Laser-Control of Ultrafast \(\pi\)-Electron Ring Currents in Aromatic Molecules: Roles of Molecular Symmetry and Light Polarization  
[PERSON]  
1 Department of Chemistry, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan;  
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to be a powerful tool not only to observe but also to drive electron motion in molecules, which is much faster than nuclear motion and thus steers chemical reactions [15; 16]. In analogy to nuclear dynamics, the fundamental mechanism of laser-induced ultrafast electron dynamics is to create a coherent superposition of electronic eigenstates, i.e., an electronic WP. The bandwidth as well as the central frequency of an attosecond laser pulse produced by high harmonic generation [17; 18; 19; 20; 21; 22] lies in the extreme ultraviolet (XUV) region, which enables simultaneous transitions from the electronic ground state to many excited states. The electronic wave function of the superposition state varies in time periodically according to the energy gaps between the eigenstates involved, giving rise to large-scale electron motion in molecules. The period of the electronic state change is only a few femtoseconds for visible-light energy gaps or even of an attosecond order for UV energy gaps, where nuclei are almost static. The instantaneous and dramatic change in molecular electronic states caused by attosecond laser irradiation may lead to the control of molecular functionalities and chemical reactions, which is one of the ultimate goals of chemistry. The development of the new discipline \"attosecond chemistry\" will bring innovation to various areas, such as materials science and nanotechnology. In line with this direction, a number of theoretical researches have been carried out on laser-induced ultrafast electron dynamics in molecules [23; 24; 25; 26; 27; 28; 29; 30; 31; 32; 33; 34; 35; 36; 37]. For diatomic and triatomic systems, quantum simulations of electron localization in lithium hydride LiH [25; 26; 27] and lithium cyanide LiCN [23] were performed. Electron localization in such linear molecules can be regarded as a switching of electric dipole moments. A similar localization phenomenon was also predicted for benzene [34; 35; 36; 37], which is a larger, aromatic molecule. Gathering delocalized \(\pi\) electrons to specific atoms or bonds in an aromatic ring results in the weakening of aromaticity. In addition to these works dealing with electronic WPs of neutral molecules, investigations have also been conducted on electron (or charge) dynamics in cations [38; 39; 40; 41; 42; 43; 44; 45; 46; 47]. The hole density evolutions initiated upon the sudden removal of an electron from \(N\)-methylacetamide [41] glycine [43; 45], and small peptides [44] by attosecond laser pulses were calculated. The hole migrates from one end of the molecules to the other on attosecond to few-femtosecond time scales in line with the above expectation of the electronic state change for visible-light or UV energy gaps. The theoretical proposal for ultrafast charge migration was put into practice in 2014 for phenylalanine by pump-probe spectroscopy [48]. A sub-300-as XUV pump pulse triggers ionization and subsequent charge migration; a 4-fs visible/near-IR probe pulse then generates an ammonium dication fragment whose yield exhibits a 4-fs oscillation as a sign of the charge migration. To the best of our knowledge, this is the first experimental application of attosecond laser pulses to charge migration in molecules. Shortly thereafter, control of charge migration was demonstrated for pre-oriented iodoacetylene HCCI by using intense near-IR pulses (instead of attosecond XUV pulses) [49; 50]. 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 thus governs the fate of hole propagation.