Femtosecond X-ray Laser induced transient electronic phase change observed in fullerene C60

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.