Intensity-dependent three-body Coulomb explosion of methane in femtosecond laser pulses

We experimentally study three-body Coulomb explosion of methane molecules, i.e., ${\mathrm{CH}}_{4}{}^{3+}\ensuremath{\rightarrow}{\mathrm{H}}^{+}+{\mathrm{H}}^{+}+{\mathrm{CH}}_{2}{}^{+}$ in 800-nm laser pulses with different intensities. We find that, at a relatively low intensity, the three-body breakup channel is dominated by a concerted pathway with a high kinetic energy release (KER). However, at a high laser intensity, an extra pathway with a low KER appears. The pathway with a low KER can be explained as a sequential pathway, indicating the potential existence of a stable intermediate ${\mathrm{CH}}_{3}{}^{2+}$ dication during the three-body Coulomb explosion of methane molecules in strong laser pulses. The intensity dependence of the pathway is explained as the increasing population on the excited state of ${\mathrm{CH}}_{3}{}^{2+}$ with the increase of the laser intensity, which has a longer lifetime than that of the ground state.