Search for resonances in the scattering of low-energy positrons from atomsand molecules

In a search for quasi-bound states, or resonances, high-resolution measurem ents (DeltaE similar to 25 meV) of the total positron scattering cross sect ions have been carried out in the energy region of the first electronically excited states of H-2, N-2, CO and Ar. In the case of H-2, a recent calcul ation by Varella et al predicts such a resonance in the total elastic cross section near the excitation threshold for the B (1)Sigma (+)(u) state. We find no experimental evidence for the existence of this resonance and also find similar negative results for the other atomic and molecular targets.