Electron Dynamics upon Ionization of Polyatomic Molecules: Coupling to Quantum Nuclear Motion and Decoherence

Abstract

Knowledge about the electronic motion in molecules is essential for our understanding of chemical reactions and biological processes. The advent of attosecond techniques opens up the possibility to induce electronic motion, observe it in real time, and potentially steer it. A fundamental question remains the factors influencing electronic decoherence and the role played by nuclear motion in this process. Here, we simulate the dynamics upon ionization of the polyatomic molecules paraxylene and modified bismethylene-adamantane, with a quantum mechanical treatment of both electron and nuclear dynamics using the direct dynamics variational multiconfigurational Gaussian method. Our simulations give new important physical insights about the expected decoherence process. We have shown that the decoherence of electron dynamics happens on the time scale of a few femtoseconds, with the interplay of different mechanisms: the dephasing is responsible for the fast decoherence while the nuclear overlap decay may actually help maintain it and is responsible for small revivals. DOI: 10.1103/PhysRevLett.118.083001 Electronic motion initiates specific rearrangements of atoms in molecules that are responsible for chemical reactions and biological processes. Because of the advent of attosecond techniques [1,2], it is possible to induce electron dynamics in molecules. Observing and potentially steering electronic motion on its natural time scale may provide novel pathways towards controlling chemical processes [3][4][5][6][7][8]. Since the electron distribution is usually considered to be changing much faster than the nuclear geometry, many theoretical studies treat molecular electron dynamics upon ionization as a purely electronic process, at a single static nuclear geometry [9][10][11][12]: long-lived oscillatory charge migration is then predicted. The fixed-nuclei and single-geometry approximations have however limited validity [13][14][15][16][17][18][19]. The fundamental challenge is to understand to what extent the electronic wave packet retains its coherence, i.e., how long the oscillations in the electronic density survive, in the presence of interactions with the nuclear degrees of freedom.Using a semiclassical description for the coupled systembath evolution, Fiete and Heller identified three processes that contribute to decoherence of the quantum system [20]: (i) system wave packet displacement, (ii) bath overlap decay, and (iii) phase jitter. In the context of molecular electron dynamics, the "system" consists of the electrons and the "bath" of the nuclei. The three mechanisms above can respectively be interpreted as (i) change in the electronic state populations, (ii) decrease of the overlap between the nuclear wave packets on different electronic states, and (iii) dephasing of the different wave packet components. The importance of these mechanisms on the coherent electron dynamics upon molecular ionization remains an outstanding question, that we aim to address in the present Letter.Previous works showed that the n...

Quantum modeling of ultrafast photoinduced charge separation

“…Finally, MCTDH is not suited for the description of the complex dynamics of nearly chaotic systems, especially when different PES topologies (bound vs. unbound states) need to be considered. It is worth mentioning that with the advent of the Direct Dynamics variational method (DD-vMCG [223]) this situation may evolve rapidly in the near future [224,225].…”

Control of Nuclear Dynamics through Conical Intersections and Electronic Coherences

“…Such calculations predict long-lived coherences and electron hole migrations driven by electron correlation [5,[7][8][9]. However, recent quantum-dynamical studies show that the motion of nuclei cannot be neglected and that nuclear motion can lead to electronic decoherence within a few femtoseconds [10][11][12][13][14][15].The interplay of electronic and nuclear motion becomes especially relevant in the presence of strong nonadiabatic couplings, as the Born-Oppenheimer separation breaks down and the time scales of electronic and nuclear motion become comparable [16]. Nonadiabatic couplings are particularly strong at conical intersections (C.I.…”

“…), which are abundant in the potential energy landscape of polyatomic molecules [17,18]. First insight into the influence of C.I.s on electronic coherence was obtained recently with a quantum-dynamical treatment of paraxylene and BMA [5,5], but a systematic understanding remains elusive [14].Nonadiabatic couplings and C.I.s are already exploited in control schemes employing femtosecond laser pulses. The underlying processes are typically well understood and the nuclear wave packet can be steered to desired reaction products [16,[19][20][21][22].…”

“…), which are abundant in the potential energy landscape of polyatomic molecules [17,18]. First insight into the influence of C.I.s on electronic coherence was obtained recently with a quantum-dynamical treatment of paraxylene and BMA [5,5], but a systematic understanding remains elusive [14].…”

“…The numerical accuracy of the simulations is assured by adjusting the number of single-particle functions (SPF) used such that the natural weight of the highest SPF is below 10 −4 [32]. A basis-independent measure for the electronic coherence is given by the electronic purity Trðρ 2 Þ [14,15], where ρ is the reduced density matrix of the electronic subsystem expressed as…”