  Time-dependent and constituent-specific spectral changes in soft near edge
X-ray spectroscopy (XAS) of an [Fe/MgO]$_8$ metal/insulator heterostructure
upon laser excitation are analyzed at the O K-edge with picosecond time
resolution. The oxygen absorption edge of the insulator features a uniform
intensity decrease of the fine structure at elevated phononic temperatures,
which can be quantified by a simple simulation and fitting procedure presented
here. Combining X-ray absorption spectroscopy with ultrafast electron
diffraction measurements and ab initio calculations demonstrate that the
transient intensity changes in XAS can be assigned to a transient lattice
temperature. Thus, the sensitivity of transient near edge XAS to phonons is
demonstrated.
