Experimentally resolving and decomposing the transfer of charge through a molecule on a single-femtosecond timescale

The rate limiting steps of charge-transfer (CT) reactions have been well understood for decades thanks to Markus theory, however, the motion of the electrons that is responsible for the CT is not considered separately. Using XUV Attosecond Transient Absorption Spectroscopy we have captured the sub-femtosecond electronic dynamics of CT in CF₃I⁺ which we find to be mediated by a conical intersection (CoIn). The CT manifests itself as a drop in the transient absorption at one photon energy and a rise at another. With the help of state-of-the art calculations of the iodine N_4,5 absorption spectrum and in-situ mass spectrometry, we are able to identify the two electronic states involved in the CT process as the B ²A₂ fluorine-hole state and the E ²A₁ iodine-hole state.

Thanks to the extremely short, σ = 1 fs, experimental cross correlation afforded by ATAS, the rise of the E state absorption is measured to be σ = 2.34 ± 0.36 fs. Furthermore, the sigmoid rise of the absorption is found to be significantly asymmetric, exhibiting a negative skew. The origin of this skew is understood through a Landau-Zener-like model, and found to originate from the modulation of the non-adiabatic coupling as the nuclear wavepacket passes through the vicinity of the CoIn. Finally, a 1.46 ± 0.41 fs delay is observed between the fall in the B state and the rise in the E state transient absorption – a yet unobserved signature of the non-adiabatic effects driving electron motion.

Along with establishing a purely experimental method for transiently probing the non-adiabatic coupling, these observations allow us to propose three separate timescales that are involved in electronic CT modulated by conical intersections; the “non-adiabatic CT time” - the time taken for the electron to travel through the molecule, the “adiabatic CT time” - the time taken for the diabatic population of a single state to rise/fall, and the “CT rearrangement time” the time taken for the vibrational wavepacket of the system to reach the CoIn.