Evidence for hybrid classical-quantal behaviour in state- and angle-resolved atom-diatom scattering

We present measurements of the state-to-state differential scattering cross section (DCS) for X (1) Sigma(g)(+) Na-2 with Xe using a new sub-Doppler spectroscopic method and compare results with those obtained using molecular beam techniques on the same system; the agreement is good. Criteria for comparison are based on the recent finding (McCaffery A J and Wilson R J 1996 Phys. Rev. Lett. 77 48) that simple quasiclassical vector relations determine the direction of the initial relative velocity vector and the scattering angle. The incident velocity direction is such that the perpendicular component just opens the inelastic channel and the parallel component is scattered. The relationship is Newtonian but is modified by the molecule's characteristic quantum structure. Angular distributions from a range of experiments are found to obey the vector relation, suggesting that atom-molecule collisions are controlled by a quantum-modified Newtonian mechanics, the modification arising as a consequence of the internal energy level structure of the molecule. We introduce the term quantum-constrained kinematics to describe the hybrid mechanics and suggest that this may also influence other collisional events.