Development of Fluorescence-yield Wavelength-dispersive Soft X-ray Absorption Spectroscopy for Real-time Observation of Surface Chemical Reaction

Abstract

A novel X-ray absorption spectroscopy technique in the soft X-ray region has been developed, which enables the real-time observation of surface chemical reactions under near ambient pressures by sequentially recording the absorption spectra without scanning the photon energy of the incident X-rays. The wavelength-dispersed soft X-rays, in which the wavelength (energy) continuously changes as a function of position, illuminate the sample, and the fluorescence soft X-rays emitted at different position on the sample are separately corrected by an imaging optics consisting of two spherical mirrors. By using the developed system, a series of X-ray absorption spectra was recorded during the oxidation reaction of copper surface under air pressure of up to 5000 Pa at a data acquisition time of 10 s for each spectrum. The curve-fitting analysis for the recorded spectra indicates that only the CuO species increases with 500 Pa air, while Cu 2 O appears at 5000 Pa.

Time- and Depth-Resolved Chemical State Analysis of the Surface-to-Subsurface Oxidation of Cu by X-ray Absorption Spectroscopy at Near Ambient Pressure

“…Figure b shows the experimental setup in side view for the wavelength-dispersive measurements. Wavelength-dispersive soft X-rays, in which the wavelength (photon energy) continuously changes as a function of position, are obtained at the exit slit of the soft X-ray monochromator ,, and are reimaged onto the sample by a toroidal mirror. The soft X-rays are absorbed at each position of the sample, and the fluorescence soft X-rays emitted from the sample are collected by the fluorescence X-ray imaging system consisting of two spherical mirrors, so that X-ray fluorescence emitted from each position on the sample is focused onto the detector separately.…”

“…Despite this advance, in conventional XAS measurements, the absorption intensity is measured at each energy in a step-by-step fashion using a monochromator, which makes in situ , real-time measurements impossible. In response, the wavelength-dispersive XAS technique was developed, in which the sample is irradiated with a range of soft X-ray wavelengths simultaneously. − In addition, the detection of soft X-ray fluorescence has recently been realized, , which further extends the range of applicable gas pressures. Very recently, non-destructive depth-resolved measurements have been combined with dispersive XAS, and this enables depth profiles of chemical states having a resolution of 1 nm (that is, the near-surface region) in real time.…”