Scattering of High-Velocity Neutral Particles. XII. He–CH4; He–CF4. CH4–CH4 and CF4–CF4 Interactions

Abstract

Collision cross sections have been measured for helium atoms with energies between 500 and 2100 ev, scattered in room-temperature CH4 and CF4. The results have been analyzed to obtain the average potential between a helium atom and a CH4 molecule, 〈V(r)〉Av=602/r9.43ev,for r between 1.92 and 2.37 A, and the average potential between a helium atom and a CF4 molecule, 〈V(r)〉Av=6.18×106/r17.51ev,for r between 2.43 and 2.74 A. This He–CH4 potential is consistent with one valid at larger separation distances which has been obtained by combining potentials derived from high-temperature viscosity and second virial coefficients of helium, and from high-temperature viscosity coefficients of methane. A procedure which assumes the centers of force to reside in the peripheral H or F atoms has been used to analyze these average atom-molecule interactions in terms of the effective He–H and He–F interatomic potentials. These interatomic potentials have been combined with the potential previously determined for He–He to obtain effective H–H and F–F potentials for atoms in different CH4 and CF4 molecules. These effective interatomic potentials were then summed and averaged over all molecular orientations to yield the average potential between two CH4 molecules, 〈V(r)〉Av=5.64×106/r15.47ev,for r between 2.47 and 3.06 A, and the average potential between two CF4 molecules, 〈V(r)〉Av=1.17×1022/r39.27ev,for r between 3.43 and 3.77 A. This CH4–CH4 potential is consistent with one valid at larger separation distances which has been derived from measurements on gaseous viscosity at high temperatures.