Abstract
The migration of hydrogen atoms resulting in the isomerization of hydrocarbons is an important process which can occur on ultrafast timescales. Here, we visualize the light-induced hydrogen migration of acetylene to vinylidene in an ionic state using two synchronized 4 fs intense laser pulses. The first pulse induces hydrogen migration, and the second is used for monitoring transient structural changes via Coulomb explosion imaging. Varying the time delay between the pulses reveals the migration dynamics with a time constant of 54 ± 4 fs as observed in the H+ + H+ + CC+ channel. Due to the high temporal resolution, vibrational wave-packet motions along the CC- and CH-bonds are observed. Even though a maximum in isomerization yield for kinetic energy releases above 16 eV is measured, we find no indication for a backwards isomerization - in contrast to previous measurements. Here, we propose an alternative explanation for the maximum in isomerization yield, namely the surpassing of the transition state to the vinylidene configuration within the excited dication state.
Highlights
Xie et al showed that they can control the fragmentation pathways in ethylene by varying the laser pulse duration,[6] and Alnaser et al showed that they could steer the directional emission of the deprotonation from acetylene with the carrier-envelope phase (CEP) of few-cycle laser pulses.[7]
Both areas can be separated at a kinetic energy release (KER) of 13 eV, indicated by the black dashed line, which is later used for energy ltering
We have presented the molecular response in acetylene at various times a er excitation by a near single-cycle pulse using the Coulomb explosion imaging technique
Summary
Paper fragmentation of hydrocarbons can be in uenced by tuning the laser parameters. Xie et al showed that they can control the fragmentation pathways in ethylene by varying the laser pulse duration,[6] and Alnaser et al showed that they could steer the directional emission of the deprotonation from acetylene with the carrier-envelope phase (CEP) of few-cycle laser pulses.[7] The second example was interpreted with a vibrational state coupling model. CEPcontrol has been extended to the isomerization of acetylene and allene, demonstrating the general importance of the vibrational state coupling mechanism.[8]
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