Abstract
ABSTRACTWe 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 -electrons in model polyenes (hexatriene and decapentaene) and -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.
Highlights
With the rapid development of ultrafast imaging techniques, which are currently reaching time resolution of a few attoseconds (10−18 s), tracking electron dynamics in atoms and molecules becomes a realistic possibility [1,2,3,4,5,6,7]
Since the correct definition of | (r, t) includes the outgoing electron, an explicit treatment of the continuum is highly desirable in order to obtain accurate transition probabilities, approximations such as Dyson orbitals are employed. Such an approach is quite limited in its capabilities – it only allows one to model charge migration initiated by the given pulse shape, which may result in nearly no electron dynamics being observed and does not provide any direct route to optimise the pulse [25]
As we have demonstrated in other work [19,20,21], it is not the mean-field molecular geometry change that leads to fast decoherence of electron dynamics, but rather the spread of the nuclear wave packet in the neutral ground state that causes the issue
Summary
In attempts to model molecular electronic dynamics in cationic systems, the initial conditions have been mainly constructed from chemical intuition [10,11,12,13,14,15,16,17,18,19,20,21,22,23] In those studies, the initial wave packet was generally a superposition of two cationic eigenstates, which resulted in a localised charge. Since the correct definition of | (r, t) includes the outgoing electron, an explicit treatment of the continuum is highly desirable in order to obtain accurate transition probabilities, approximations such as Dyson orbitals are employed Such an approach is quite limited in its capabilities – it only allows one to model charge migration initiated by the given pulse shape, which may result in nearly no electron dynamics being observed and does not provide any direct route to optimise the pulse [25].
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