To collect a 2D spectra at a fixed waiting time (Tw), both pump pulse delays (τ1) and probe pulse delays (τ3) needed to be scanned to frequency resolve the two axes. A normal scan for single 2D spectra with 4 fs step from 0 to 5000 fs and 27-step phase cycling at each grid point (τ1, τ3) for both axes is not practically possible. Therefore, we made several modifications for data collection scheme. First, each time axis was reduced to 44 points. This reduced total collection time at the cost of lower but acceptable frequency resolution (from 6.67 cm -1 to about 13 cm -1 (Re cation) or 22 cm -1 (protonated caffeine)). Second, a rotating frame of 1400 cm -1 (protonated caffeine) or 1900 cm -1 (Re cation) was used to phase shift the first (first pump) and last (second probe) pulse in pulse train. Briefly, a rotating frame applies an additional constant frequency phase shift to the pulses, which rotate and downshift the collected signal. The signal will therefore oscillate at a lower frequency and a larger step size can be used while avoiding undersampling.