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.