Modified statistical electron–gas calculations using the methods of Gordon, Kim, Rae, Cohen, and Pack are carried out to obtain the interaction energy of Ar with H2 as a function of geometry. The results are combined with the accurate pairwise interactions, the long‐range nonpairwise interaction, and the potential LeRoy and van Kranendonk fit to spectral data on the van der Waals’ complex to obtain a potential energy surface which is as accurate as possible at all geometries. This surface and the pairwise additive surface are then used in a Monte Carlo quasiclassical trajectory study of the cross sections (under shock‐tube high‐energy collision conditions) for complete dissociation, for production of quasibound states of H2, and for V–T, R–T, and V–R–T energy transfer. Except for R–T energy transfer, the accurate surface yields smaller cross sections than the pairwise additive surface does. The cross sections for dissociation are much smaller than predicted by the available‐energy hard‐sphere model but ar...

Monte Carlo trajectory study of Ar+H2 collisions. I. Potential energy surface and cross sections for dissociation, recombination, and inelastic scattering

Pseudo-spectral dipole oscillator strength distributions (DOSDs) are reported for ground state C2H6, C3H8 and normal-C4H10, C5H12, C6H14, C7H16 and C8H18. The pseudo DOSDs, which are a discrete representation of original recommended continuous DOSDs, can be used to evaluate the dipole properties of the alkanes in a very efficient and economical way without loss of accuracy. The pseudo DOSDs, together with a previous pseudo DOSD for CH4, are used to evaluate the orientation averaged dipole-dipole dispersion energy coefficient, C6(A, B), the corresponding relativistic coefficient, W 4(A, B), and the triple-dipole dispersion energy coefficients, C 9(A, B, C), for all pairs and triple of molecules taken from CnH 2n+2, n = 1(1)8. The results are used to make self consistent tests of various semiempirical formulae for C6, W 4 and C9.

Pseudo-spectral dipole oscillator strength distributions for the normal alkanes through octane and the evaluation of some related dipole-dipole and triple-dipole dispersion interaction energy coefficients

We have measured the frequency splitting between the $(2S,F=1∕2)$ and $(2S,F=3∕2)$ hyperfine sublevels in atomic deuterium by an optical differential method based on two-photon Doppler-free spectroscopy on a cold atomic beam. The result ${f}_{\mathrm{HFS}}^{(D)}(2S)=40\phantom{\rule{0.2em}{0ex}}924\phantom{\rule{0.2em}{0ex}}454(7)\phantom{\rule{0.3em}{0ex}}\mathrm{Hz}$ is the most precise value for this interval to date. In comparison to previous radio-frequency measurements we have improved the accuracy by a factor of 3. The specific combination ${D}_{21}=8{f}_{\mathrm{HFS}}^{(D)}(2S)\ensuremath{-}{f}_{\mathrm{HFS}}^{(D)}(1S)$ of metastable and ground state hyperfine frequency intervals in deuterium derived from our measurement agrees well with the value for ${D}_{21}$ calculated from quantum electrodynamics.

2S hyperfine structure of atomic deuterium

The hyperfine pressure shift (HPS) of hydrogen atoms in a helium atmosphere is calculated over a wide temperature range (50–700°K). Results are not very sensitive to the exact form of the H–He potential, but depend strongly on the values of the spin density at the hydrogen nucleus at different H–He interatomic separations. Spin densities are evaluated by the spin‐optimized self‐consistent‐field (SOSCF) method and are corrected for the long‐range Van der Waals interaction. It is argued that long‐range spin density shifts are indeed negative as suggested by several previous calculations, in contrast with a recent five‐configuration treatment. Our results are 20%–25% higher than experimental values. Most of the error is attributed to the uncertainties concerning the method of including long‐range effects.

Hyperfine Pressure Shift of Hydrogen in Helium

The R−6, R−8, and R−10 terms in the long‐range expansion for the hyperfine frequency shift are calculated for hydrogen in the presence of He, Ne, Ar, Kr, and Xe. The R−6 terms are based on the dipole oscillator strength sums. For helium, the R−8 and R−10 terms are based on quadrupole and octupole oscillator strength sums. For the heavier inert gases, the results for the R−8 and R−10 terms are obtained from the sum rules and the static polarizabilities. Upper bounds are also determined for the R−8 and R−10 terms.

Dipole, quadrupole, and octupole terms in the long‐range hyperfine frequency shift for hydrogen in the presence of inert gases

A one-centre expression is derived for the long-range dispersion contribution to the hyperfine pressure shift of a paramagnetic atom in an inert buffer gas, and the use of oscillator strength sums in the evaluation of the leading term is discussed. Accurate calculations are carried out for atomic hydrogen in each of the inert gases.

W D Davison

Papers

Long-range interactions and the hyperfine pressure shift: Hydrogen in an inert gas