# Spectroscopic Studies of Laser-Based Far-Ultraviolet Plasma Light Source
## 2 Experimental Setup  
We described the experimental setup employed in this study, as done in previous publications, which can be seen for example in Refs. [4; 6; 17; 20; 21]. Briefly, in the present study, two 1.0 \(\upmu\)m lasers were used separately, a Q-switched Nd:YAG solid-state laser producing 10 ns FWHM pulses at 100 Hz repetition rate and a fiber laser operating with adjustable pulse width and repetition rate, e.g., 60-100 ns FWHM and 2-20 kHz, respectively. The output of these laser pulses was focused in vacuum by a 25.4 mm diameter, 60 mm focal length lens onto fast moving planar solid targets to create plasmas. The laser spot size diameters on the target for the 10 ns and 60-100 ns FWHM pulses were measured to be \(\approx\)120 \(\upmu\)m (1/e\({}^{2}\) width \(\approx\)200 \(\upmu\)m for an ideal Gaussian pulse) and 80 \(\upmu\)m (1/e\({}^{2}\) width \(\approx\)135 \(\upmu\)m), respectively. The focused beam of these two lasers generated plasmas in a pressure of \(\approx\)1.3 \(\times\) 10\({}^{-4}\) Pa vacuum chamber from a rotating target having a rotational speed of \(\approx\)5 RPM. The surface of each target was aligned to the focal plane of the laser with a combination of translation and rotation stages. Linear and rotational motion was in the plane of the target surface to provide a new target surface for each laser shot. However, the target rotational speed was not fast enough to provide a fresh sample in each laser shot for the 60-100 ns beam having a 2-20 kHz repetition rate. The plasma spectral features recorded at an angle \(\leq\) 15\({}^{\circ}\) from the target's normal were characterized using an aberration corrected 234/302 McPherson Seya-Namioka style spectrometer, with a 1200 groove/mm grating, 200 mm focal length, 0.1 nm achievable spectral resolution (tested at the wavelength of \(\approx\)185 nm with the 10 \(\upmu\)m wide slit), and 4 nm/mm dispersion. The spectrometer was customized with a fiber optic face plate, coupled micro-channel plate (MCP), with a cesium iodide (CsI) photocathode at the output image plane. This photocathode limits the spectral response of the spectrometer to the wavelengths shorter than \(\approx\)164 nm. A chilled charged-couple-device (CCD) array (169 \(\upmu\)m\({}^{2}\) 1024-pixel detector) with a fiber optic face plate was coupled to the MCP's fiber optic face plate. The spectrometer and its optical system were calibrated with a standard Deuterium (D\({}_{2}\)) lamp (X2D2, L9841 Hamamatsu) over an equivalent distance between the source and the slit. The lamp provides the absolute calibrated irradiance data 100 cm from the lamp over the wavelength region from \(\approx\)115 to 300 nm. Furthermore, the wavelength of the spectrometer is calibrated using the spectral features of the standard D\({}_{2}\) lamp. The spectral irradiance of the standard lamp is shown in Figure 1 (right ordinate in units of \(\upmu\)W cm\({}^{-2}\) nm\({}^{-1}\)) over the wavelength region of 120-170 nm. Using the lamp data, the instrument response correction, the so-called, \"absolute radiometric response correction curve\" is tabulated. An absolutely calibrated Si plasma radiation created by the 10 ns FWHM, 100 Hz laser pulse at the irradiance (\(I\)) of \(\approx\)2.5\(\times\)10\({}^{11}\) W cm\({}^{-2}\) is also shown in Figure 1 (left ordinate in units of \(\upmu\)W cm\({}^{-2}\) nm\({}^{-1}\)). The Si spectra were obtained by accumulating 12,000 laser shots, each on a fresh target, i.e., the acquisition time was 120 s. In all calibration processes and laser plasma experiments, the background noise signal is subtracted from the real data. It should be emphasized that the background noise signal was recorded for each experiment over the same acquisition time. This means that the noise signal is recorded over the same acquisition time in the absence of plasma. The spectral features of the calibrated Si spectrum may be identified (Figure 1 and Table 1) using the atomic database on the National Institute of Standards and Technology (NIST) website [22].