Excitingly, the bleach signals from both features oscillate with pump-probe waiting time with a period of ~400 fs. This oscillation period is consistent with the difference frequency between the two vibrational peaks (80 cm -1 , 417 fs). This observation suggests the presence of coherent coupling between the symmetric and asymmetric carbonyl stretch pair. Metal-carbonyl complexes have been well characterized in the condensed phase using ultrafast transient and 2D IR spectroscopies. Symmetric and asymmetric carbonyl stretch pairs in these systems yield relatively strong off-diagonal cross-peak signals. In transient absorption experiments, where only the probe axis is resolved, the measured signals are the sum of the contributions from the diagonal pathways depicted in Fig. and those from the cross-peak pathways. Rephasing cross-peak bleach pathways for a coupled two-oscillator system are depicted in Fig. , with the complete set of pathways given in Fig. . Numerous cross-peak pathways involve states |A⟩⟨S| during the waiting time Tw, where A denotes asymmetric and S symmetric stretch, respectively. These states are generated by the pump interactions, where the first pump interaction excites the asymmetric stretch and the second pump interaction excites the symmetric stretch, or vice versa. During the waiting time Tw, these states evolves at the difference frequency ωAS between the coupled oscillators, e -iω AS T w . Indeed, strong oscillations with periods of 1 2πω AS ⁄ have been reported in condensed-phase transient and 2D IR spectra of metal-carbonyl complexes. The intensity evolution of the C≡O peaks across all waiting times collected is presented in Fig. . Importantly, the largest signal modulations were observed at pump-probe waiting times Tw corresponding to the expected 417 fs period. This observation confirms that the recorded N2 tag-loss action signals derive from nonlinear pathways and that underlying cross-peak signals are present. We note that the cross peaks in fac-Re(CO)3(CH3CN)3 + are expected to be much stronger in the ZZYY polarization scheme (when pump and probe pulses have perpendicular polarizations) since the symmetric and asymmetric carbonyl stretches have orthogonal transition moments. The current set-up, however, is limited to the ZZZZ (parallel polarization) scheme. The bleach signals also appear to show a small overall decay in signal with waiting time Tw. In non-polar solvents, carbonyl stretches in metal complexes exhibit very long vibrational lifetimes (10's ps). With no solvent present in the gas phase (except for the single N2 tag) and very few low-frequency modes to couple to, the bleach signals for fac-Re(CO)3(CH3CN)3 + are expected to show minimal decay dynamics throughout the measured waiting time series. Without solvent, the ground state of isolated molecular ions will never be repopulated and the ions will remain vibrationally hot following excitation. Shifts in vibrational frequencies might be observed in species where transitions exist that are highly sensitive to excitation of lower-frequency modes via IVR. ESA transitions would be the direct and ideal reporters of vibrational lifetime dynamics. Given that the ESA2 pathway ends in the |2⟩⟨2| population state while all other pathways end in |1⟩⟨1|, this path could be isolated if a strong-binding tag or a multiple tag-loss mass channel are utilized which require energy greater than ħω10 to occur. Measured vibrational dynamics are also limited to the tag-loss timescale. Given typical IVR timescales, interesting vibrational dynamics should still be accessible using the tag-loss method. One intriguing possibility is to measure rotational reorientation dynamics as a method to elucidating the structures of large molecular systems.