Fig. 4: Pump-probe delay dependent \({\rm H}_{3}^{+}\) yields.

The \({\rm H}_{3}^{+}\) yields are shown for CH₃OD (red), CH₃Cl (green), and CH₃NCS (blue). Black lines indicate fits of the experimental data to Eq. (3). The resulting values of parameter τ_rise, displayed in the panels, indicate the times of formation of \({\rm H}_{3}^{+}\) from the three molecules. The \({\rm H}_{3}^{+}\) yields for the CH₃OD²⁺ and CH₃Cl²⁺ transients level off below the baseline (ion yield = 1), indicating fragmentation of the \({\rm H}_{3}^{+}\) ion while it is still confined within the focal volume. In the case of the CH₃NCS²⁺ transient, the long pump-probe delay asymptote is at a value above the baseline, which suggests that the pump pulse has populated an excited state of the corresponding monocation that has subsequently been ionized by the weak probe to the dication state, resulting in \({\rm H}_{3}^{+}\) production from an additional channel. All data are included in the Source Data folder.