the expansion axis of the plasma. It is evident that the temperatures were not homogeneous in the whole plasma volume since the map presented the inner zone with lower values with respect to those at the edge of the plasma (30% hotter). At longer delay times, the temperatures tended to reach homogeneous values all over the plasma volume. On the other hand, from the plasma images, the plasma volume as a function of time could be estimated. This information can be very important if the ablated mass can be measured independently, for example, by measuring the ablated volume of the crater because it allows the calculation of the ablated mass from the density of the target [17]. When the total ablated mass and the plasma volume are known, the initial number density can be determined. Moreover if the initial number density and the plasma temperature is known, the Saha equation [18] can be applied and the temporal and spatial evolution of the number density of the species in the plasma can be determined. The details of these calculations are reported in Ref. [7]. The traditional OES on plasma emission, although it allows good temporal resolution, does not permit spatial resolution and, consequently, the estimated temperature is spatially integrated along the all emitting volume.