Scattering of thermal He beams by crossed atomic and molecular beams. II. The He–Ar van der Waals potential

Abstract

Differential cross sections for He-Ar scattering at room temperature have been measured. The experimental consistency of these measurements with others performed in different laboratories is demonstrated. Despite this consistency, the present van der Waals well depth of 1.78 meV, accurate to 10%, is smaller by 20% to 50% than the experimental values obtained previously. These discrepancies are caused by differences between the assumed mathematical forms or between the assumed dispersion coefficients of the potentials used in the present paper and those of previous studies. Independent investigations have shown that the previous assumptions are inappropriate for providing accurate potentials from fits to experimental differential cross section data for He-Ar. We use two forms free of this inadequacy in the present analysis: a modified version of the Simons-Parr-Finlan-Dunham (SPFD) potential, and a double Morse-van der Waals (M 2 SV) type of parameterization. The resulting He-Ar potentials are shown to be equal to within experimental error, throughout the range of interatomic distances to which the scattering data are sensitive. The SPFD or M 2 SV potentials are combined with a repulsive potential previously determined exclusively from fits to gas phase bulk properties. The resulting potentials, valid over the extended range of interatomic distances r $ 2.4 A., are able to reproduce all these bulk properties quite well, without adversely affecting the quality of the fits to the DCS.

Scattering of thermal He beams by crossed atomic and molecular beams. IV. Spherically symmetric intermolecular potentials for He+CH4, NH3, H2O, SF6

“…6. 4,20 Although some of the dispersion coefficients have been calculated for the interactions of the three lighter molecular partners with He, 25 • 26 we felt it best to fit this region of the potential empirically, for reasons discussed below. By fixing c 8 = c 10 =0 in Eqs.…”

“…The crossed molecular beam apparatus used in this investigation has been described previously, 4 • 8 c,lz and only its major components are discussed here. The modulated He primary beam comes from a room-temperature nozzle source, and is angularly well collimated in two stages of differential pumping.…”

“…1 and 2 has been described previously. 4 It consists of a standard weighted nonlinear least-squares optimization 16 of the parameters of mathematical functions chosen to represent the intermolecular potential, as discussed in Sec. Ill. As in previous work, appropriate velocity and angle averaging is included in the…”

“…3 c· 4 • 5 Potentials for such systems are now well characterized, enabling experimental data of many different kinds to be correlated and accurately described. 4 • 6 • 7 However, relatively little is known about atom-molecule or molecule-molecule van der Waals potentials, especially with regard to their orientation dependence. a-u…”

“…The first three molecules represent an isoelectronic series, progressing from nonpolar CH 4 to the highly polar NH 3 and H 2 0. The He+ SF 6 system is an example of the interaction between an atom and a highly symmetric large molecule.…”