Fig. 1: The roaming model for the production of \({\rm H}_{3}^{+}\) following the strong-field double ionization of methanol.

The upper half of the figure illustrates the process of generating a molecular dication by doubly ionizing methanol via electron rescattering. The electric field of the ultrafast laser pulse (represented by the dashed line) bends methanol’s potential (shown in yellow), resulting in tunnel ionization which creates a high-energy electron that accelerates back toward the molecule when the field reverses its direction. The returning electron has sufficiently high kinetic energy to remove the second electron from the molecule within the same optical cycle, producing the CH₃OH²⁺ dication. Given the short pulse duration (∼ 35 fs), the dynamics of the CH₃OH²⁺ species, shown in the lower half of the figure, are not influenced by the electric field of the laser. The dication in its lowest singlet state ejects a neutral H₂ fragment consisting of the hydrogen atoms labeled as H(2) and H(3), which then roams the remaining CHOH²⁺ moiety until it abstracts the proton corresponding to the hydrogen atom H(1) bound to the carbon, leading to the formation of \({\rm H}_{3}^{+}\).