  Long-lived oscillations in 2D spectra of chlorophylls are at the heart of an
ongoing debate. Their physical origin is either a multi-pigment effect, such as
excitonic coherence, or primarily stems from localized vibrations. In the
present work, we analyze distinguishing characteristics of relative phase
difference measured between diagonal- and cross-peak oscillations. While direct
discrimination between the two scenarios is obscured when peaks overlap, their
sensitivity to temperature provides a stronger argument. We show that
vibrational (vibronic) oscillations change relative phase with temperature,
while electronic oscillations are only weakly dependent. This highlights that
studies of relative phase difference as a function of temperature provide a
clear and easily accessible method to distinguish between vibrational and
electronic coherences.

Chemical Physics