. However, because the coherence direction of the rephasing pathway is reversed compared to the nonrephasing pathway, i    will flip its sign during measurement when the negative frequency of the rephasing pathway becomes positive. Therefore, the rephasing and nonrephasing spectra can be collected together without resulting in phase mismatch errors. If it is desirable to collect the rephasing and nonrephasing spectra independently, one can adopt different phase cycling schemes, including the four-frame phase cycling scheme described in Section S1.1 of the Supporting Information. The probe pulse is split into three pulses using the front and back reflections from an uncoated 3° CaF2 wedge: E3 (transmitted, 95%), the tracer (reflected, 2.5%), and the LO (reflected, 2.5%). A mechanical delay stage (ANT-95 Aerotech Inc) controls the waiting time (t2) between the pump and probe pulses. The two pump pulse pairs (E1 and E2; E1' and E2'), E3, and the tracer are focused onto the sample area by a 10-cm off-axis parabolic mirror. The tracer beam follows the path of the generated photon echo signal from the sample and is blocked during data collection in the BoxCARS geometry. The photon echo signal and LO field interfere at a ZnSe BS to recover the amplitude and phase of the signal. The LO timing is controlled via a mechanical stage (MPS-50SL, Aerotech Inc) with a 3° CaF2 wedge paired with a stationary CaF2 wedge for fine control with femtosecond resolution and a manual stage for coarse timing. The heterodyned signal and LO are detected simultaneously on a 128-pixel × 128-pixel IR camera (Catalina, Teledyne Nova Sensors) and the detection area is binned vertically. This hybrid BoxCARS 2D IR setup can be easily transformed to the pump-probe geometry by blocking one pump arm, E3, and the LO, while using the tracer beam as the probe.