Production and detection of m j -resolved samples of ND 3

Scattering processes can be highly sensitive to the orientation of collision partners. Preparing the colliders in an initially oriented state and probing the state-resolved orientation of collision products offers up one of the richest pictures of the underlying scattering dynamics. We have previously shown how a Stark decelerator can produce ND3 molecules oriented or aligned to an external electric field, quantified using THz transitions. Here, we investigate a more convenient approach: resonance-enhanced multiphoton ionisation with variable laser polarisation to probe the mj-composition of molecules emerging from the decelerator. We derive the two-photon absorption polarisation sensitivity, generalising the work of Mo and Suzuki [J. Chem. Phys. 109, 4691 (1998)] to include elliptical polarizations. With this framework, we identify transitions maximally sensitive to the mj-composition, enabling high-fidelity probing of mj-populations. We show that j|k|±=11− ND3 molecules can be prepared selectively in mj = 1 or 0 levels with near-unity purity across a wide velocity range. Together, this work represents a crucial step towards performing scattering measurements with velocity- and orientation-control over molecules prior to the collision, while probing how the degree of orientation evolves during the collision process.