Exploring charge transfer mediated by rotational dynamics with Timepix3-driven velocity map imaging

Intermolecular charge transfer is fundamental to many complex chemical processes. It is, therefore, vital to gain a detailed understanding of the process at the molecular level. One aspect concerns how the probability of a charge transfer to occur relates to the position of involved nuclei. Past experiments studied the dependence of the charge transfer probability on the distance of two molecular fragments [1-6]. Those found that a classical over-the-barrier model [7] is applicable – implying a critical distance beyond which no further charge transfer is possible for a given charge-state configuration. Here, we present the results of a recent experiment that took the next step: For dihalomethanes, it is well-known that cleavage of a halogen-carbon bond triggers a dissociation, which subsequently causes the CH2 group to rotate around the remaining halogen [8,9]. As a result of the rotation, the critical distance may be crossed several times for specific charge configurations [6]. In those cases, the classical over-the-barrier model implies a temporary recovery of the ability to transfer charges. Experimentally, we investigate the delay-dependent charge transfer ability in different dissociating molecules. We can extract features resulting from fragment rotation and test those against the classical over-the-barrier model. The ability to transfer charges (or lack thereof) can be measured using intense soft X-ray pulses for site-selective ionization of the dissociating halogen atom. When charge transfer is impossible, no recoil occurs due to Coulomb forces between the fragments, as the other fragment remains neutral. Consequently, only the ionized fragment is measured with low kinetic energy. We employed 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. [1] Erk et al., Science 345, 288-291 (2014). [2] Schnorr et al., PRL 113, 073001 (2014). [3] Boll et al., Structural Dynamics 3, 043207 (2016). [4] Amini et al., Structural Dynamics 5, 014301 (2018). [5] Allum et al., Faraday Discuss., 2021, 228, 571-596. [6] Köckert et al., J. Phys. B: At. Mol. Opt. Phys. 55 (2022) 014001. [7] Niehaus, J. Phys. At. Mol. Opt. Phys. 19, 2925–2937 (1986). [8] Burt et al., Phys. Rev. A 96, 043415 (2017). [9] Murillo-Sánchez et al., Phys. Chem. Chem. Phys., 2018, 20, 20766.