The ultrafast 2D IR optical setup has been described in detail previously . In brief, a 1 kHz Ti:sapphire regenerative amplifier (Astrella, Coherent Inc.) was used to pump an OPA/DFG unit (Light Conversion Inc.) to generate 100-fs mid-IR pulses centered at 5.8 µm. The delay between the two pump pulses was generated using a Ge-based pulse shaper (Quickshape, PhaseTech Inc.). The time delay between the pump pulses (t 1 ) was numerically Fourier transformed to generate the pump-frequency axis (ω pump cm -1 ). The pump and probe pulses were overlapped onto the sample in a pump-probe geometry, after which the probe pulse spectrum was measured using a spectrograph equipped with a 128-pixel MCT array (Teledyne), and the probe frequency (ω probe cm -1 ) was measured directly in the frequency domain. The delay between the two pump pulses was scanned from 0 to 3000 fs in 20 fs steps. The delay time between the pump and probe pulses (t 2 ) was measured from 150 fs to 2000 fs, with 11 times in total. Each spectrum was averaged over 5 million laser shots to obtain cleaner background. In addition to phase cycling, all spectra were measured under perpendicular polarization condition to minimize scattered light at the detector. An illustration of the pump-probe 2D IR pulse configuration and measurements is shown in Figure .