Molecular Orientation-Induced Second-Harmonic Generation: Deciphering Different Contributions Apart.

We demonstrate and explore an all-optical technique for direct monitoring of the orientation dynamics in gas-phase molecular ensembles. The technique termed "MOISH" utilizes the transiently lifted inversion symmetry of polar gas media and provides a sensitive and spatially localized probing of the second-harmonic generation signal that is directly correlated with the orientation of the gas. Our experimental results reveal selective electronic and nuclear dynamical contributions to the overall nonlinear optical signal and decipher them apart using the "reporter gas" approach. "MOISH" provides new crucial means for implementing advanced coherent rotational control via concerted excitation by both terahertz and optical fields.