Long Lived Electronic Coherences in Molecules Studied with Pulse Shape Spectroscopy

Electronic coherences tend to decay much more rapidly in molecules than in atoms as a consequence of averaging over the different rates of phase advance for different internuclear separations associated with non-local nuclear wave functions. However, pairs of states whose potential energy surfaces are parallel can exhibit long lived electronic coherences. We use few cycle pulses from an ultrafast pulse shaper to study such long lived coherences. Our experiments measure the photoelectron spectrum as a function of pulse shape, which show clear modulations in the yield as a result of interfering contributions from excited states. We interpret our measurements using strong-field ionization dynamics calculations, which include multiphoton resonance, dynamic Stark shifts, as well as vibrational dynamics during the ionization process. The calculations illuminate how pairs of nearly parallel excited states maintain electronic coherence for hundreds of femtoseconds.