# Spectroscopic Studies of Laser-Based Far-Ultraviolet Plasma Light Source
## 4 Results and Discussion
Typical calibrated FUV spectral irradiances (in units of \(\upmu\)W cm\({}^{-2}\) nm\({}^{-1}\)) of the Si (\(Z\) = 14), Zn (\(Z\) = 30), and tin (Sn, \(Z\) = 50) plasmas at a distance of 100 cm from the target over a wavelength region of \(\approx\)123 to 164 nm are shown in Figure 3. The laser irradiance is estimated using the measurement of the average laser power for each experiment by an OPHIR laser power meter (serial number 517845). We recorded the plasma FUV emission (e.g., using the Zn solid target) for the laser irradiance as low as \(\approx\)5 \(\times\) 10\({}^{6}\) W cm\({}^{-2}\) using 100 ns FWHM and 20 kHz pulses. A high repetition rate of 20 kHz enables improving the signal-to-noise ratio at this low laser irradiance, i.e., to detect a clear FUV spectral emission over the wavelength region of \(\approx\)123-165 nm. This laser irradiance gives a F\({}_{\rm th}\approx\) 0.5 J cm\({}^{-2}\). This value is equal to the calculated theoretical plasma threshold fluence for the Zn metal target, as can be seen in Ref. [24]. It is worth mentioning that the previous report [24] shows that there is approximately no significant difference in F\({}_{\rm th}\) using single-shot irradiation with each pulse hitting a fresh sample spot or accumulating ten pulse shots on the same target surface. Our experimental and atomic physics calculations [6; 17; 21] show that the Zn plasma radiates strong FUV spectral band with a CE as high as \(\approx\)3%/2\(\pi\)sr at a laser irradiance of \(\approx\)5 \(\times\) 10\({}^{9}\) W cm\({}^{-2}\) for 2 kHz, 60 ns pulse. The corresponding Zn spectrum is shown in Figure 3 (right ordinate: the spectrum is marked with a rectangle). The unresolved transitions arrays from 3d\({}^{9}\) 4l-3d\({}^{9}\) 4\(l^{\prime}\) and 3d\({}^{8}\) 4\(l\)-3d\({}^{8}\) 4\(l^{\prime}\) orbitals in Zn III and Zn IV ions produce an efficient Zn plasma FUV light source. Here, \(l\) and \(l^{\prime}\) are the orbital quantum numbers (i.e., s, p, d, f,...). An inspection of Figures 1, 2b and 3 reveals the main charge states responsible for the Si and Zn plasma FUV radiations are the Si III, Si IV and Zn III, Zn IV ions, respectively. Furthermore, our study shows that the Sn III, Sn IV, and Sn V ions are major contributors to the FUV emission in Sn plasma. Figure 3 shows the Si spectra is somewhat more powerful (with a CE \(\approx\) 1%/2\(\pi\)sr) than the emission from Zn and Sn plasmas at the laser irradiance of \(\approx\) 10\({}^{9}\) W cm\({}^{-2}\) for 60 ns pulse at 2 kHz operation. A reason may come from the fact that the spectral lines of Si plasma are strong resonance lines in Si III and Si IV ions that are radiatively coupled to the ground states, whereas for Zn III and Zn IV ions, the transitions are generated among the singly excited states. Additionally, the low-ionized Si III-Si IV charge states remain stable over a wider electron temperature region than Zn III-Zn IV and Sn III-Sn V ions. For example, the average ionic charge states of Si and Sn are \(\approx\)3.9 and \(\approx\)6.2 at the electron temperature of \(\approx\)10 eV, respectively. Thus, a plasma over-ionization effect is less severe for the Si than Zn and/or Sn plasmas. Comparison of the Si spectra in Figures 1 and 3 show the two strong spectral lines around 140 nm (i.e., transitions of 2p\({}^{6}\) 3s-2p\({}^{6}\) 3p in Si IV) are broadened and merged in the plasma created by the longer pulse of 60 ns width at a lower laser irradiance of \(\approx\)10\({}^{9}\) W cm\({}^{-2}\). The reason may be understood based on the calculation using the CR solver for an artificial homogeneous plasma. Figure 2b shows the spectral lines of Si IV around 140 nm can become optically thick (i.e., no object at a given temperature can emit more than black-body emission limit) at a high density region or a long photon path length owing to a larger plasma size for the 60 ns than 10 ns pulse.