  The direct observation of non-adiabatic dynamics at conical intersections is
a long-standing goal of molecular physics. Novel time-resolved spectroscopies
have been proposed which are sensitive to electronic coherences induced by the
passage of an excited state wavepacket through a region of conical
intersection. Here we demonstrate that inclusion of the geometric phase effect,
and its manifestations, is essential for the correct description of the
transient electronic coherences that may or may not develop. For electronic
states of different symmetry, electronic coherences are suppressed by the
geometric phase. Conversely, for states of the same symmetry, appreciable
electronic coherences are possible, but their magnitude depends on both the
topography of and direction of approach to the conical intersection. These
general results have consequences for all studies of electronic coherences at
conical intersections.
