  Collisions between cold polar molecules represent a fascinating research
frontier, but have proven hard to probe experimentally. We report measurements
of inelastic cross sections for collisions between NO and ND 3 molecules at
energies between 0.1 and 580 cm-1 , with full quantum state resolution. At
energies below the 100 cm-1 well depth of the interaction potential, we
observed backward glories originating from peculiar U-turn trajectories. At
energies below 0.2 cm-1, we observed a breakdown of the Langevin capture model,
which we interpreted in terms of a suppressed mutual polarization during the
collision, effectively switching off the molecular dipole moments. Scattering
calculations based on an ab initio NO-ND3 potential energy surface revealed the
crucial role of near-degenerate rotational levels with opposite parity in
low-energy dipolar collisions.
