♦ Laser Control of Attosecond Electron Dynamics Nonadiabatically Coupled with Femtosecond Molecular Vibrations

The continuous progress in laser technology has been vigorously promoting applications of attosecond/several-femtosecond laser pulses to control and observation of ultrafast intramolecular electron dynamics, which governs chemical reactions. Each of the parameters that characterize a laser pulse plays a decisive role in the excitation of molecules; for example, the light intensity of the pulse affects the number of molecules excited and the central frequency specifies the average energy absorbed by the molecules. Among the parameters, laser polarization is attracting more and more attention as a key controlling factor of molecular motions. It can, in general, be classified into three categories: linear, circular, and elliptical polarizations, in which the tip of the electric field vector describes a straight line, circle, and ellipse, respectively, in the plane perpendicular to the propagation direction of the light as time progresses. So far, the characteristic oscillating features of polarized laser pulses have been extensively utilized to manipulate molecular rotational states (molecular alignment and orientation) on a picosecond time scale.
The present status of the application of polarized lights is entering into the next stage, that is, control of ultrafast dynamics such as valence-electron motions and molecular vibrations with shorter attosecond/several-femtosecond polarized laser pulses. In fact, the generation of polarization-shaped femtosecond UV/Vis laser pulses has recently become realizable. An aromatic molecule is characterized by π electrons delocalized over its ring, which are movable and can be excited by UV/Vis lights. Ring current, which is induced by π electrons flowing in either a clockwise or counterclockwise direction along an aromatic ring, is a model case to investigate the manipulation of ultrafast electron dynamics in complex polyatomic molecules. Previous theoretical studies proposed that π electrons of a highly symmetric aromatic molecule such as benzene can be rotated around its ring by applying a circularly polarized UV laser pulse. In contrast, we demonstrated that if the molecular symmetry is lowered, e.g., by introducing functional groups, ring currents can be driven in the aromatic molecule by a nonhelical, linearly polarized UV laser pulse.
Thereafter, laser-induced electron dynamics in ring-shaped systems have been reported by other groups as well; however, they all neglected nuclear motions. To address this important issue, we theoretically and numerically analyzed the nonadiabatic coupling between laser-driven ring currents and molecular vibrations in aromatic molecules, and revealed that the vibrational amplitudes strongly depend on the rotation direction of π electrons. Furthermore, we extended our theory to arbitrary laser polarizations including linear, circular, and elliptical ones, and generally predicted the laser-polarization dependence of ring currents and molecular vibrations. These findings suggest that laser-induced attosecond electron dynamics significantly affects femtosecond molecular vibrations, which may offer a new way to observe ultrafast intramolecular electron motions from vibrational spectroscopy on a longer time scale.