Mark Keil

Empirical potential for the He+CO2 interaction: Multiproperty fitting in the infinite‐order sudden approximation

An empirical intermolecular potential for the interaction of He with CO2 is obtained via data reduction of phenomenological cross sections. The infinite order sudden approximation is used to calculate the total differential, total integral, and spectral line broadening cross sections, and diffusion, viscosity, thermal conductivity, thermal diffusion factors, and second virial coefficients. Second order Chapman–Cowling corrections were used to determine some of the transport coefficients, and quantum corrections to the classical virial coefficients were included. The empirical potential obtained simultaneously fits all nine different types of experimental data to within their experimental error and the computational accuracy.

On obtaining interatomic potentials from multiproperty fits to experimental data

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 measure...

Damping of total differential cross sections: Observations and empirical anisotropic potentials for HeC2H2 and HeOCS

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, ...

Interatomic potentials for HeAr, HeKr, and HeXe from multiproperty fits

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 A (±0.03 A) 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 an...

Energy transfer as a function of collision energy. III. State‐to‐state cross sections for rotational‐to‐translational energy transfer in HF+Ar

State‐to‐state cross sections for rotationally inelastic collisions of HF (v,J) with Ne, Ar, and Kr have been measured. Laser pumping of the molecular beam to the initial states v = 1, J = 1–6, and v = 2, J = 2, followed by infrared fluorescence, permitted measurements of relative cross sections with ‖ ΔJ ‖⩽8. The collision energy was varied between 4 and 16 kcal/mol. These cross sections could be fitted well using an inverse‐power dependence on the rotational energy gap [due to Pritchard and co‐workers; J. Chem. Phys. 70, 4155 (1979)] for rotational energy transfers of up to 55% of the initial translational energy. The energy‐corrected sudden approximation was used to determine an ’’effective’’ collision length for rotationally inelastic scattering. The scattering is thought to occur predominantly on the repulsive wall of the intermolecular potential, except for the J = 1→J′ = 0 transition, which is shown to be sensitive to the depth of the van der Waals attractive well.

Vibrationally and rotationally resolved angular distributions for F+H2→HF(ν,j)+H reactive scattering

Angular distributions for individually resolved ν, j states from the F+H2→HF(ν,j)+H chemical reaction are measured for the first time. Vibrational and rotational resolution is achieved simultaneously by applying laser+bolometer detection techniques to crossed-beam reactive scattering. In addition to backward-scattering HF(ν=1, j=6) and HF(ν=2, j=5), we also observe HF(ν=1, j=6) products scattered into the forward hemisphere. The results are in qualitative agreement with fully three-dimensional exact quantum reactive scattering calculations [Castillo et al., J. Chem. Phys. 104, 6531 (1996)] which were conducted on an accurate potential-energy surface [Stark and Werner, J. Chem. Phys. 104, 6515 (1996)]. However, the forward-scattered HF(ν=1, j=6) observed in this experiment is not reproduced by quasi-classical calculations [Aoiz et al., Chem. Phys. Lett. 223, 215 (1994)] on the same potential-energy surface.

Anisotropic intermolecular potentials for HeC2H2, HeC2H4, and HeC2H6, and an effective spherical potential for HeCHF3 from multiproperty fits

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.

Differential cross sections for rotationally state‐resolved inelastic scattering of HF by argon

We present differential cross section (DCS) measurements for scattering of HF by Ar. These crossed‐beam experiments employ rotational state sensitivity, allowing determination of the DCS as a function of the scattered HF rotational state. The initial HF rotational distribution is generated by nozzle expansion, without further state selection. Its composition is mostly J=0 and J=1, with small admixtures for J>1. The DCS for each final state J’ is measured using a stabilized cw HF chemical laser, in conjunction with a rotatable liquid He‐cooled bolometer. Measurable signals are obtained for scattering into 0≤J’≤5, where J’=6 is the thermodynamic limit for our collision energy of 120 meV. The measured DCS’s show a strong forward peak, largely from elastic scattering. In addition, the DCS’s evolve from a broad shoulder in the θ≊25°–40° region for J’=0—through a flattening of the wide‐angle scattering for J’=2 and J’=3—to an increase in the scattering beyond ∼40° for J’=4. The DCS for scattering into J’=5 also...

Anisotropic repulsive potential energy surfaces from Hartree–Fock calculations for HeCO2 and HeOCS

Hartree–Fock calculations are presented for the repulsive interactions of He with CO2 and OCS. The results are well described by parametrizing the anisotropic potential energy surface as a sum over interactions between He and each atom of the molecule. The interaction of He with the oxygen ends of both molecules is almost identical, thereby reducing the number of potential fitting parameters required. The analytic surfaces obtained yield good agreement with pressure broadening measurements, which probe the anisotropy while being independent of the van der Waals attraction. It is suggested that the sum‐over‐sites parametrization may be useful in constructing semiempirical surfaces that do include the van der Waals attraction. The sum‐over‐sites parametrization is also particularly well suited to describing the vibrational dependence of the repulsive anisotropy.

The HeNe interatomic potential from multiproperty fits and Hartree–Fock calculations

New high‐resolution differential scattering cross sections are reported for the HeNe interaction. These experimental results are combined with Hartree–Fock calculations in constructing a highly accurate interatomic potential. The new potential is capable of reproducing all available experimental data judged to be sufficiently reliable. This includes properties that are highly sensitive to the very weak attractive well and its outer bowl, in addition to the weakly repulsive wall. The potential is compared to those previously proposed for HeNe, particularly to one obtained by direct inversion of differential cross section data of similarly high quality. The potential crosses through zero at σ=2.699 A; its minimum occurs at rm=3.029 A with a depth of e=1.83 meV.