Jörn Manz

Quantum dynamics simulations reveal how a linearly polarized laser pulse can induce attosecond chirality flips in achiral triatomic molecules via electronic superposition states—a simple, robust approach to ultrafast chiral control.

The vibronic ground state of HO–H–OH− represents axial Ra and Sa enantiomers, with equal probabilities. It can be prepared thermodynamically at low temperatures (4K). If the chirality is measured to be Ra, then wavefunction collapse induces periodic quantum stereo-mutation from Ra to Sa to Ra to Sa and so on. Similarly, the observation of Sa induces chirality flips from Sa to Ra to Sa to Ra and so on. The period of stereo-mutation is τ = 2.554 ps. The phenomenon is supported by the low energy barrier 174.6 hc cm−1 between the enantiomers and by the light masses of the hydrogen atoms, which interchange positions during stereo-mutation. Interchanges of heavier atoms and higher energy barriers would prohibit Schrödinger cat ground states. This is demonstrated for the counter example, isotopically substituted 12CDH13CH2–Si–12C13C. The results are obtained by means of quantum chemical calculations and quantum dynamics simulations.

Chirality is an important topic in biology, chemistry and physics. Here we show that ultrashort circularly polarized laser pulses, which are chiral, can be fired on achiral oriented molecules to induce chirality in their electronic densities, with chirality flips within femtoseconds or even attoseconds. Our results, obtained by quantum dynamics simulations, use the fact that laser pulses can break electronic symmetry while conserving nuclear symmetry. Here two laser pulses generate a superposition of three electronic eigenstates. This breaks all symmetry elements of the electronic density, making it chiral except at the periodic rare events of the chirality flips. As possible applications, we propose the combination of the electronic chirality flips with Chiral Induced Spin Selectivity.

We develop a general method to study ultrafast laser induced charge migration in molecules, which includes both electronic and nuclear dynamics. The method can be applied to relatively large systems. A detailed analysis of charge migration in pyrene is performed. Decoherences and recoherences of charge migration in pyrene are found and explained in terms of nuclear motions.

Electron symmetry determines many important properties of molecules, from selection rules for photoelectron spectroscopy to symmetry selection rules for chemical reactions. The original electron symmetry is broken if a laser pulse changes the initial state, typically the ground state g, to a superposition of g and an excited state e with different irreducible representations (IRREPs). Quantum dynamics simulations for two examples, the oriented benzene and LiCN molecules, show that the original electron symmetry can be restored by means of a reoptimized π-laser pulse which transfers the component in the excited state e to another state e′, or to several others with the same IRREP as the ground state. This method lends itself to much easier experimental applications than all previous ones because it allows the healing of electron symmetry immediately, without any attosecond constraint on the timing of the second pulse.

Quantum simulations of the electron dynamics of oriented benzene and Mg-porphyrin driven by short (6h and D_4h 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.

Charge migration moves electrons from one molecular site to another, in the typical time domain from few hundred attoseconds to few femtoseconds. On this timescale, the nuclei stand practically still, implying that nuclear point group symmetry is conserved. Since 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 non-degenerate 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 A_1+B_2 superposition state of the oriented H2O molecule (C_2v versus C_2v ---> C_s).

This work suggests an approach to a new target of laser control of charge migration in molecules or molecular ions. The target is motivated by the fact that nuclear motions can not only cause decoherence of charge migration, typically within few femtoseconds, but they may also enable the reappearance of charge migration after much longer times, typically several tens or even hundreds of femtoseconds. This phenomenon is called recoherence of charge migration, opposite to its decoherence. The details depend on the initiation of the original charge migration by an ultrashort strong intense pump laser pulse. It may reappear quasiperiodically, with reference period T_r. We show that a well-designed pump-dump laser pulse can enforce recoherences of charge migration at different target times T_c, for example, at T_c ≈ T_r/2. The approach is demonstrated by quantum dynamics simulations of the laser driven electronic and nuclear motions in the oriented linear cation HCCI⁺. First, the concept is explained in terms of a didactic one-dimensional (1D) model that accounts for the decisive CI stretch. The 1D results are then confirmed by a three-dimensional model for the complete set of the CH, CC, and CI stretches.