Determination of molecular anisotropy from diffraction and rotational inelastic scattering of He from C 2H 6 using two simple models

Abstract

For the scattering of He from C 2H 6 at 73 meV diffraction undulations in the angular distribution of the total signal and the inelastic energy loss from time-of-flight spectra have been measured in the range of center-of-mass angles 3° < ϑ < 65°. The data are analyzed for the anisotropy of the repulsive potential by two complementary models in which the He atoms are treated either as wave packets or as discrete particles. With a modified Fraunhofer model the observed diffraction anomalies can be attributed to a rotationally symmetric ellipsoid and provides information on the semi-axis difference a - b. A classical analysis of the energy transfer based on an impulsive model reveals scattering from the CH 3 groups. The effective mass is related by a simple formula to the anisotropy of an effective hard shell and this agrees with the Fraunhofer prediction: a - b = 1 2 r e where r e is the CC bond length.