Attosecond Charge Migration Can Break Electron Symmetry While Conserving Nuclear Symmetry

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 proof-of-principle, namely, periodic attosecond charge migration in the σ_g + σ_u superposition state of the aligned H₂⁺ cation (nuclear point group D_∞h, but electron symmetry breaking D_∞h → C_∞v) and in the A₁ + B₂ superposition state of the oriented H₂O molecule (C_2v vs C_2v → C_s).