Real-time observation of a correlation-driven sub 3 fs charge migration in ionised adenine

Abstract

Sudden ionisation of a relatively large molecule can initiate a correlation-driven process dubbed charge migration, where the electron density distribution is expected to rapidly move along the molecular backbone. Capturing this few-femtosecond or attosecond charge redistribution would represent the real-time observation of electron correlation in a molecule with the enticing prospect of following the energy flow from a single excited electron to the other coupled electrons in the system. Here, we report a time-resolved study of the correlation-driven charge migration process occurring in the nucleic-acid base adenine after ionisation with a 15-35 eV attosecond pulse. We find that the production of intact doubly charged adenine - via a shortly-delayed laser-induced second ionisation event - represents the signature of a charge inflation mechanism resulting from many-body excitation. This conclusion is supported by first-principles time-dependent simulations. These findings may contribute to the control of molecular reactivity at the electronic, few-femtosecond time scale.

Fig. 1. Experimental results: pump–probe scan.

a Schematic of the experiment: a molecular beam is injected into a VMI operated in the ion time-of-flight mode. Adenine ions are accelerated towards the detector and the ion yield is measured as a function of the XUV-pump IR-probe delay. b Normalised yield of several ions (shown with a vertical offset) as a function of the XUV-pump NIR-probe delay. The yield of the ionic fragments exhibits a distinct positive or negative step-like behaviour, while the adenine parent dication (67.5 u/e) is fitted to have an exponential risetime of τ₁ = 2.32 ± 0.45 fs (68 % confidence interval). The decay lifetime (τ₂) is significantly shorter for the dication (green curve) than for the cations. The grey shading indicates the standard error of the mean of seven successive scans. c Example of calculated time evolution of bond lengths in the first 10 fs following XUV ionisation, with an electron removed from the fourth highest occupied molecular orbital (HOMO − 3). All the bonds start elongating only after 3 fs. The theoretical simulations for bond elongation are performed with TDDFT. d The bond numbering used in the theoretical work (for more details see Supplementary Methods, section 7, Supplementary Figs. S8 and S9).

Fig. 2. Overview of the molecular dynamics: the shake-up process.

a Following XUV photoionisation, a hole is created in the inner valence. The hole decays in a characteristic transition time and, due to electronic correlations, this can lead to excitation of a second electron to a bound excited state, called shake-up state. If optimally time-delayed from the XUV, the NIR control pulse extracts the excited electron, hence doubly ionising the molecule. b Transition times to a given shake-up state calculated with a Fermi’s Golden rule approach. A special shake-up state (LUMO+6) is highlighted in green and shows a characteristic time of 2.5 fs. The states are ordered by energy and grouped (shades of grey) by the number of NIR photons (1, 2 or 3) required to ionise a second electron. The LUMO+6 state falls in the two NIR-photon group.

Fig. 3. Theoretical results: shake-up and charge inflation.

a Time-dependent occupations of the adenine bound excited states after photoionisation by the XUV pulse, calculated with the ab-initio non-equilibrium Green’s function method. The special state (LUMO+6), highlighted in green, is populated via the shake-up process and its population rises over several femtoseconds to one of the largest values. b Integrated time-dependent electron density more than 3 Å away from the molecular plane. The grey shaded area represents the time-window of the pump pulse, having its peak at t_pump = 0.48 fs. c Left panel: the adenine molecule and the planes defining the integration region. Right panel: snapshots of the variation of the electronic density with respect to the density immediately after the XUV pulse. We observe that the electronic density inflates considerably away from the molecular plane. The noticeable out-of-plane charge migration can be attributed to the increasing population of the correlated LUMO+6 state (Supplementary Fig. S17). These results have been obtained with the non-equilibrium Green’s function method.

Fig. 4. Role of the NIR probe pulse.

a Temporal evolution of the LUMO+6 state depletion due to the combined action of XUV and NIR pulses as a function of the delay. b Simplified readout with the state depletion averaged in a 1 fs window after the NIR pulse. The signal is absent at zero delay although half the NIR pulse then exposes the sample after the centre of the XUV pulse, it then shows a significant onset in the window of 2–4 fs and keeps increasing with larger delays. The trend reported in b reproduces, remarkably, the one of the time-dependent yield measured for the adenine dication. These results have been obtained with the non-equilibrium Green’s function method.