Attosecond Charge Migration Can Break Electron Symmetry While Conserving Nuclear Symmetry

Abstract

Charge migration moves electrons from one molecular site to another, in a typical time domain from few hundred attoseconds to few femtoseconds. On this timescale, the nuclei stand practically still, implying that the nuclear point group symmetry is conserved. Because electrons move ultrafast, this can lead to a surprising effect, namely, breaking the spatial symmetry of the electron density in spite of the conservation of nuclear framework symmetry. We demonstrate theoretically that attosecond charge migration achieves this electron symmetry breaking if the electrons are prepared in a coherent superposition of nondegenerate electronic ground and excited states which transform according to different irreducible representations. Two simple examples provide a proofof-principle, namely, periodic attosecond charge migration in the σ g + σ u superposition state of the aligned H 2 + cation (nuclear point group D ∞h , but electron symmetry breaking D ∞h → C ∞v ) and in the A 1 + B 2 superposition state of the oriented H 2 O molecule (C 2v vs C 2v → C s ).

Electron Symmetry Breaking during Attosecond Charge Migration Induced by Laser Pulses: Point Group Analyses for Quantum Dynamics

“…We shall show below that this offers new opportunities for laser control of electron symmetry breaking. The new phenomenon calls for the present significant extension of the previous theory and applications [1][2][3][4][5].…”

“…This sets the stage for a list of the next challenges which we try to master in the rest of this paper. The historical overview centers attention on just five publications [1][2][3][4][5]. To the best of our knowledge, these are the only ones that assign different point group symmetries of the relevant one-electron densities of the initial states of the oriented molecules-this is always the electronic ground state, and its electronic symmetry is the same as the nuclear symmetry -and after symmetry breaking by the laser pulse.…”

“…Ref. [5] refers to preparations of electronic superposition states with different IRREPs in oriented molecules by laser pulses, but without explicit quantum dynamics simulations. The superposition states present attosecond charge migration.…”

“…Ref. [5] has the first rigorous derivation of the point group symmetries of the one-electron densities of the target states. The applications are for rather simple prototypes, namely D ∞h → C ∞v electron symmetry breaking and attosecond charge migration in the σ g + σ u superposition state of oriented H 2 + , with robust nuclear symmetry D ∞h , and by analogy for C 2v → C s electron symmetry breaking and attosecond charge migration in the A 1 + B 2 superposition state of oriented H 2 O, with robust nuclear symmetry C 2v .…”

“…The pioneering papers [1][2][3][4][5] call for the following extensions: (a) Rigorous assignments of electron symmetry breaking in supplementary examples for oriented benzene and for oriented Mg-porphyrin. For this purpose, we design linearly polarized laser pulses that initiate selective types of attosecond charge migration with corresponding electron symmetry breaking while conserving nuclear symmetry.…”