Soft X-ray photoelectron spectroscopy (XPS) is a surface-sensitive probe of the chemical state of materials. A prime example where the characteristic of surface sensitivity is utilized is chemical reactions on catalyst surfaces. For improving the performance of catalysts, it is essential to understand the chemical reaction processes on their surfaces, where analysis using XPS provides valuable information. However, performing XPS measurements in a gas environment presents challenges. First, soft X rays are easily absorbed by gas molecules in the air, causing a significant reduction in their intensity before reaching the sample. Second, photoelectrons emitted from the sample also undergo strong scattering by gas molecules, making it difficult to guide them to the detector. Vacuum environments solve both problems, and hence XPS measurements have typically been performed under ultra-high vacuum.

In recent years, advances in photoelectron measurement technique and synchrotron X-ray technology have led to the development of XPS in gas environments. Such XPS instruments are now installed at synchrotron facilities around the world and are used for research of surface chemistry. However, the gas pressure is limited to 0.13 bar at maximum [1], and many chemical reactions that occur at atmospheric pressure have not been covered. To address this problem, we developed an ambient-pressure soft X-ray photoelectron spectroscopy (APXPS) instrument that operates at atmospheric pressure [2] at NanoTerasu, a synchrotron radiation facility recently constructed on the Tohoku University campus (Fig. 1).

The key to realize XPS at ambient pressure is to suppress the attenuation of soft X-rays and photoelectrons by gas. To achieve this, it is important to bring the soft X-ray outlet and photoelectron entry as close to the sample as possible, thereby minimizing the distance that soft X rays and photoelectrons travel through the gas. We first prepared a soft X-ray introduction path consisting of a vacuum tube to avoid soft X-ray attenuation [Fig. 2(a)]. At the tip, we attached a SiN thin film (200 nm thick) with high transmittance in the soft X-ray region. By combining this soft X-ray introduction path with a movable stage, we created an environment where the SiN film could be brought very close to the sample ($\sim$ 5 mm). On the other hand, to suppress the scattering of photoelectrons by gas, an aperture cone [Fig. 2(b)] was attached to the tip of the analyzer. We used focused ion beam processing to create an aperture with a diameter of as small as 24 µm, which considerably reduces the conductance and makes it possible to introduce ambient-pressure gas to the analysis chamber while maintaining high vacuum inside the electron analyzer. By bringing the sample very close (comparable to the aperture-cone diameter) to the tip of the aperture cone during measurement, the distance that photoelectrons travel in the gas can be reduced.

Combining these elements [Fig. 2(c)], and owing to high photon flux available at NanoTerasu BL08U, we succeeded in observing Au 4$f$ photoelectron peaks under H₂ atmosphere even at 1.0 bar [Figs. 2(d),(e)]. The time spent to acquire the spectrum in Fig. 2(e) is only 8 minutes, which is quick enough to trace chemical reactions in a real-time fashion. The developed APXPS system [2] dramatically expands the capability of XPS and opens up new applications to real functional materials under operation.

References

• [1] S. Kaya et al., Catal. Today 205, 101 (2013).
• [2] T. Wada et al., Appl. Phys. Express 18, 036504 (2025).

Authors

• M. Horio, T. Wada, Y. Liu, Y. Murano, H. Sakurai, T. Sumi^a, M. Miyamoto, S. Yamamoto^a, and I. Matsuda
• ^aTohoku University