## 3 Results  
NEXAFS spectra can be generated from our transmission mode spectra using the Lambert-Beer law, \(\ln\)(I\({}_{0}\)/I), where I and I\({}_{0}\) are the transmission spectra recorded with and without sample gas, respectively. Figure 2a shows three typical FEL pulse spectra (I\({}_{0}\)) recorded at BL1. The photon energy distributions have spike structures, and are different for each shot. These instable spectra may be attributed to the SASE-FEL which starts from shot noise in the electron beam. These spike structures are also observed for third- and fifth-order harmonics in the FEL radiation. In Figure 2b, the evolution of the spectrum as a function of averaging is shown. For 100 shots or less, the spike structure remains, but it can be seen that sufficient averaging is provided by averaging over 500 shots or more. Figure 2c shows 1000-shot average spectra. The bottom three spectra were measured at 3 min intervals, and the upper spectrum was recorded 66 min after the \"0 min\" spectrum. Since a thicker aluminum filter was used to record these spectra, it has been scaled to facilitate comparison. While the photon energy distributions are different for each shot, the average spectra are very similar and have a high reproducibility. There was little change in central light energy and width, even at 1 h apart. With the current experimental setup, we are unable to record both I and I\({}_{0}\) shot-by-shot, and here we could only compare multiple-shot average I and I\({}_{0}\) recorded consecutively. For soft X-ray absorption spectra of Ar and CO\({}_{2}\) gas shown below, we averaged 3000 shots for each spectrum. Figure 3a shows average incident (I\({}_{0}\)) and transmitted (I) spectra for Ar, recorded near the 2p edge. The photon energy of the FEL pulse was set to 82 eV, giving a third-order energy of 246 eV. Figure 1: Schematic drawing of the experimental setup for soft X-ray absorption spectroscopy in transmission mode at SACLA BL1. Figure 2: Typical (**a**) single-shot spectra, (**b**) the evolution of the spectrum as a function of the number of averaging, and (**c**) 1000-shot average spectra of incident free-electron laser (FEL) pulses. While the intensity of the third-order harmonics is only around 0.3% [23] of the fundamental FEL radiation, the transmission geometry allows us to directly record the transmitted FEL pulses. For the transmitted spectrum I, several sharp dips can be seen. These correspond to core excitations of Ar 2p \(\rightarrow\) nl Rydberg states. Using the Lambert-Beer law, we can obtain the transmission mode absorption spectrum from ln(I\({}_{0}\)/I), which is shown in Figure 3b. The sharp peaks correspond to Ar 2p \(\rightarrow\) nl core excitations, and the overall structure of the absorption spectrum is very similar to that reported in the previous work [30]. The observed peak width of 2p\({}_{3/2}\)\(\rightarrow\) 4s at 244.4 eV is 200 meV, whereas the natural width is 116 meV [30]. We can thus estimate the resolution E/AE of our spectrometer to be around 1200. The spectral width of the FEL pulses is around 3% (\(\Delta\)E/E), corresponding to 8 eV at a photon energy of 246 eV. This energy width allows us to record a NEXAFS spectrum without changing the FEL conditions or scanning the photon energy. We also recorded the carbon and oxygen K-edge NEXAFS spectra of CO\({}_{2}\) molecules using the same technique. For the C 1s spectra, we set the FEL pulse energy to 98 eV, and used the third order (294 eV). For the O 1s spectra, we used 108 eV and the fifth order (540 eV). Figure 4 show the resulting C 1s and O 1s NEXAFS spectra. The strong peaks at photon energies of 291 and 533 eV are due to carbon and oxygen 1s \(\rightarrow\)\(\pi^{\ast}\) core excitations. The spectra also show C 1s \(\rightarrow\) 3s Rydberg and O 1s \(\rightarrow\)\(\sigma^{\ast}\) core excitations. Due to the lower cross-sections and the lower photon intensity, the spectra are of lower quality than that recorded for Ar 2p.