Potential energy curves of HCl. Solid black curves: initial and final state of the two-photon transition. Dashed curves: ionic states correlating to H + Cl⁺ (red) and H⁺ + Cl (blue). Solid coloured curves: neutral Rydberg states converging to H + Cl⁺ (red) and H⁺ + Cl (blue). Red arrows illustrate the pathway for producing Cl⁺ ions, blue arrows illustrate one of several pathways that produce H⁺ ions. Dashed and dotted black straight lines denote the 2, 3, and 4 photon energy levels and the ionization limit, respectively. The asymptotic value for the H⁺ + Cl⁻ ion pair remains between H*() + Cl and H + Cl*(4s).