ENERGY-TRANSFER IN ACETYLENE DIMER HELIUM COLLISIONS

In a crossed molecular beams experiment differential energy loss spect ra have been measured for (C2H2)(2) + He collisions at two energies, 6 6.4 and 106.7 meV. In the backward direction up to 60% of the availabl e energy is transferred into internal degrees of freedom. Classical tr ajectory calculations based on an additive, anisotropic potential mode l show that most of the energy flows into the end-over-end-rotation of the dimer and the angular motion of the molecules against each other. The dimer stretching motion is excited to a lesser extent. The direct comparison with the measured time-of-flight spectra gives a good over all agreement, but reveals some weakness in predicting the large energ y transfer in the backward direction.