  Ultrafast laser excitation can create coherent superpositions of electronic
states in molecules and trigger ultrafast flow of electron density on few- to
sub-femtosecond time scales. While recent attosecond experiments have addressed
real-time observation of these primary photochemical processes, the underlying
roles of simultaneous nuclear motions and how they modify and disturb the
valence electronic motion remain uncertain. Here, we investigate coherent
electronic-vibrational dynamics induced among multiple vibronic levels of ionic
bromine (Br$_2^+$), including both spin-orbit and valence electronic
superpositions, using attosecond transient absorption spectroscopy. Decay,
revival, and apparent frequency shifts of electronic coherences are measured
via characteristic quantum beats on the Br-$3d$ core-level absorption signals.
Quantum-mechanical simulations attribute the observed electronic decoherence to
broadened phase distributions of nuclear wave packets on anharmonic potentials.
Molecular vibronic structure is further revealed to be imprinted as discrete
progressions in electronic beat frequencies. These results provide a future
basis to interpret complex charge-migration dynamics in polyatomic systems.
