Fig. 1: Schematic of ion-pair formation in H₂.

The potential energy curves are taken from ref. ⁷. a For the production of low kinetic energy H^─ ions and (b) for the high kinetic energy ions. The potential energy scale is about the v = 0 state of the electronic ground state of H₂. The red dotted curves are the H₂⁺ ground and first excited states. The black dashed curve is the ion-pair curve, and the blue curves are the neutral excited states from the Rydberg series converging to the cation ground state in (a) contributing to the low kinetic energy ion-pair signal and from the Q₁ series in (b) contributing to the higher kinetic energy ion-pair signal. The blue curves from the Q₁ series are partly shown as dashed-dot, indicating that the autoionisation is active in this region. The grey thick-shaded curves indicate the paths taken by the \({\,}^{1}\mathop{\Sigma} \nolimits_{g}^{+}\) and \({\,}^{1}\mathop{\Sigma}\nolimits _{u}^{+}\) states after excitation by the incoming electrons leading to the ion-pair formation (see the text). Vertical purple dashed lines indicate the Franck-Condon region with the vibrational ground level of the electronic ground state of H₂. The verticle black dotted line separates the linear x-axis scale from the logarithmic x-axis scale for larger internuclear separation.