Time-Resolved Fragmentation of Methane in Strong Laser Fields

The ionization and dissociation dynamics of methane (CH4) in strong laser fields are studied in a series of near-infrared (NIR) pump-probe experiments with a Reaction Microscope (ReMi) [1]. Upon few-cycle strong NIR pump pulses, methane is ionized to different cationic states and undergoes the Jahn-Teller distortions, leading to multiple dissociation pathways. A time-delayed NIR probe pulse then induces Coulomb explosion (CE), leading to the rapid dissociation of the molecule into two or three fragments. In CE, the initial internuclear distances can be determined by measuring the final kinetic energy release (KER). Combined with the angular distribution of the fragments, four distinct fragmentation processes are identified and analyzed. Through the time-resolved study of methane photofragmentation, ‘molecular movies’ are reconstructed, enabling the investigation of the ultrafast molecular rearrangement dynamics. In two-body fragmentation, atomic hydrogen dissociates from methane as either neutral hydrogen (H^0) or a proton (H^+), each exhibiting distinct angular emission characteristics. Comparing features observed in atomic and molecular hydrogen dissociation provides insight into molecular structural dynamics. Moreover, while molecular hydrogen is observed as a product in two-body fragmentation, it can also act as an intermediate species, contributing to the three-body dissociation channel (CH_4 → CH_2^+ + H^+ + H^+). Finally, the unexpected detection of H_3^+ raises fundamental questions regarding its formation mechanism under strong-field conditions, suggesting new directions for future research.