Electronic coherence dynamics in trans-polyacetylene oligomers

Abstract

Electronic coherence dynamics in trans-polyacetylene oligomers are considered by explicitly computing the time dependent molecular polarization from the coupled dynamics of electronic and vibrational degrees of freedom in a mean-field mixed quantum-classical approximation. The oligomers are described by the Su-Schrieffer-Heeger Hamiltonian and the effect of decoherence is incorporated by propagating an ensemble of quantum-classical trajectories with initial conditions obtained by sampling the Wigner distribution of the nuclear degrees of freedom. The electronic coherence of superpositions between the ground and excited and between pairs of excited states is examined for chains of different length, and the dynamics is discussed in terms of the nuclear overlap function that appears in the off-diagonal elements of the electronic reduced density matrix. For long oligomers the loss of coherence occurs in tens of femtoseconds. This timescale is determined by the decay of population into other electronic states through vibronic interactions, and is relatively insensitive to the type and class of superposition considered. By contrast, for smaller oligomers the decoherence timescale depends strongly on the initially selected superposition, with superpositions that can decay as fast as 50 fs and as slow as 250 fs. The long-lived superpositions are such that little population is transferred to other electronic states and for which the vibronic dynamics is relatively harmonic.

Reduced purities as measures of decoherence in many-electron systems

“…3D, the model that best adjusts to the observed behavior is M5 indicating that during photoexcitation the system is best described as an incoherent mixture between states |Φ 0 , |Φ 1 and |Φ 2 . This is because of the fast electronic decoherence timescale that is characteristic of the model and method employed 30,32 .…”

“…Note that the coherences or phase relationship between electronic eigenstates (the off-diagonal elements) in ρ e are determined by the overlaps S nm (t) = χ m |χ n between the environmental states associated with the electronic eigenstates. Thus, the loss of coherences inρ e (t) can be interpreted as the result of the decay of the S nm during the coupled electron-bath evolution [27][28][29][30] . Standard measures of decoherence capture precisely this.…”

“…Since s 10 = 1, both P 1 and P 2 follow the coherence decay, and the fall of P 2 is (N − 1) times larger than the one of P 1 . The partial recurrences in the purities signal vibronic evolution of the chain 30 . After 200 fs the system is well described as an incoherent mixture.…”

“…Specifically, we consider an oligoacetylene chain with 4 carbon atoms and 4 π electrons as described by the SuSchrieffer-Heeger (SSH) Hamiltonian 37 , a tight-binding model with electron-vibrational interactions. The details of the Hamiltonian and the Ehrenfest mixed quantumclassical method employed to follow the vibronic dynamics have been specified before 29,30,32 . What is of relevance to this discussion is that the system consists of 4 noninteracting π electrons distributed among 4 spectrally isolated molecular orbitals |ǫ n of energy ǫ n that allow for double occupancy, for a total of 19 possible N -particle levels (without counting spin-degeneracies) subject to decoherence.…”

Nature of Quantum States Created by One Photon Absorption: Pulsed Coherent vs Pulsed Incoherent Light

“…Rather, the observed fast coherence loss arises , in the incoherent light case, from the nature of the two wave packets as they move away from one another. That is, the molecular vibration serves as the decohering environment [16], and it is particularly effective in this case due to the highly unstructured and choppy nuclear wave functions created by incoherent light. If, in addition, coupling to an external environment is present, the decay of electronic coherence will also reflect this coupling [12].…”