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

Abstract

Quantum simulations of the electron dynamics of oriented benzene and Mg-porphyrin driven by short (<10 fs) laser pulses yield electron symmetry breaking during attosecond charge migration. Nuclear motions are negligible on this time domain, i.e., the point group symmetries G = D6h and D4h of the nuclear scaffolds are conserved. At the same time, the symmetries of the one-electron densities are broken, however, to specific subgroups of G for the excited superposition states. These subgroups depend on the polarization and on the electric fields of the laser pulses. They can be determined either by inspection of the symmetry elements of the one-electron density which represents charge migration after the laser pulse, or by a new and more efficient group-theoretical approach. The results agree perfectly with each other. They suggest laser control of symmetry breaking. The choice of the target subgroup is restricted, however, by a new theorem, i.e., it must contain the symmetry group of the time-dependent electronic Hamiltonian of the oriented molecule interacting with the laser pulse(s). This theorem can also be applied to confirm or to falsify complementary suggestions of electron symmetry breaking by laser pulses.

Funneling dynamics in a phenylacetylene trimer: Coherent excitation of donor excitonic states and their superposition

“…With laser pulses of about 15fs, one observes a delay in the occupation of the localized sites of about 5fs and therefore a reduction of the motion amplitude due to the effect of the baths. If the target is to increase the lifetime of the asymmetry and thus of the coherence, other strategies should be tried, for instance, ultra-short attosecond pulses 113,114 , short half-cycle pulses 38 , succession of kick pulses already used for molecular orientation and alignment 115 , light-induced nonadiabaticity 116 , Floquet engineering 117,118 or exploit the optimal control tools with environment 119 or deap learning new proposals 120 .…”

Laser-Induced Electron Symmetry Restoration in Oriented Molecules Made Simple

“…2−14 The effect can also be rationalized by rigorous point group analyses of the laser-driven quantum dynamics. 15 It opens new opportunities, but it also poses new challenges. On the one hand, it paves the way to laser control of electron symmetry.…”

“…On the one hand, it paves the way to laser control of electron symmetry. 15 On the other hand, electron symmetry breaking is undesirable if the original symmetry has favorable properties, e.g., for the selection rules in photoelectron spectroscopy 16 or for electronic symmetry control of chemical reactions. 17 Electron symmetry breaking then calls for prompt symmetry restoration.…”

“…Electron symmetry is broken by a laser pulse which creates a superposition of |1Γ g ⟩ and an excited state |nΓ e ⟩ with different IRREP Γ g ≠ Γ e . The original electron symmetry G is then broken to a subgroup S of G. 15 The concept is to restore electron symmetry S → G by means of a reoptimized π-laser pulse which transfers the component in the excited state |nΓ e ⟩ into one or more excited states |n′Γ e′ ⟩ with IRREP Γ e′ = Γ g . This approach will be demonstrated by…”

“…Laser pulses can break electron symmetry in many ways. 15 Here, we employ a reoptimized x-polarized π/2-laser pulse with duration T b which transfers the initial state |1Γ g ⟩ into the superposition of |1Γ g ⟩ and target state |nΓ e ⟩ with equal populations, P nΓ e (T b ) = P 1Γ g (T b ) = 1/2. Likewise, electron symmetry is restored by means of a reoptimized x-polarized πlaser pulse with duration T r which is designed to transfer |nΓ e ⟩ This symmetry is also imposed on the one-electronic density of the initial electronic ground state |1Γ g ⟩ = |1Σ + ⟩ (cf.…”