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

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.

Nonadiabatic Electron Dynamics in Tunneling Junctions: Lattice Exchange-Correlation Potential.

The search for exchange-correlation functionals going beyond the adiabatic approximation has always been a challenging task for time-dependent density-functional theory. Starting from known results and using symmetry properties, we put forward a nonadiabatic exchange-correlation functional for lattice models describing a generic transport setup. We show that this functional reduces to known results for a single quantum dot connected to one or two reservoirs and furthermore yields the adiabatic local-density approximation in the static limit. Finally, we analyze the features of the exchange-correlation potential and the physics it describes in a linear chain connected to two reservoirs where the transport is induced by a bias voltage applied to the reservoirs. We find that the Coulomb blockade is correctly described for a half-filled chain, while additional effects arise as the doping of the chain changes.

Efficient computation of the second-Born self-energy using tensor-contraction operations.

In the nonequilibrium Green's function approach, the approximation of the correlation self-energy at the second-Born level is of particular interest, since it allows for a maximal speed-up in computational scaling when used together with the generalized Kadanoff-Baym ansatz for the Green's function. The present day numerical time-propagation algorithms for the Green's function are able to tackle first principles simulations of atoms and molecules, but they are limited to relatively small systems due to unfavorable scaling of self-energy diagrams with respect to the basis size. We propose an efficient computation of the self-energy diagrams by using tensor-contraction operations to transform the internal summations into functions of external low-level linear algebra libraries. We discuss the achieved computational speed-up in transient electron dynamics in selected molecular systems.