Real-Time Observation of Surface Chemical Reactions Proceeding in the Depth Direction by Wavelength-Dispersive Soft X-ray Absorption Spectroscopy

It has long been a challenging task to observe surface chemical reactions proceeding in the depth direction without stopping the reaction (i.e., real-time observation), with subnanometer depth resolution. X-ray absorption spectroscopy (XAS) in the soft X-ray region is one of the most crucial methods to analyze the chemical states of light elements; however, simultaneous timeand depth-resolved XAS measurements during the reaction have not been realized before. Herein, we show the real-time observation of the time-evolving oxidation process of a Co thin film with a fluorescence-yield wavelength-dispersive XAS method, which also has subnanometer depth resolution. The series of Co L-edge XAS spectra show that the oxide component increases from the surface to deeper regions as the reaction proceeds. The progress of oxidation in the depth direction was interpreted by the reaction with O-2 gas at the surface and the interlayer reaction by oxygen migration.

Automated summary

In this study, real-time observation of the evolving oxidation process of a Co thin film was achieved using fluorescence-yield wavelength-dispersive X-ray absorption spectroscopy, revealing the increase of oxide component from the surface to deeper regions as the reaction progresses. The oxidation progress in the depth direction was interpreted by the reaction with O-2 gas at the surface and the interlayer reaction by oxygen migration.