Abstract
Being motivated by the recent progress in attosecond laser technology, we theoretically explore the strategy of inducing ultrafast electron dynamics inherent to aromatic molecules, i.e., ring currents by means of polarized laser pulses. The main topic of discussion is how to control the direction of ring currents in an aromatic molecule of low symmetry, for which the design of an efficient control pulse cannot be achieved intuitively. We first consider a system with a single aromatic ring and show that coherent π-electron angular momentum, which oscillates with time, can be produced and controlled by a polarized laser pulse with its ellipticity and orientation properly chosen. Nonadiabatic couplings with molecular vibration gradually weaken the angular momentum, while the vibrational amplitude strongly depends on the polarization of incident light. This suggests the conversion of the polarization dependence of ring current into that of subsequent vibration, which may open a way to detect laser-driven ultrafast electron dynamics by vibrational spectroscopy. The laser-control scheme for the ring current is then extended to a molecule with two aromatic rings, which exhibits characteristic phenomena absent in that with a single ring. We demonstrate that two-dimensional switching of the direction of angular momentum is possible in such molecules. In addition, ring current can be localized at a specific ring by tailored lasers. The application of the present control method to polycyclic aromatic hydrocarbons will lead to the development of next-generation organic optical switching devices.
Highlights
Chemical reaction can be viewed as a drastic change in nuclear positions, accompanied by electron transfer or migration between atoms in matter.In photochemical reactions, nuclear motion is initiated by the generation of a coherent superposition of vibrational eigenstates, i.e., a quantum vibrational wave packet (WP) with light
We performed nuclear WP simulations within the Born-Oppenheimer approximation (BOA) [83], where the WPs propagated on the individual potential energy surfaces (PESs), and found that the amplitude of lz (t) exhibits periodic reduction and recovery in turn owing to the temporal change in the WP overlap but never decays monotonically [56,57,58]
We have focused on several fundamental issues of angular momentum of π electrons in a low-symmetry aromatic ring molecule
Summary
Chemical reaction (i.e., chemical bond formation and cleavage) can be viewed as a drastic change in nuclear positions, accompanied by electron transfer or migration between atoms in matter. The linear polarization direction chosen perpendicular or parallel to its molecular axis affects the superposition of cationic ground and excited states created by multiphoton ionization and governs the fate of hole propagation This experiment tells us that laser control of ultrafast electron dynamics in molecular systems is becoming reality. Numerical simulations showed that much stronger ring currents can be produced in benzene [51] and Mg porphyrin [52,53,54] by a circularly polarized laser pulse than by a static magnetic field available with present technology For these molecules the direction of electron flow is determined by the helicity of circular polarization.
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