The XUV Spectra of KMgF₃ and NaMgF₃

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By using synchrotron radiation as the light source, the near-normal incidence reflection spectra of KMgF₃ and NaMgF₃ were measured in the XUV region, from which the spectra of imaginary part of the dielectric constant were obtained by means of the Kramers-Kronig transformation. The largest sharp peak observed at 11.9 eV in KMgF₃ was assigned to Γ₁₅(upper)→Γ₁ exciton (A exciton) and the small hump at 13.5 eV to the Γ₁₅(lower)→Γ₁ exciton (B exciton). The line shapes of the A and B excitons were calculated theoretically by taking into account the interference effects between the A and B channels. It was found that the interchannel mixing due to the electron-hole Coulomb and exchange interactions has an important effect on the experimentally observed line shapes.