Feasibility Study of Thin Film Surface Analysis Using Synchrotron Low-Angle Incidence Conversion Electron Mössbauer Spectroscopy

A feasibility study was conducted on thin film surface analysis using conversion electron Mössbauer spectroscopy (CEMS) measured at incident angles more than the critical angle and less than several degrees, utilizing a synchrotron Mössbauer source. To identify the minimum incident angle yielding a CEMS spectrum with experimentally negligible nuclear specular reflection effects, γ-ray reflection and CEMS of a ⁵⁷Fe [90%] enriched thin film were studied by grazing-incidence synchrotron Mössbauer spectroscopy, varying the incident angle in the range of 0.3 to 3°. Since the incident angles were larger than the critical angle (θ_c ∼ 0.22°) of the iron film for 14.4 keV X-rays, the photons passing through the nuclear resonant channel, rather than the electronic non-resonant channel, predominantly contributed to the specular reflection. Consequently, at θ_in = 0.3°, strong scattering peaks appeared at nuclear resonance energies, and then CEMS showed an anomalous spectral profile. At θ_in > θ_c, the nuclear specular reflection gradually decreased with increasing θ_in, whereas nuclear resonance peaks were persistently observed up to θ_in = 1.5°. Finally, the reflection signals were nearly extinguished at around θ_in = 3°, and the CEMS showed an experimentally normal spectral profile in usual use. Low-angle incidence CEMS with negligible nuclear specular reflection is advantageous for surface analysis of thin films because the penetration depth of γ-rays is still shorter than the conversion electron escape depth. Furthermore, a surface analysis of an annealed ⁵⁷Fe-rich film was conducted using low-angle incidence CEMS with circularly polarized synchrotron Mössbauer source. The present paper also describes the theoretical background and additional benefits in instrument development.