  Two-photon excitation (TPE) proceeds via a "virtual" pathway, which depends
on the accessibility of one or more intermediate states, and, in the case of
non-centrosymmetric molecules, an additional "dipole" pathway, involving the
change in the permanent dipole moment. Here, we control the quantum
interference between these two optical excitation pathways by using
phase-shaped femtosecond laser pulses. We find enhancements by a factor of up
to 1.75 in the two-photon-excited fluorescence of the photobase FR0-SB in
methanol after taking into account the longer pulse duration of the shaped
laser pulses. Simulations taking into account the different responses of the
virtual and dipole pathways to external fields and the effect of pulse shaping
on two-photon transitions are found to be in good agreement with our
experimental measurements. The observed quantum control of TPE in condensed
phase may lead to enhanced signal at a lower intensity in two-photon
microscopy, multiphoton-excited photoreagents, and novel spectroscopic
techniques that are sensitive to the magnitude of the contributions from the
virtual and dipole pathways to multiphoton excitations.
