Pathway-specific nonlinear vibrational action spectroscopy with mixed frequency-time domain pulse shaping

Nonlinear spectroscopies utilizing ultrafast, broadband laser pulses are now widely used techniques for the study of molecular structures, interactions, and dynamics with ever increasing molecular specificity. Broadband laser pulses, however, result in signals produced by many overlapping nonlinear pathways that can be challenging to separate. To overcome this, we introduce a mixed time-frequency domain pulse-shaping approach that uses phase-controlled, Boxcar-frequency-filtered pulses to isolate specific nonlinear pathways in a vibrational action-spectroscopy framework. By filtering each pulse to excite a single normal mode and exploiting narrow transitions of cryogenically cooled gas-phase molecular ions, we selectively prepare and detect rephasing, nonrephasing, and two-quantum coherence pathways, the latter of which provides anharmonic information that is typically inaccessible in action-based nonlinear experiments. This strategy establishes a clear and simplified platform for resolving pathway-specific dynamics, laying the foundation for future studies of higher-order quantum control in complex molecular systems in both gas and condensed phases.