Long-lived oscillatory incoherent electron dynamics in molecules: trans-polyacetylene oligomers

Ignacio Franco,Angel Rubio,Paul Brumer

doi:10.1088/1367-2630/15/4/043004

Abstract

We identify an intriguing feature of the electron–vibrational dynamics of molecular systems via a computational examination of trans-polyacetylene oligomers. Here, via the vibronic interactions, the decay of an electron in the conduction band resonantly excites an electron in the valence band, and vice versa, leading to oscillatory exchange of electronic population between two distinct electronic states that lives for up to tens of picoseconds. The oscillatory structure is reminiscent of beating patterns between quantum states and is strongly suggestive of the presence of long-lived molecular electronic coherence. Significantly, however, a detailed analysis of the electronic coherence properties shows that the oscillatory structure arises from a purely incoherent process. These results were obtained by propagating the coupled dynamics of electronic and vibrational degrees of freedom in a mixed quantum-classical study of the Su–Schrieffer–Heeger Hamiltonian for polyacetylene. The incoherent process is shown to occur between degenerate electronic states with distinct electronic configurations that are indirectly coupled via a third auxiliary state by vibronic interactions. A discussion of how to construct electronic superposition states in molecules that are truly robust to decoherence is also presented.

Highlights

We identify an intriguing feature of the electron–vibrational dynamics of molecular systems via a computational examination of transpolyacetylene oligomers
Vibronically induced resonant electronic population transfer (VIBRET) is seen to arise between degenerate electronic states with distinct electronic configurations that are indirectly coupled via a third auxiliary state by the electron–vibration interactions in the system
A striking feature of VIBRET is that it leads to population oscillations among the relevant levels that are analogous to those observed in beatings that result from coherent superposition states

Summary

Model and methods

In the SSH model, the PA is described as a tight-binding chain, where each site represents a CH unit, in which the π-electrons are coupled to distortions in the oligomer backbone by a parameterized electron–vibrational interaction. Decoherence effects are incorporated by propagating an ensemble of quantum-classical trajectories with initial conditions selected by importance sampling of the ground-state nuclear Wigner distribution function [12, 23] of the oligomer obtained in the harmonic approximation. In this way, the dynamics reflects the initial nuclear quantum distribution and is subject to the level broadening and internal relaxation mechanism induced by the vibronic couplings. Because of the electron–hole symmetry in the Hamiltonian, the orbital energies are always such that for each orbital in the valence band of energy − i there is an orbital in the conduction band of energy i

Results and discussion

Electronic coherence during the VIBRET

Conclusions