Detection of collision-induced rotational alignment in counterpropagating beam scattering

Collision-induced rotational alignment of NH₃ scattered from He is detected in a counterpropagating pulsed molecular beam scattering experiment. Starting from an isotope distribution of the initial rotational angular momentum, the generation of a highly aligned product ensemble due to inelastic scattering is observed. For all probed state-resolved differential cross sections the alignment changes from negative values for backward scattering to positive values at small deflection angles which is in qualitative agreement with predictions from a geometric apse model. The degree of alignment strongly depends on the final rotational state, indicating a correlation with the involved energy transfer.