Using molecular alignment to track ultrafast collisional relaxation

Field-free molecular alignment has been used in order to track the collisional relaxation occurring in a molecular gas. CO2 molecules were initially irradiated by a short linearly polarized laser pulse resulting in the increase of their rotational energy. The evolution of the subsequent ultrafast relaxation process was optically probed after irradiating the sample with a second, weaker, short pulse leading to the alignment of the preheated molecules. Using classical molecular dynamic simulations, we were able to quantitatively reproduce the experimental shapes and amplitudes of the recorded revival transients for a time interval extending from 25 to 500 ps until thermalization of the gas sample is reached.