Electric field-resolved nonlinear spectroscopy of aligned molecules

We report the measurement of third-order nonlinear electronic response from impulsively aligned molecules. A moderately strong femtosecond near-infrared laser pulse is used to impulsively align gas-phase molecules, and a set of weaker femtosecond pulses subsequently probe the nonlinear response using degenerate four-wave mixing (DFWM). We show that by measuring both the amplitude and phase of the emitted electric field, additional information about the excited rotational wavepacket can be accessed. By comparing these measurements from two linear molecules, nitrogen and carbon dioxide, we show that our measurement contains information about the electronic symmetries of the molecules, beyond their rotational response which is generally similar for all linear molecules. The emitted electric field is measured using an interferometry technique for measuring ultraweak femtosecond pulses, called TADPOLE, which is further enhanced using a lock-in detection-based spectrometer. We then discuss similar measurements on electronically excited molecules to study their ultrafast dynamics after VUV excitation.