Charge migration engineered by localisation: electron-nuclear dynamics in polyenes and glycine

Abstract

We demonstrate that charge migration can be 'engineered' in arbitrary molecular systems if a single localised orbital-that diabatically follows nuclear displacements-is ionised. Specifically, we describe the use of natural bonding orbitals in Complete Active Space Configuration Interaction (CASCI) calculations to form cationic states with localised charge, providing consistently well-defined initial conditions across a zero point energy vibrational ensemble of molecular geometries. In Ehrenfest dynamics simulations following localised ionisation of π-electrons in model polyenes (hexatriene and decapentaene) and σ-electrons in glycine, oscillatory charge migration can be observed for several femtoseconds before dephasing. Including nuclear motion leads to slower dephasing compared to fixed-geometry electron-only dynamics results. For future work, we discuss the possibility of designing laser pulses that would lead to charge migration that is experimentally observable, based on the proposed diabatic orbital approach.

Onset of ionic coherence and ultrafast charge dynamics in attosecond molecular ionisation

“…Whereas quantum chemistry packages implementing sophisticated methods to compute bound electronic molecular states are well-established, comparable tools for the molecular continuum are not yet widely available. For this reason, most theoretical studies of ultrafast charge dynamics in molecules have either ignored the ionisation step [24][25][26][27][59][60][61][62][63][64][65][66] or modelled it by disregarding electron correlation effects 18,67 .…”

Excited-state electronic coherence in vinyl bromide ions

“…Ehrenfest dynamics, combined with TDDFT or other approaches to describe the electronic motion, is often used to describe the ultrafast dynamics in molecules [2,33,[52][53][54][55]61]. Ehrenfest dynamics has recently been extended to fully quantum nuclear motion [32].…”

“…The charge migration and coherence of electronic states have often described by superpositions like in Eq. 10[ 24,25,[52][53][54][55].…”

Quantum electronic coherences by attosecond transient absorption spectroscopy: ab initio B-spline RCS-ADC study

“…The extent to which this result is affected by also explicitly taking into account the nuclear degrees of freedom, remains to be seen, and will mainly depend on the timescales of the resulting decoherence, i.e. whether the electronic coherences live long enough before they are damped as a result of the chemical change that they pre-determine 53,54 , and the spatial delocalisation of the ground-state nuclear wavefunction 55 . Nevertheless, the importance of these results is in the fact that they demonstrate, by means of advanced ab initio calculations, that the ATAS technique, combined with the use of sub-fs X-ray pulses of the kind that have become available at X-ray FELs 52 , is perfectly suitable to image the complex many-electron dynamics which emerges from attosecond ionisation of polyatomic molecules, including correlation-driven charge dynamics, and retrieve the quantum electronic coherences that are not completely washed out by the dephasing effect of nuclear motion.…”