Electron dynamics upon ionization: Control of the timescale through chemical substitution and effect of nuclear motion

Vacher, Morgane; Mendive-Tapia, David; Bearpark, Michael J.; Robb, Michael A.

doi:10.1063/1.4913515

Cited by 46 publications

(53 citation statements)

References 35 publications

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“…Previous theoretical studies have shown the system dependence of the effect of nuclear motion using the Ehrenfest method. 8,10,11 Our studies on para-xylene 12 and phenylamines 13 showed that the electron dynamics was not destroyed by the nuclear motion over the studied timeframe (∼20 fs) but the frequency and amplitude was altered, in agreement with studies on methyl substituted benzenes. 11 Only recently has the nuclear delocalization been taken into account in theoretical studies, [12][13][14] where it has been demonstrated that it, in general, will lead to a loss in electron density oscillations, with the time scale being system dependent.…”

Section: Introductionsupporting

confidence: 70%

“…In previous work, [8][9][10][11] we have shown that we can engineer electron dynamics near a conical intersection. However, we have also demonstrated that nuclear spatial delocalization, due to the zero-point energy of the neutral species, can "wash out" the electron density oscillations [12][13][14] because of the spread of the energy gaps in the nuclear wavepacket.…”

Section: Introductionmentioning

confidence: 99%

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Charge migration in polycyclic norbornadiene cations: Winning the race against decoherence

Jenkins

Vacher

Twidale

et al. 2016

The Journal of Chemical Physics

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The observation of electronic motion remains a key target in the development of the field of attoscience. However, systems in which long-lived oscillatory charge migration may be observed must be selected carefully, particularly because it has been shown that nuclear spatial delocalization leads to a loss of coherent electron density oscillations. Here we demonstrate electron dynamics in norbornadiene and extended systems where the hole density migrates between two identical chromophores. By studying the effect of nuclear motion and delocalization in these example systems, we present the physical properties that must be considered in candidate molecules in which to observe electron dynamics. Furthermore, we also show a key contribution to nuclear delocalization arises from motion in the branching plane of the cation. For the systems studied, the dephasing time increases with system size while the energy gap between states, and therefore the frequency of the density oscillation, decreases with size (obeying a simple exponential dependence on the inter-chromophore distance). We present a system that balances these two effects and shows several complete oscillations in the spin density before dephasing occurs. C 2016 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license

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Section: Introductionsupporting

confidence: 70%

Section: Introductionmentioning

confidence: 99%

Charge migration in polycyclic norbornadiene cations: Winning the race against decoherence

Jenkins

Vacher

Twidale

et al. 2016

The Journal of Chemical Physics

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“…2). In both 040502-2 molecules, the equilibrium geometry of the neutral species is in the vicinity of a crossing between the two lowest-energy electronic states of the cationic species [30,[33][34][35]. The nonzero energy gap at the equilibrium geometry of the neutral species is needed to induce the electron dynamics we study.…”

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confidence: 99%

“…These simulations predict long-lived oscillating motion in the electronic density at a well-defined frequency, but they neglect both the spatial delocalization of the nuclear wave packet and the nuclear motion. We have previously shown examples of how nuclear motion affects electron dynamics after a few femtoseconds [28][29][30]. Despré and coworkers recently simulated hole migration at a small number * mike.robb@imperial.ac.uk of distorted geometries [31].…”

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confidence: 99%

Electron dynamics following photoionization: Decoherence due to the nuclear-wave-packet width

et al. 2015

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The advent of attosecond techniques opens up the possibility to observe experimentally electron dynamics following ionization of molecules. Theoretical studies of pure electron dynamics at single fixed nuclear geometries in molecules have demonstrated oscillatory charge migration at a well-defined frequency but often neglecting the natural width of the nuclear wave packet. The effect on electron dynamics of the spatial delocalization of the nuclei is an outstanding question. Here, we show how the inherent distribution of nuclear geometries leads to dephasing. Using a simple analytical model, we demonstrate that the conditions for a long-lived electronic coherence are a narrow nuclear wave packet and almost parallel potential-energy surfaces of the states involved. We demonstrate with numerical simulations the decoherence of electron dynamics for two real molecular systems (paraxylene and polycyclic norbornadiene), which exhibit different decoherence time scales. To represent the quantum distribution of geometries of the nuclear wave packet, the Wigner distribution function is used. The electron dynamics decoherence result has significant implications for the interpretation of attosecond spectroscopy experiments since one no longer expects long-lived oscillations. The generation of attosecond pulses in the extreme ultraviolet range (using high harmonic generation in gases) [1,2] has opened up the possibility to probe dynamics in atoms, molecules, and solids with attosecond resolution [3-5]-the natural time scale of electronic motion. Attosecond techniques have since been developed and applied successfully to a range of problems, including the real-time observation of electronic relaxation in krypton atoms [6], the measurement of delays in photoemission of electrons in condensed-matter [7] and atomic [8] systems using the streaking technique, and the observation of electron dynamics in krypton atoms upon valence ionization using transient absorption spectroscopy [9].One key target of attosecond experiments remains the real-time observation and control of electron dynamics upon ionization in molecules [10][11][12][13][14][15][16][17]. The interference between electronic eigenstates, populated coherently, alternates between constructive and destructive and leads to oscillating motion of the electronic density with a period inversely proportional to the energy gap. This is "pure" electron dynamics (i.e., takes place even if the nuclei are fixed) and is often called charge migration in the literature [18] or hole migration if it is induced by electron correlation [19,20].A fascinating and outstanding question in the theoretical description of electron dynamics in molecules is the effect of the nuclei since most studies are carried out at a single fixed nuclear geometry (usually the equilibrium geometry of the neutral species) [21][22][23][24][25][26]. (Some studies include several conformers [22,27] but again with a single geometry per conformer.) These simulations predict long-lived oscillating motion in the electron...

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“…For example, in studies using Ehrenfest trajectories on the ionisation in aromatic molecules, a distortion from the equilibrium geometry was required to see charge migration in benzene, but it happens spontaneously in toluene and para-xylene [45], as it does in the non-aromatic bismethylene-adamantane [46]. "…”

Section: Model Hamiltonianmentioning

confidence: 99%

Using quantum dynamics simulations to follow the competition between charge migration and charge transfer in polyatomic molecules

et al. 2017

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Recent work, particularly by Cederbaum and co-workers, has identified the phenomenon of charge migration, whereby charge flow occurs over a static molecular framework after the creation of an electronic wavepacket. In a real molecule, this charge migration competes with charge transfer, whereby the nuclear motion also results in the re-distribution of charge. To study this competition, quantum dynamics simulations need to be performed. To break the exponential scaling of standard grid-based algorithms, approximate methods need to be developed that are efficient yet able to follow the coupled electron-nuclear motion of these systems. Using a simple model Hamiltonian based on the ionisation of the allene molecule, the performance of different methods based on Gaussian Wavepackets is demonstrated.

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Electron dynamics upon ionization: Control of the timescale through chemical substitution and effect of nuclear motion

Cited by 46 publications

References 35 publications

Charge migration in polycyclic norbornadiene cations: Winning the race against decoherence

Charge migration in polycyclic norbornadiene cations: Winning the race against decoherence

Electron dynamics following photoionization: Decoherence due to the nuclear-wave-packet width

Using quantum dynamics simulations to follow the competition between charge migration and charge transfer in polyatomic molecules

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