  The element specificity of soft X-ray spectroscopy makes it an ideal tool for
analyzing the microscopic origin of ultrafast dynamics induced by localized
optical excitation in metal-insulator heterostructures. Using [Fe/MgO]$_n$ as a
model system, we perform ultraviolet pump/soft X-ray probe experiments, which
are sensitive to all constituents of these heterostructures, to probe both
electronic and lattice excitations. Complementary ultrafast electron
diffraction experiments independently analyze the lattice dynamics of the Fe
constituent, and together with ab initio calculations yield comprehensive
insight into the microscopic processes leading to local relaxation within a
single constituent or non-local relaxation between two constituents. Besides
electronic excitations in Fe, which are monitored at the Fe L$_3$ absorption
edge and relax within 1 ps by electron-phonon coupling, soft X-ray analysis
identifies a change at the oxygen K absorption edge of the MgO layers which
occurs within 0.5 ps. This ultrafast energy transfer across the Fe-MgO
interface is mediated by high-frequency, interface vibrational modes, which are
excited by hot electrons in Fe and couple to vibrations in MgO in a
mode-selective, non-thermal manner. A second, slower timescale is identified at
the oxygen K pre-edge and the Fe L$_3$ edge. The slower process represents
energy transfer by acoustic phonons and contributes to thermalization of the
entire heterostructure. We thus find that the interfacial energy transfer is
associated with non-equilibrium behavior in the phonon system. Because our
experiments lack signatures of charge transfer across the interface, we
conclude that phonon-mediated processes dominate the competition of electronic
and lattice excitations in these non-local, non-equilibrium dynamics.
