While time-resolved photofragmentation signals have been measured on gaseous molecular ions to track reaction kinetics and excited-state lifetimes, frequency-resolved action-based ultrafast spectroscopies of isolated molecular ions has yet to be demonstrated. Extension of ultrafast nonlinear spectroscopies to molecular ions is highly desirable, as well-established mass spectrometric techniques can be used to precisely generate, manipulate, and composition-select the desired chemical species of interest. Further, soft ionization methods allow for the isolation of more diverse chemical and biological systems, providing more direct comparisons to condensed-phase studies. The very low density (<10 6 cm -3 ) of ions that can be stored, however, makes the detection of time-domain modulations in weak nonlinear signals a grand challenge to overcome. Towards this goal, we recently demonstrated the collection of linear cryogenic ion vibrational spectra using an ultrafast IR pulse pair by monitoring photodissociation of a weakly-bound N2 messenger "tag" molecule. The Fourier transform of the time-domain tag-loss interferogram, which derived from interferences in the excited vibrational coherences, yielded the linear frequency-domain spectrum. We also demonstrated the presence of detectable nonlinear signal, but did not frequency-resolve the response.