To further demonstrate that the 2D-CIVS spectra are indeed capturing nonlinear response pathways, we take a more detailed look at the asymmetric stretch bleach feature at Tw = 417 fs in Fig. . Lineshapes in 2D spectroscopies derive from both absorptive and dispersive components, leading to "phase twists" that contain positive and negative contributions. In addition, nonlinear signals derive from so-called rephasing and nonrephasing pathways. Rephasing (or photon echo) pathways are those where the coherences during τ1 and τ3 evolve in the opposite sense, with signal proportional to e -iω 10 (τ 3 -τ 1 ) . Nonrephasing pathways evolve as e -iω 10 (τ 3 +τ 1 ) . Feynman diagrams illustrating these pathways are provided in Supporting Information. Importantly, rephasing and nonrephasing signals produce lineshapes with opposite phase twists. Adding or simultaneously collecting the rephasing and nonrephasing signals produces purely absorptive lineshapes. The 27-step phase-cycling scheme allows the absorptive, rephasing, and nonrephasing signals to be isolated from the same data set, the real components of which are presented in Figs. and, respectively, for the asymmetric carbonyl stretch. The respective imaginary components are presented in Supporting Information. The rephasing and nonrephasing spectra for both the real and imaginary components display the appropriate phase twists 14,49,50 due to positive (blue) and negative (red) contributions to the lineshapes. These comparisons unequivocally show that the measured tag-loss photodissociation signal encodes the desired nonlinear light-matter interaction pathways. Conspicuously absent from the absorptive 2D-CIVS spectra are positive induced absorption peaks deriving from overtone (diagonal) and combination band (off-diagonal) transitions that are hallmarks of condensed-phase experiments. The absence of induced absorption transitions is a consequence of the action-detection scheme. The action-response signal is directly proportional to the final excited-state population that causes the action. The requirement that the final state of the system be a population state necessitates the use of an additional probe pulse in action schemes. In condensed-phase experiments, the system returns to a population state after emitting a signal field, which is directly detected, following excitation by a single probe pulse. Therefore, while conventional condensed-phase approaches are 3 rd -order experiments, action-based methods are actually 4 th -order experiments. This important difference results in a unique set of light-matter interaction pathways in action-detection experiments, the Feynman diagrams for which are given in Supporting Information. While action experiments result in pathways analogous to the 3 rd -order ground-state bleach, stimulated emission, and induced absorption pathways, there is an additional excited-state pathway not accessible in 3 rd -order experiments. This new pathway, which corresponds to a bleach of the excited state, destructively interferes with the induced absorption pathway. As a result, only bleaching signals are expected in action-based TA and 2D spectra. Although important molecular and dynamical information provided by induced absorption transitions are unavailable, the presence of bleach-induced absorption peak pairs in condensed-phase measurements often leads to significant spectral congestion in 2D IR spectra. This is particularly true for large molecules, like peptides, in the important fingerprint region. The lack of induced absorption features, along with the much sharper spectral transitions generally obtained with CIVS, could greatly aid in the interpretation of 2D spectra. As an example, the 2D-CIVS spectrum of protonated caffeine (C8H10N4O2H + ) in the 1650-1850 cm -1 region is presented in Fig. . The linear spectrum is given in Fig. along with the calculated stick spectrum. There are three closely spaced transitions: the stretching modes of the two carbonyl groups and a mixed NH + bending/CN stretching mode. Note that the two carbonyl stretches in neutral caffeine produce an asymmetric/symmetric stretch normal mode pair that results in strong 2D IR cross peak signals in solution. The calculated displacement vectors shown in Fig. , however, indicate that protonation acts to largely decouple the carbonyl stretch transitions. The NH + bend/CN stretch normal mode also appears mostly isolated from the carbonyl stretches. Despite the expectation of weaker coupling compared to neutral caffeine, the 2D-CIVS spectrum of protonated caffeine reveals relatively strong cross-peak signals between all three transitions at both 200 fs (Fig. ) and 3 ps (Fig. ) waiting times. This observation indicates the presence of anharmonic coupling between the three transitions, not unlike the coupling observed in condensed-phase 2D IR between amide I (CO stretch) and amide II (NH bend/CN stretch) modes in peptides. Importantly, all 2D bleach features are well resolved and free from significant spectral overlap and lineshape distortions.