Photoelectron shield for the first mirror of a soft X-ray beamline

At a soft X-ray beamline with an undulator source, significant heat generation at the first-mirror chamber and light emission at the viewport were found, which can be explained by photoelectrons from the mirror. The chamber temperature increases up to approximately 50°C over a period of several hours. A photoelectron shield consisting of thin copper plates not only prevents the heat generation and light emission but also improves the pressure of the vacuum chamber, if a voltage of a few tens of V is applied to the shield. The total electron yield of the shield reached as much as 58 mA under high heat-load conditions, indicating the emission of numerous photoelectrons from the first mirror. Heat-balance analyses suggest that approximately 30% or more of the heat load on the first mirror is transferred to the surroundings.

Soft X-ray refractive index by reconciling total electron yield with specular reflection: experimental determination of the optical constants of graphite

The complex refractive index of many materials is poorly known in the soft X-ray range across absorption edges. This is due to saturation effects that occur there in total-electron-yield and fluorescence-yield spectroscopy and that are strongest at resonance energies. Aiming to obtain reliable optical constants, a procedure that reconciles electron-yield measurements and reflection spectroscopy by correcting these saturation effects is presented. The procedure takes into account the energy- and polarization-dependence of the photon penetration depth as well as the creation efficiency for secondary electrons and their escape length. From corrected electron-yield spectra the absorption constants and the imaginary parts of the refractive index of the material are determined. The real parts of the index are subsequently obtained through a Kramers–Kronig transformation. These preliminary optical constants are refined by simulating reflection spectra and adapting them, so that measured reflection spectra are reproduced best. The efficacy of the new procedure is demonstrated for graphite. The optical constants that have been determined for linearly polarized synchrotron light incident with p- and s-geometry provide a detailed and reliable representation of the complex refractive index of the material near π- and σ-resonances. They are also suitable for allotropes of graphite such as graphene.

Spectral Distribution of Oscillator Strength for Core-to-Valence Transitions probed by using X-ray Absorption and Total Electron Yield Modes

Changes in spectral distribution of oscillator strength for core-to-valence transitions probed by X-ray absorption and total electron yield (TEY) modes are studied in more detail. The quantitative analysis of their redistribution in TEY due to the saturation effect is performed by applying the model $\tilde M$ -function method suggested by Flesch et al. J. Chem. Phys. 138 (2013) 144302. The model $\tilde M$ -function method is generalized to account for the saturation-induced distortion of the line shape of the transitions embedded into the core ionization continua and located below them. It is shown that TEY measurements produce essential changes not only in the relative intensity of the transitions but also shift upward the transition energy, broaden the Lorentzian and Gaussian widths and enhance the line asymmetry. The Gaussian width of the TEY signal undergoes the most substantial broadening though the Lorentzian width increases significantly too. The saturation-induced distortion of the line shapes is not negligible even if the well-known escape-to-penetration condition is valid.

Effect of reflection and refraction on NEXAFS spectra measured in TEY mode

The evolution of near-edge X-ray absorption fine structure in the vicinity of theK-absorption edge of oxygen for HfO2over a wide range of incidence angles is analyzed by simultaneous implementation of the total-electron-yield (TEY) method and X-ray reflection spectroscopy. It is established that the effect of refraction on the TEY spectrum is greater than that of reflection and extends into the angular region up to angles 2θc. Within angles that are less than the critical angle, both the reflection and refraction strongly distort the shape of the TEY spectrum. Limitations of the technique for the calculation of optical constants from the reflection spectra using the Kramers–Kronig relation in the limited energy region in the vicinity of thresholds are discussed in detail.