  We report a conceptually straightforward witness that isolates coherent
electronic oscillations from their vibronic counterparts in nonlinear optical
spectra of molecular aggregates: Coherent oscillations as a function of waiting
time in broadband pump/broadband probe spectra correspond to coherent
electronic oscillations. Oscillations in individual peaks of 2D electronic
spectra do not necessarily yield this conclusion. Our witness is simpler to
implement than quantum process tomography and potentially resolves a
long-standing controversy on the character of oscillations in ultrafast spectra
of photosynthetic light harvesting systems.
