# Spectroscopic Studies of Laser-Based Far-Ultraviolet Plasma Light Source
## 4 Results and Discussion
Thus, in the case of a collision-dominated plasma or an optically thick medium [25], it is expected that these two resonance spectral lines broadened and merged together at a typical areal density of Si plasma \(\approx\)\(\rho\times\) d \(\geq\) 10\({}^{-5}\) g cm\({}^{-2}\) and a temperature of 5 eV. Note that, in the CR calculation, we used the Voigt spectral line shape that includes the collisional (i.e., the impact pressure) and Doppler broadening mechanisms (Figure 2b) [23]. It is worth noting that the instrumental broadening is not taken into account in the calculation. There are a few discrepancies between the calculated and the measured Si spectral emission. Particularly, the calculated continuum emission is much less than the measured spectra. Figure 3 shows the Si spectral lines around 130 nm and at shorter wavelengths are stronger than the corresponding lines in Figure 1. The reason is due to the lower temperature of Si plasma in conditions relevant to Figure 3 compared to Figure 1 owing to the lower laser irradiance. Thus, the spectral radiations originated from the lower charge state of Si III are stronger than Si IV. Moreover, the calculations using the CR solver show (Figure 2b) the ratio of spectral lines around 130-140 nm increases by raising the plasma opacity, i.e., by increasing the plasma density or the photon path length. The spectral features of several targets were also generated with 10 ns FWHM, 100 Hz laser at different laser irradiances in the range of \(I\approx\) 10\({}^{10}\) to 2.5 \(\times\) 10\({}^{11}\) Wcm\({}^{-2}\). Note that at this laser condition, the target rotated fast enough to provide a fresh sample in each laser shot. Figure 4a presents the calibrated spectral plasma irradiances (in units of \(\mu\)W cm\({}^{-2}\) nm\({}^{-1}\)) of the Si, Zn, Mo, Sn and tantalum (Ta, Z = 73) planar targets at a distance of 100 cm from the target over the wavelength region of \(\approx\)123-164 nm. Here, the laser irradiance was \(\approx\)2.5 \(\times\) 10\({}^{11}\) Wcm\({}^{-2}\). This irradiance is well beyond the plasma threshold fluence of all refractory metals. At this laser irradiance, all spectra were calculated after averaging \(\approx\)3000 pulses. As previously discussed, the main spectral features of the Si and Zn plasmas were mainly created from the low-ionized charge states of Si III-Si IV and Zn III-Zn IV ions, respectively. Inspection of Figures 3 and 4a reveals the main spectral lines of Si IV (around \(\approx\)140 nm) and Zn IV (\(\leq\)140 nm) are brighter than Si III (around \(\approx\)130 nm) and Zn III (\(\geq\)140 nm) in plasma generated by a 10 ns pulse than 60 ns due to the higher laser irradiance, consequently, a higher temperature. Additionally, as stated before, the low-ionized Sn III, Sn IV, and Sn V ions are main contributors in the measured Sn spectra (Figure 4a). Figure 4a shows that the Mo and Ta plasmas generate a weaker spectral FUV emission compared to the Zn and Sn ions. The reason may be due to plasma over-ionization,  
Figure 3: Measured FUV spectral irradiances of the Si, Zn and Sn plasmas at a distance of 100 cm from the target over the \(\approx\)123–164 nm wavelength region. Plasmas were generated by a 1.0 μm laser at two operation regimes: with pulse widths of 60 and 100 ns FWHM and the repetition rates of 2 and 20 kHz. Note that the right ordinate is used for one of the Zn spectra (marked with a filled rectangle) for better view. Here, the laser irradiances (pulse energies) for 2 and 20 kHz rep-rates are \(\approx\)10\({}^{9}\) W cm\({}^{-2}\) (\(\approx\)3 mJ), \(\approx\)5 \(\times\) 10\({}^{9}\) W cm\({}^{-2}\) (\(\approx\)15 mJ), and \(\approx\)10\({}^{8}\) W cm\({}^{-2}\) (20 kHz, 100 ns FWHM laser pulse energy \(\approx\)0.6 mJ), respectively. Positions of Si spectral lines are shown (on top) using the NIST atomic database [22]. i.e., the Mo and Ta plasma radiations may be increased at the lower laser irradiance region.