Fully Coherent Triple Sum Frequency Spectroscopy of a Benzene Fermi Resonance

In this paper we present a new multiresonant coherent multidimensional spectroscopy (CMDS) technique employing a pathway that is both fully coherent and necessarily unique. This technique is based on a Triple Sum Frequency (TSF) coherence pathway with three excitation pulses having frequencies ω1, ω2, and ω3 and the phase matching condition k⃗1 + k⃗2 + k⃗3. Two-dimensional spectra are created by independently tuning the ω1 and ω2 pulses across vibrational resonances while monitoring the intensity of a visible output beam created by a Raman transition induced by the ω3 pulse. Two-dimensional plots of the coherent dynamics are created by independently scanning the τ21 and τ31 delay times between the different frequency excitation pulses over all time orderings. TSF CMDS separates fundamental and overtone/combination band states uniquely onto the ω1 and ω2 axes when τ21 ≠ 0. TSF is valuable in its ability to probe states of complementary parity to those seen in Doubly Vibrationally Enhanced Four-Wave Mixing (DOVE-FWM), the other fully coherent mixed electronic/vibrational CMDS method. This capability is demonstrated through the use of neat benzene as a model system, where the center of inversion imposes strict parity selection rules.