Polarisation potentials for positron-molecule collisions: positron-H, scattering
Model polarisation potentials for use in calculating positron-molecule cross sections are often based on approximations in which terms that depend on the sign of the charge of the projectile are neglected. Thus, these potentials do not fully distinguish between electron and positron scattering. The validity of using such potentials for positron- H2 collisions is investigated by performing scattering calculations using electron- and positron-H, model polarisation potentials. Results show that, because of differences in the way electrons and positrons distort the target charge cloud, sign-dependent terms in the adiabatic potential can significantly influence the total cross section. Low-energy cross sections are also shown to be rather sensitive to how non-adiabatic polarisation effects are taken into account. Using a cut-off fully adiabatic positron-H, polarisation potential, theoretical total cross sections in reasonably good agreement with recent measurements are obtained.
Comparison of one and two centre studies of low energy scattering by diatomic molecules
The elastic, momentum transfer and rotational excitation cross sections for low energy positrons scattered by hydrogen molecules are calculated using respectively, spherical and spheroidal coordinates and harmonic expansions. Both the spherical and the spheroidal calculations are done in the adiabatic-nuclei approximation. The harmonic expansions in the two coordinate systems are both truncated at corresponding order (up to and including d-waves). The calculations are identical, save for the fact that partial spherical analysis is used in one case and partial spheroidal analysis in the other. The two sets of results so obtained turn out to be almost identical.
The first ground cricket (Orthoptera: Trigonidiidae: Nemobiinae) from mid-Cretaceous Burmese amber
Abstract A new genus and species, Birmaninemobius hirsutus gen. et sp. nov., is described on the basis of a well-preserved orthopteran specimen in mid-Cretaceous Burmese amber and attributed to the subfamily Nemobiinae (ground crickets) of the family Trigonidiidae (Grylloidea). As the first record of Nemobiinae from mid-Cretaceous Burmese amber, this new taxon not only provides novel morphological diversity in ground crickets but also indicates that the ground crickets lived in Burma (Tethyan islands) as early as the mid-Cretaceous.