High-resolution differential cross section (DCS) and accurate new limiting diffusion measurements for all the unlike-pair He+rare-gas systems are combined in constructing new multiproperty interatomic potentials. The new potentials predict most properties available for these systems, including independent high-resolution DCS measurements. Remaining discrepancies with earlier multiproperty potentials for HeKr and HeXe are attributed to incompatibilities among data sets used in the multiproperty fitting procedure. It is also shown that the 5% difference in well depths between two recently proposed potentials for HeXe is due to some of the data used in constructing these potentials, and that the DCS measurements of those studies are mutually consistent. Finally, the present potentials are refined slightly for agreement with high-energy cross section measurements. At the present level of reliability for DCS and dilute-gas data, it seems likely that high-resolution DCS and accurate (limiting) diffusion measurements will assist in determining He+molecule potentials. These two properties are particularly useful because they are independent of uncertainties in the corresponding molecule+molecule potentials.
Crossed molecular beam measurements of differential cross sections (DCS) are reported for elastic scattering of He by Ar, Kr, and Xe at high resolution. Interatomic potentials are determined by simultaneously fitting the DCS’s, as well as mixture viscosity and interaction second virial data. Bias due to systematic and potential model errors are examined and are used to estimate the accuracy of the potential energy curves obtained. Attractive well depths are 2.59, 2.67, and 2.64 meV±3% for HeAr, HeKr, and HeXe, respectively, agreeing with the best available HeAr potential and a previously proposed HeKr potential, but significantly deeper than previously reported potentials for HeXe. The HeXe attractive well is also considerably broader than previously reported. Attractive minimum positions are 3.48, 3.70, and 4.00 Å (±0.03 Å) for HeAr, HeKr, and HeXe, respectively. Including the accurate diffusion data of Dunlop and co-workers [Physica A 95, 561 (1979)] and the absolute integral cross sections of Pirani and Vecchiocattivi [J. Chem. Phys. 66, 372 (1977) and revisions thereto] verify the error bounds for all three potentials.
Differential cross section (DCS) measurements are reported for scattering of a He atomic beam by crossed beams of Ar, C2H2, CO2, and OCS. Relative to the HeAr diffractive structure, the HeC2H2 DCS is moderately damped at small scattering angles and severely damped at large scattering angles; the HeCO2 and HeOCS DCS’s are severely damped for all scattering angles. This damping directly reflects increasing anisotropy of the scattering partner from Ar (none) to C2H2 (moderate) to CO2 and OCS (strong). Even though the present data do not resolve elastic from inelastic contributions, the HeCO2 results are consistent with partially resolved inelastic DCS measurements [U. Buck, H. Meyer, M. Tolle, and R. Schinke, Chem. Phys. 104, 345 (1986)] and therefore complement them. Consequently the data are used to obtain empirical anisotropic intermolecular potentials for HeC2H2 and HeOCS within the infinite-order-sudden approximation. These reproduce the total (unresolved elastic +inelastic) DCS measurements very well, even though the angular asymmetry of OCS is ignored in the analysis.
Differential cross section (DCS) measurements are reported for scattering of a He atomic beam by crossed beams of C2H2, C2H4, C2H6, and CHF3. In addition, interaction virial measurements and accurate limiting diffusion measurements are presented for these systems. Damping of the DCS diffraction oscillations is used to extract anisotropic intermolecular potentials, which are constrained in multiproperty fits to accurately reproduce the dilute gas data. The radial anisotropies determined are in the sequence C2H6>C2H4∼C2H2>CHF3, as sampled by the He probe.
Very low-energy scattering experiments are described for He atoms colliding with an LiF(001) surface. The apparatus is capable of performing experiments for beam energies down to 4meV. The He-LiF results are compared with close-coupling calculations on the interaction potential of Celli et al. (J. Chern. Phys., 1985, 83, 2504. Very good agreement between experiment and theory is found, but there are small discrepancies which should allow the potential to be refined.
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