  We show that time-resolved x-ray scattering from molecules prepared in a
superposition of electronic states moving through an avoided crossing has new
features not found in diffraction from the corresponding classical mixed state.
Photoabsorption in molecular iodine at 520 nm produces a superposition of two
dipole-allowed nearly degenerate electronic states, which interact due to
non-adiabatic coupling. We show experimental evidence that the mixing of the
nuclear wavepackets from the two electronic states at the avoided crossing
leads to ultrafast changes in the angular composition of the scattering
pattern. This provides a novel means to study transitions in excited molecular
systems. We reconstruct a movie of the nuclear probability density arising from
this interference.
