Probing Electronic Coherence with Pulse Shape Spectroscopy*

Electronic coherence decays much more rapidly in molecules than atoms as a consequence of averaging over the different rates of phase advance for different internuclear separations associated with nonlocal nuclear wave functions. We make use of an ultrafast pulse shaper to produce few cycle phase locked pulse pairs with independent control over the phase and delay between pulses and show via interference that the electronic coherence of a wave function in a molecule that undergoes internal conversion via nonadiabatic (non-Born-Oppenheimer) coupling between electronic states is maintained.  We show that this coherence can be used to control the population of different electronic states and determine the electronic coherence times for two similar molecules.