  Soft x-rays were applied to induce graphitization of diamond through a
non-thermal solid-to-solid phase transition. This process was observed within
poly-crystalline diamond with a time-resolved experiment using ultrashort soft
x-ray pulses of duration 52.5 fs and cross correlated by an optical pulse of
duration 32.8 fs. This scheme enabled for the first time the measurement of a
phase transition on a timescale of ~150 fs. Excellent agreement between
experiment and theoretical predictions was found, using a dedicated code that
followed the non-equilibrium evolution of the irradiated diamond including all
transient electronic and structural changes. These observations confirm that
soft x-rays can induce a non-thermal ultrafast solid-to-solid phase transition
on a hundred femtosecond timescale.
