  Coupling of nuclear and electronic degrees of freedom mediates energy flow in
molecules after optical excitation. The associated coherent dynamics in
polyatomic systems, however, remain experimentally unexplored. Here, we
combined transient absorption spectroscopy with electronic population control
to reveal nuclear wavepacket dynamics during the S2-S1 internal conversion in
beta-carotene. We show that passage through a conical intersection is
vibrationally coherent and thereby provides direct feedback on the role of
different vibrational coordinates in the breakdown of the Born-Oppenheimer
approximation.
