  Transient structural changes of Al$_2$O$_3$ on subatomic length scales
following irradiation with an intense x-ray laser pulse (photon energy: 8.70
keV; pulse duration: 6 fs; fluence: 8$\times$10$^2$ J/cm$^{2}$) have been
investigated by using an x-ray pump x-ray probe technique. The measurement
reveals that aluminum and oxygen atoms remain in their original positions by
$\sim$20 fs after the intensity maximum of the pump pulse, followed by
directional atomic displacements at the fixed unit cell parameters. By
comparing the experimental results and theoretical simulations, we interpret
that electron excitation and relaxation triggered by the pump pulse modifies
the potential energy surface and drives the directional atomic displacements.
Our results indicate that high-resolution x-ray structural analysis with the
accuracy of 0.01 \AA is feasible even with intense x-ray pulses by making the
pulse duration shorter than the timescale needed to complete electron
excitation and relaxation processes, which usually take up to a few tens of
femtoseconds.
