Positron-HF collisions: Prediction of a weakly bound state.

Ab initio molecular R-matrix calculations are presented for collisions of low-energy positrons with the hydrogen fluoride molecule as a function of HF internuclear separation. Calculations presented are for Z total symmetry with and without polarization eA'ects. These calculations mirror recent electron-HF scattering calculations by Morgan and Burke. Strong evidence is obtained for the presence of a bound state of the e+HF system. The experimental consequences of this result are discussed. A striking difference between the behavior of low-energy electrons and positrons in collision with atoms and molecules is the absence in positron scattering of the sharp structures often observed with electrons. These structures are associated with compound states, either bound or resonant, of the total system and often dominate elastic and inelastic cross sections in the low-energy region. Although the possibility of compound states in positron-molecule systems has long been discussed, ' to our knowledge there is as yet no confirmed observation of such a state. It is well known from both theory and experiment that the cross sections for very-low-energy electrons in collision with strongly dipolar systems such as HF and HC1 display sharp peaks near threshold. Recent work on HF by Morgan and Burke (MB) suggests that these structures are due to the presence of a very weakly bound state of HF. MB found a bound state for all the geometries of HF they studied, including small internu-clear separations for which the dipole moment of HF is subcritical, i.e. , less than 0.639 a.u. For this case Craw-ford4 has shown that no bound states are supported by a potential of simple dipolar form, a conclusion which is unaltered by the inclusion of rotation. Actual molecular potentials, however, contain short-range terms due to static repulsion (damping of the dipole) and polarization effects. As the asymptotic potential of these polar systems is equally attractive to both electrons and positrons, they would seem good candidate systems which might support positron-molecule bound states. Such bound states would have immediate consequences for positron scattering off the system, as the associated nuclear excited Feshbach resonances — caused by vibrationally (or rota-tionally) excited states of the positron-molecule complex — should lead to very rapid positron annihilation. Indeed a number of small molecules have been observed to be very efficient positron annihilators, having an effective number of electrons available for annihilation (Z,tr) as high as 15000! What causes these anomalously high values of Z,p remains unclear. …