Impulsive Laser Induced Alignment of Molecules Dissolved in Helium Nanodroplets

We report the experimental observation of revival structures in the alignment of a ground-state rotational wave packet following nonresonant excitation of I2 molecules by an intense picosecond laser pulse. The revivals appear at characteristic time delays following the excitation by the pump laser pulse, and show a significant narrowing of the angular distribution during a few picoseconds. The interaction with the pump laser also leads to a steady-state alignment of the molecule, due to rotational pumping.

Random orientation of molecules within a sample leads to blurred observations of chemical reactions studied from the laboratory perspective. Methods developed for the dynamic imaging of molecular structures and processes struggle with this, as measurements are optimally made in the molecular frame. We used laser alignment to transiently fix carbon disulfide molecules in space long enough to elucidate, in the molecular reference frame, details of ultrafast electronic-vibrational dynamics during a photochemical reaction. These three-dimensional photoelectron imaging results, combined with ongoing efforts in molecular alignment and orientation, presage a wide range of insights obtainable from time-resolved studies in the molecular frame.