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Abstract

The break-up of methane molecules exposed to intense laser fields has been studied using a combination of a femtosecond pump-probe setup and a reaction microscope. The time evolution of a particular fragmentation reaction (e.g. one- or few-electron dissociative ionization) induced by a ‘pump’ pulse (4·1014 W/cm2, ≈ 9 fs, central wavelength 780 nm) was probed with the identical ‘probe’ pulse arriving after a variable time delay. Using the ‘Coulomb Explosion Imaging’ technique we were able to visualize the motion of the dissociating molecular fragments. The dissociating dynamics of singly ionized methane molecule was found to depend strongly on the final localization of the charge, which is reflected in very different fragment angular distributions for the two reaction pathways (CH+ 4 → CH3+H+ and CH+ 4 → CH+ 3 +H). Exploiting this dependence for the channel separation, we map the ionization probability for one of these pathways as a function of the delay between the pump and the probe pulse, and thus, of the internuclear distance. Moreover, similar to earlier results on the CH4 fragmentation, we observe that double ionization often leads to the formation of a hydrogen molecular ion. Possible mechanisms of this bond-forming reaction will be discussed.