## 4 EUV Pulse-Preserving Monochromator  
The EUV generated from the HHG process has a broad spectrum with equally spaced harmonics. In order to select a single harmonic for time-resolved Spin-ARPES, a pulse preserving EUV monochromator is used after the HHG chamber. The monochromator utilizes gratings in the off-plane mount (OPM). In this design, the EUV incident plane is almost parallel to the grooves of the grating.  
Figure 3: Schematic of the HHG EUV source: (1) input laser pulse; (2) focusing lens; (3) entrance windows with anti-reflective (AR) coating; (4) gas nozzle; (5) HHG EUV source; (6) EUV and laser output to monochromator; and (7) gas input pipe.  
Grating monochromators for EUV ultrafast pulses can be realized using two different designs. The \"time-delay compensated\" design indicates a monochromator with two grating stages. The first stage performs the monochromatization on an intermediate slit, while the second stage compensates for the pulse front-tilt introduced by the diffraction from the first grating. In this case, the instrument is time-delay compensated and the final temporal response is close to the Fourier limit. Temporal response as short as 5 fs have been recently measured on monochromatized pulses at 42 eV, being essentially limited by the intrinsic duration of the generation process [40].  
The \"time-preserving\" design indicates a single-stage grating monochromator in which the operating parameters are chosen so that the pulse temporal broadening (due to the pulse-front tilt) does not alter considerably the pulse duration. In the design here presented, the pulse-front tilt is limited in the few-to-several tens of femtoseconds. The adopted configuration, time preserving in off-plane mount, has the main advantages in the high throughput as well as geometric simplicity [26; 27].  
The monochromator consists of two toroidal mirrors, a plane grating and a slit with adjustable width, as shown in Figure 4. The first toroidal mirror collimates the beam, while the second one focuses the diffracted beam on the exit slit plane. The monochromator has a set of three interchangeable gratings with different groove density for different spectral ranges and resolutions. Table 1 shows the specifications of toroidal mirrors and three gratings. The width of the slit is adjustable so that the bandwidth of a single harmonic can be controlled to some extent.  
The overall EUV conversion efficiency of the beamline is estimated as \(\sim\)2 \(\times\) 10\({}^{-7}\)/Harmonic, considering a laser to HHG efficiency of \(\sim\)10\({}^{-6}\)/Harmonic and a beamline overall transmission  
\begin{table}
\begin{tabular}{l l l} \hline \hline Toroidal Mirrors & Incident angle & 3\({}^{\circ}\) \\  & Input/output arms & 500 mm \\ \hline Gratings & Altitude & 3.5\({}^{\circ}\) \\ \hline \multirow{3}{*}{G1} & Energy region & 10.8 eV–40 eV \\  & Groove density & 150 gr/mm \\  & Bandwidth (100-\(\upmu\)m slit) & \(\Delta\)E = 0.4 eV @ 20 eV \\ \hline \multirow{3}{*}{G2} & Energy region & 10.8 eV–40 eV \\  & Groove density & 300 gr/mm \\  & Bandwidth (100-\(\upmu\)m slit) & \(\Delta\)E = 0.2 eV @ 20 eV \\ \hline \multirow{3}{*}{G3} & Energy region & 30 eV–100 eV \\  & Groove density & 600 gr/mm \\ \cline{1-1}  & Bandwidth (100-\(\upmu\)m slit) & \(\Delta\)E = 0.7 eV @ 50 eV \\ \hline \hline \end{tabular}
\end{table}
Table 1: Characteristics of the monochromator gratings. The energy resolution has been calculated on a 100-\(\upmu\)m slit width.  
Figure 4: EUV monochromator. (**a**) off plane mount geometry; (**b**) laboratory picture: (1) toroidal mirrors; (2) gratings mounted on translation stage; (3) grating rotation stage.  
efficiency of \(\sim\)0.20. The photon flux at the sample needs anyway to be reduced in order to avoid space charge effects, as discussed in Section 8.5.