Thermoreflectance and temperature dependence of theL2,3soft-x-ray threshold in Si

We report thermoreflectance and temperature-dependent reflectance data for the 100-eV $L_{2,3}$ soft-x-ray threshold of Si. The thermoreflectance data show that the threshold energy shift with temperature accounts for nearly all features of the observed line shape. Only minor contributions arise from changes in broadening parameters. The reflectance data show that the total shift of the $j=\frac{3}{2}$ threshold to lower energy as the temperature is increased from 150 to 600 K is surprisingly large, 250±50 meV. The major contribution to this shift appears to be the change of the electrostatic potential at the $2p$ core sites resulting from the reduction of bond charge with increasing temperature. This large temperature dependence is incompatible with the lattice-mode model, which could explain the data only if a single core hole could destroy $2e$ of bond charge.