STATE-RESOLVED DIFFERENTIAL CROSS-SECTIONS FOR CROSSED-BEAM AR-NO INELASTIC-SCATTERING BY DIRECT ION IMAGING

State-to-state differential cross sections for inelastic collisions of NO with Ar have been measured in a crossed-beam experiment using time-of-flight ion imaging. Rotational rainbow peaks are observed in the angular distributions, and these move to backward scattering angles with increasing final rotational level. The images are analyzed using a Monte Carlo forward convolution program that accounts for the transformation from the center-of-mass differential cross sections to the experimental image. The results are interpreted using a simple two-dimensional hard ellipse model to provide quantitative insight into the anisotropy of the potential energy surface. For NO (j' = 18.5), two rainbow peaks are observed. These double rainbows have been predicted for scattering of atoms from heteronuclear molecules, but they have not previously been directly observed in the angular distributions. The analysis is also used to obtain the eccentricity of the hard ellipse potential from the positions of the two rainbow peaks. The angular distributions for the spin-orbit conserving collisions and spin-orbit changing collisions are remarkably similar, even though they were thought to involve two different potential energy surfaces. An alternative mechanism is proposed to account for the spin-orbit changing collisions through non-Born-Oppenheimer coupling of nuclear and electronic motion.