  X-ray Free-Electron Lasers (XFELs) deliver X-ray pulses with a coherent flux
that is approximately eight orders of magnitude greater than that available
from a modern third generation synchrotron source. The power density in an XFEL
pulse may be so high that it can modify the electronic properties of a sample
on a femtosecond timescale. Exploration of the interaction of intense coherent
X-ray pulses and matter is of both intrinsic scientific interest, and of
critical importance to the interpretation of experiments that probe the
structures of materials using high-brightness femtosecond XFEL pulses. In this
letter, we report observations of the diffraction of an extremely intense 32 fs
nanofocused X-ray pulses by a powder sample of crystalline C60. We find that
the diffraction pattern at the highest available incident power exhibits
significant structural signatures that are absent in data obtained at both
third-generation synchrotron sources or from XFEL sources operating at low
output power. These signatures are consistent with a highly ordered structure
that does not correspond with any previously known phase of crystalline C60. We
argue that these data indicate the formation of a transient phase that is
formed by a dynamic electronic distortion induced by inner-shell ionisation of
at least one carbon atom in each C60 molecule.
