Mapping the interplay between rotational dynamics and charge transfer ability by velocity map imaging

Charge transfer is fundamental to many complex chemical processes. Precise modeling requires a detailed understanding of the process at the molecular level. One crucial aspect concerns the interplay between the position of nuclei and the probability of transferring charges. Past experiments studied the dependence of the charge transfer probability on the distance of two molecular fragments [1-4]. Those found that a classical over-the-barrier model [5] is applicable – implying a critical distance beyond which no further charge transfer is possible. Here, we present the results of a recent experiment that took the next step: For dihalomethanes, it is well-known that UV-excitation triggers a dissociation, which subsequently causes the CH2 group to rotate around the remaining halogen [6,7]. For those molecules, the critical distance may be crossed several times for certain charge states. For this, the classical over-the-barrier model implies a temporary recovery of the ability to transfer charges. The presented data tests for such behavior. The (in)ability to transfer charges can be measured by site-selective ionization. When no charge transfer is possible, no recoil due to Coulomb forces between the fragments occurs because the other one stays neutral. Consequently, only the ionized fragment is measured with low kinetic energy. We used a velocity map imaging spectrometer with an MCP-phosphor-stack detector and a Timepix3-based optical ns-timestamping camera to isolate the kinetic energy-, delay-, mass-, and charge-dependent signals. With this detection setup, capturing an average of ca. 100 ions per pump-probe-pulse-pair at an intratrain repetition rate of 94 kHz was feasible. 1. Erk et al., Science 345, 288-291 (2014) 2. Boll et al., Structural Dynamics 3, 043207 (2016) 3. Amini et al., Structural Dynamics 5, 014301 (2018) 4. Allum et al., Faraday Discuss., 2021, 228, 571-596 5. Niehaus, J. Phys. At. Mol. Opt. Phys. 19, 2925–2937 (1986) 6. Burt et al., Phys. Rev. A 96, 043415 (2017) 7. Murillo-Sánchez et al., Phys. Chem. Chem. Phys., 2018, 20, 20766