## 2 Ultrafast Deep-UV and XUV Beamline  
The femtosecond DUV and XUV beamline consists of three chambers: source, delay, and target chamber. The schematic diagram is presented in Figure 1a. The beamline is driven by carrier-envelope-phase (CEP) stabilized sub-5 fs NIR pulses close to the Fourier-transform limit (flat spectral phase) with a pulse energy of 0.25 mJ at a 3 kHz repetition rate. The spectral phase and resulting pulse duration was characterized by the dispersion scan method [23]. The driving NIR laser pulses are focused into quasi-static gas cells filled with noble gases using a 500 mm focal length spherical mirror for collinear third- and high-order harmonic generation (THG and HHG, light-yellow panel in Figure 1a) [24]. The THG gas cell is placed in front of the focus, whereas the HHG gas cell is placed about 3 mm after the focus. The cell is a 3 mm inner-diameter ceramic tube and has a 200 um diameter hole for the laser beam. One end of the tube is sealed with epoxy. A ceramic tube is chosen because it is robust against damage from the focused laser pulse. Two motorized three-axis stages are used to independently control the two gas cell positions, allowing for fine optimization of the harmonic yields. The driving NIR spectrum is displayed in Figure 1b and the generated DUV and XUV spectra are shown in Figure 1c,d, respectively. With careful optimization of the cell position and gas pressure in consideration of the nonlinear effect during the THG, a DUV pulse with an energy of \(\sim\)1 uJ has been obtained. The optimization reveals that the THG yield from an Ar gas is the highest at a pressure of 0.8 bar in the cell. The HHG gas cell is supplied with Ne gas of less than 200 mbar. During the THG and HHG process, all chambers are kept under vacuum, and any residual gas is evacuated with turbo molecular pumps and a backing scroll pump. The corresponding vacuum level of each chamber is indicated in Figure 1a. The DUV spectrum in Figure 1c supports a Fourier-transform limited pulse duration of 2.6 fs. Since the driving NIR pulse is near-Fourier-transform-limited, we estimate the resulting DUV pulse to be close to the Fourier-transform limit as well, since the group velocity dispersion in a gaseous nonlinear medium is negligible compared to a solid medium. Temporal characterization of the XUV pulses can be done via streaking in the future. Based on previous work with comparable driving pulses and XUV spectra, the XUV pulse duration is expected to be in the order of a few 100 as [25]. The photon flux of the XUV source is \(\sim\)2.5 \(\times\) 10\({}^{7}\) photons/pulse at the target chamber, taking into account the losses from the metallic filters and rhodium mirrors. After the source chamber, the generated DUV, XUV, and residual fundamental NIR pulses co-propagate into the delay chamber where a home-built double-layer Mach-Zehnder interferometer system provides a variable, actively phase-stabilized pump-probe delay. Inherently different divergences of the fundamental NIR, DUV, and XUV pulses allow us to separate them spatially using a perforated dichroic beam separator (DBS1 in  
Figure 1: Layout of the experimental setup and the spectra of the generated light sources. (**a**) Scheme for collinear harmonics generation and the double-layer interferometer method for pump-probe delay and active stabilization. The upper layer contains a reference interferometer for active phase stabilization; the lower layer contains the main beam path for the pump-probe experiment using a DUV and an XUV beam. (**b**) Driving few-cycle NIR laser spectrum. (**c**) Measured DUV and (**d**) XUV spectrum. THG: third-order harmonic generation, HHG: high-order harmonic generation, DBS: dichroic beam separator, BS: HeNe beamsplitter, M: silver mirror, RM: triple rhodium mirror (75\({}^{\circ}\) angle of incidence per segment), DM: dielectric mirror, TM: toroidal mirror, HWP: half-wave plate, QWP: quarter-wave plate, ND: neutral-density filter, PD: photodiode.