Absence of Quasibound Negative-Ion Ground States of He andH2in Electron Scattering

A search for quasibound negative-ion ground states in electron scattering from He and ${\mathrm{H}}_2$ in transmission has been made with negative results. The half-width of the electron beam used in these experiments was about 0.05 eV and was independent of electron energy in the interaction region. The lower limit for detection of cross-section changes in this experiment was about 2 or 3 parts in ${10}^4$. The series of elastic vibrational resonances in ${\mathrm{N}}_2$ which are probably due to the quasibound negative-ion ground state of ${\mathrm{N}}_2$ are discussed.