Differential cross sections for collisions of hexapole state-selected NO with He.

The first measurements of differential inelastic collision cross sections of fully state-selected NO (j=12, Omega=12, epsilon= -1) with He are presented. Full state selection is achieved by a 2 m long hexapole, which allows for a systematic study of the effect of parity conservation and breaking on the differential cross section. The collisionally excited NO molecules are detected using a resonant (1+1') REMPI ionization scheme in combination with the velocity-mapped, ion-imaging technique. The current experimental configuration minimizes the contribution of noncolliding NO molecules in other rotational states j, Omega, epsilon--that contaminates images--and allows for study of the collision process at an unprecedented level of detail. A simple method to correct ion images for collision-induced alignment is presented as well and its performance is demonstrated. The present results show a significant difference between differential cross sections for scattering into the upper and lower component of the Lambda-doublet of NO. This result cannot be due to the energy splitting between these components.