RIXS reveals sub-20-femtosecond lattice relaxation around two core excitons in MgO and MgF₂

Soft X-ray emission at the Mg L_2,3 edge of MgO and MgF₂ reveals weak continuous energy-loss tails (ELTs) adjacent to the elastic scattering lines. These ELTs are attributed to lattice relaxation around core excitons. This effect is observed at two distinct resonances, peaking at 54.4 eV and 57.0 eV, and assigned to s- and d-symmetry core excitons, respectively. The maximum ELT widths are about 2 eV and 5 eV, respectively. Resonant inelastic X-ray scattering (RIXS) enabled the identification of this otherwise dark d-exciton in both systems. The total fluorescence yield (TFY) measurements of MgF₂ revealed a well-resolved and Lorentzian broadened spin-orbit split L_2,3 doublet at 54.4 eV. The lifetime broadening extracted from the MgF₂ TFY spectrum indicates a 17 fs lifetime for the bright s-exciton. In contrast, the same s-exciton state in MgO is not resolved in the TFY spectrum, even though clear signatures of lattice relaxation are observed in the RIXS data. The short lifetimes of these excitonic states strongly suppress the high-energy-loss region of the ELTs compared with previously reported systems. Consistently, the RIXS spectral profile of MgO can be reproduced within a simple vibronic-coupling model, provided that a short lifetime in the range of 13-20 fs is assumed.