Anisotropy of the repulsive intermolecular potential from rotationally inelastic scattering

Structure is observed in recoil velocity distributions of potassium atoms scattered inelastically from N₂ and CO molecules at CMS angles?>π/2 and collision energies 0.34≦E≦1.24 eV. It is caused by rotational excitation mainly. Individual rotational transitions are not resolved. The quasi-continuous recoil velocity distributions extend between well defined bounds, the upper corresponding to elastic scattering, the lower marking a largest amount of energy transferred into molecular rotation at given? andE. This maximal transfer increases with angle and — forE?0.64 eV — in near proportion toE. At all? andE it is larger for CO than forN ₂, amounting to about 0.42E forN ₂ and 0.64E for CO at?=150°. At or very close to each of their bounds the distributions exhibit pronounced maxima. A third intermediate maximum is present for CO, but missing for N₂. The scattering in these experiments is dominated by the repulsive core of the interaction potential. On the basis of classical scattering from a rigid, ellipsoidal, initially nonrotating potential shell, the observations can be qualitatively understood as follows: The recoil velocity dependence of the orientation averaged cross section at fixed observation angle will exhibit classical, integrable singularities at extremal amounts of rotational energy transfer. If the center of symmetry of the shell coincides with the center of mass of the molecule, there will be two such extremal transfers,ΔE=0 and a largest transfer possible at the chosen angle. If the centers do not coincide, the singularity at the largest transfer will split into two, one corresponding to the short, the other to the long “lever arm” of the shell. K + N₂ is an example of the centrical, K+CO of the excentrical case. The finite energy losses, at which the singularities occur, are — at each angle — strongly dependent on the anisotropy and excentricity of the shell.