Delayed fragmentation of methane induced by femtosecond laser pulses

We experimentally investigated the delayed fragmentation process of methane (CH4) molecules in a strong linearly polarized femtosecond laser field by cold target recoil-ion momentum spectroscopy. The prompt fragmentation and delayed fragmentation processes of the CH42+→H++CH3+ channel were unambiguously identified in the coincident photoion-photoion time-of-flight spectrum. By solving Newton's equations, the survival times of long-lived methane dications have been obtained, with a corresponding statistical lifetime of 304 ± 9 ns. The branching ratios have also been evaluated, with the relative yield of delayed fragmentation being much lower than that of prompt fragmentation. Momentum distributions and kinetic energy release spectra have been obtained for both prompt and delayed processes. The possible formation mechanism of delayed fragmentation was also discussed.