To collect transient absorption spectra, a pump pulse preceding the probe pair was generated with the pulse shaper (Fig. ). At each pump-probe waiting time Tw between the pump pulse and first probe pulse, the probe pair delay time τ was scanned and modulations in the N2 tag-loss signal were monitored. The pulse interactions act to modulate population in the vibrational excited states as a function of the delay times Tw and τ. The measured tag-loss signal is directly proportional to the excited vibrational population following the final pulse interaction. A potential issue using the messenger tag method is the timescale on which tag loss occurs. The tag-loss mechanism proceeds through intramolecular vibrational redistribution (IVR), which will depend on the coupling between the excited vibrational modes and lower-frequency modes, and, ultimately, the low-frequency ion-tag intermolecular vibrational mode. IVR typically occurs on the tens of picoseconds to nanosecond timescale in the gas phase. Tag-loss effects between the pulse delays, therefore, are not expected to contribute in the current experiments.