Electron and nuclear dynamics following ionisation of modified bismethylene-adamantane

Abstract

We have simulated the coupled electron and nuclear dynamics using the Ehrenfest method upon valence ionisation of modified bismethylene-adamantane (BMA) molecules where there is an electron transfer between two π bonds. We have shown that the nuclear motion significantly affects the electron dynamics after a few fs when the electronic states involved are close in energy. We have also demonstrated how the non-stationary electronic wave packet determines the nuclear motion, more precisely the asymmetric stretching of the two π bonds, illustrating "chargedirected reactivity". Taking into account the nuclear wave packet width results in the dephasing of electron dynamics with a half-life of 8 fs; this eventually leads to the equal delocalisation of the hole density over the two methylene groups and thus symmetric bond lengths.

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

“…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.…”

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

“…In recent work, we showed that allowing the nuclei to move has little effect on the dephasing time. 14 …”

“…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. In this article, we study norbornadiene (NBD) and poly-cyclicnorbornadiene (PLN) systems (see Figure 1), where the charge migration occurs between two ethylenic cation moieties.…”

“…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. In the neutral ground vibrational state, the nuclear positions are delocalized.…”

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

“…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]. "…”