Translation electron spectroscopy study of the ¹P⁰ shape resonance of H^- excited in H^- rare-gas collisions (0.5-6 keV)

The study of electron spectra in H^- -rare-gas collisions (0.5-6 keV) at low observation angles ((5') reveals structure due to decay of the H^- * (2s2p ¹ P⁰ ) shape resonance to H (n=2). Projectile kinematics allow the transformed cross section in the H^-frame to be obtained with a resolution that can be as high as 1 meV which then yields the resonance parameters. At very low angles ((1') a cusp is observed due to zero-energy electrons in the projectile frame, implying a nonzero cross section at threshold. This behaviour originates from a long range potential interaction which can be the dipole interaction between the excited hydrogen atom and the detached electron. Furthermore the authors have observed a target effect on the angular distribution of these near-zero energy electrons which could be related to the target polarizability. Very weak structure in the electron spectra corresponding to energies below 4 meV could be the first observation of the predicted dipole oscillations.