Resonances in low-energy positron scattering from Li, Na and K

A number of resonances in the L=0, 1 and 2 channels have been found in close-coupling calculations of positron-alkali collisions. In contrast to the corresponding electron scattering case the s-wave phaseshifts for positrons colliding with Li, Na and K start from a positive multiple of pi rad at zero energy which indicates the existence of true alkali-positron bound states. The widths for the various resonances found in the region of the atomic excitation threshold vary between 0.2 and 130 meV. The inclusion of the electronic 3d orbital is observed to be crucial in order to obtain fully developed resonances below the 'resonant' ns-np threshold.