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Abstract

Combining a femtosecond laser with a Reaction Microscope (ReMi), time-resolved fragmentation measurements of methane (CH₄) have been done in a pump-probe scheme. By reconstructing the threedimensional momentum for different reaction products, various dissociation pathways have been observed showing distinct angular distributions. For a better understanding of these pathways, also single-pulse measurements were examined, serving as valuable references. The main focus lay on two-body Coulomb explosions of CH₃⁺ + H⁺ and CH₂⁺ + H₂⁺. For both channels two processes were observed: One timeindependent Coulomb explosion and, in the pump-probe experiment, another process depending on the time delay of the second pulse in respect to the first one. The latter is due to a two-step Coulomb explosion with both pulses being involved. Thus, during the delay-time between both the molecule starts to dissociate on an intermediate potential curve. The angular dependence of those processes was compared with the single ionized dissociations. As a result, the time delayed Coulomb explosions as well as the two dissociation channels of CH₃⁰ + H⁺ and CH₂⁰ + H₂⁺ show a break-up strongly favored along the laser polarization axis. For CH₃⁺ +H⁰ and CH₂⁺ +H₂⁰ an isotropic distribution was observed. Therefore the first two dissociation channels are possible intermediate states for the time-delayed Coulomb explosion.