Measurement of molecular-frame electronic nonlinear response in aligned carbon dioxide and nitrogen molecules

We report the measurement of the molecular frame third-order nonlinear response tensor (second hyper-polarizability) in impulsively aligned carbon dioxide and nitrogen molecules. A moderately strong femtosecond near-infrared laser pulse is used to impulsively align gas molecules, and a set of weaker femtosecond pulses subsequently probe the nonlinear response using non-degenerate four-wave mixing (N-DFWM). The emitted electric field is directly measured, using a spectral interferometry technique for measuring ultraweak femtosecond pulses, called TADPOLE. The measured amplitude and phase of the signal electric field is combined to get the real and imaginary parts of the signal in a single measurement. By suppressing the rotational and ionization grating contributions, N-DFWM exclusively measures the electronic-only nonlinearity. By analyzing the nonlinear signal as a function of time delay between the alignment pulse and the four-wave mixing pulses, the molecular frame nonlinear response is extracted using the previously established technique of Orientation Resolution through Rotational Coherence Spectroscopy (ORRCS). Using the transformation properties of a tensor under rotation, a few tensor components measured as a function of the molecular alignment angle can be converted into multiple tensor components for a fixed molecular orientation. Knowledge of these multiple tensor components are used to distinguish the symmetries of the ground state electronic character in different molecules. We discuss these results for carbon dioxide and nitrogen. The roadmap for extending these nonlinear response tensor measurements to study ultrafast dynamics on electronically excited states in molecules is discussed.