Attosecond angular flux of partial charges on the carbon atoms of benzene in non-aromatic excited state

Spin current in the early stage of radical reactions and its mechanisms

We study the electronic spin flux (atomic-scale flow of the spin density in molecules) by a perturbation analysis and ab initio nonadiabatic calculations. We derive a general perturbative expression of the charge and spin fluxes and identify the driving perturbation of the fluxes to be the time derivative of the electron-nucleus interaction term in the Hamiltonian. We then expand the expression in molecular orbitals so as to identify relevant components of the fluxes. Our perturbation theory describes the electronic fluxes in the early stage of reactions in an intuitively clear manner. The perturbation theory is then applied to an analysis of the spin flux obtained in ab initio calculations of the radical reaction of O2 and CH3· starting from three distinct spin configurations; (a) CH3· and triplet O2 with total spin of the system set Stot=1/2 (b) CH3· and singlet O2, Stot=1/2, and (c) CH3· and triplet O2, Stot=3/2. Further analysis of the time-dependent behaviors of the spin flux in these numerical simulations reveals (i) the spin flux induces rearrangement of the local spin structure, such as reduction of the spin polarization arising from the triplet O2 and (ii) the spin flux flows from O2 to CH3· in the reaction starting from spin configuration (a) and from CH3· to O2 in that starting from configuration (b), whereas no major intermolecular spin flux was observed in that starting from configuration (c). Our study thus establishes the mechanism of the spin flux that rearranges the local spin structures associated with chemical bonds.

Short-wavelength radiation pulses with time-varying orbital angular momentum from tailored relativistic electron beams

In this Letter, we propose a novel, to the best of our knowledge, technique to generate short-wavelength radiation carrying time-varying orbital angular momentum (OAM) by tailoring relativistic beams in free-electron lasers. To manipulate the temporal properties of OAM beams, two time-delayed seed lasers with different OAM values are used to interact with the electron beam in the undulator. With this method, high-harmonic electron beam microbunching with a time-varying helical distribution can be tailored to match the time-varying instantaneous helical phase structure of the x ray beams. Theoretical and simulation results demonstrate that high-power x ray beams with time-varying OAM can be produced by the proposed technique, which opens new routes to scientific research in x ray science.

Ultrafast attosecond-magnetic-field generation of the charge migration process based on HeH2+ and H2 + electronically excited by circularly polarized laser pulses

The ultrafast process by the electron in molecular ions from one site or region to another that has come to be known as charge migration (CM), which is of fundamental importance to photon induced chemical or physical reactions. In this work, we study the electron current and ultrafast magnetic-field generation based on CM process of oriented asymmetric (HeH²⁺) and symmetric (H₂ ⁺) molecular ions. Calculated results show that they are ascribed to quantum interference of electronic states for these molecular ions under intense circularly polarized (CP) laser pulses. The two scenarios of (i) resonance excitation and (ii) direct ionization are considered through appropriately utilizing designed laser pulses. By comparison, the magnetic field induced by the scenario (i) is stronger than that of scenario (ii) for molecular ions. However, the scheme (ii) is very sensitive to the helicity of CP field, which is opposite to the scenario (i). Moreover, the magnetic field generated by H₂ ⁺ is stronger than that by HeH²⁺ through scenario (i). Our findings provide a guiding principle for producing ultrafast magnetic fields in molecular systems for future research in ultrafast magneto-optics.

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

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.

Is it really possible to control aromaticity of benzene with light?

Recent theoretical investigations claim that tailored laser pulses may selectively steer benzene's aromatic ground state to localized non-aromatic excited states. For instance, it has been shown that electronic wavepackets, involving the two lowest electronic eigenstates, exhibit subfemtosecond charge oscillation between equivalent Kekulé resonance structures. In this contribution, we show that such dynamical electron-localization in the molecule-fixed frame contravenes the principle of the indistinguishability of identical particles. This breach stems from a total omission of the nuclear degrees of freedom, giving rise to nonsymmetric electronic wavepackets under nuclear permutations. Enforcement of the latter leads to entanglement between the electronic and nuclear states. To obey quantum statistics, the entangled molecular states should involve compensating nuclear-permutation symmetries. This in turn engenders complete quenching of dynamical electron-localization in the molecule-fixed frame. Indeed, for the (six-fold) equilibrium geometry of benzene, group-theoretic analysis reveals that any electronic wavepacket exhibits a (D_6h) totally symmetric electronic density, at all times. Thus, our results clearly show that the six carbon atoms, and the six C-C bonds, always have equal Mulliken charges, and equal bond orders, respectively. However, electronic wavepackets may display dynamical localization of the electronic density in the space-fixed frame, whenever they involve both even and odd space-inversion (parity) or permutation-inversion symmetry. Dynamical spatial-localization can be probed experimentally in the laboratory frame, but it should not be deemed equivalent to charge oscillation between benzene's identical electronic substructures, such as Kekulé resonance structures.

Ultrafast X-ray photoelectron diffraction in triatomic molecules by circularly polarized attosecond light pulses

We theoretically study ultrafast photoelectron diffraction in triatomic molecules with cyclic geometry by ultrafast circular soft X-ray attosecond pulses.

Generation of extreme-ultraviolet beams with time-varying orbital angular momentum

Light fields carrying orbital angular momentum (OAM) provide powerful capabilities for applications in optical communications, microscopy, quantum optics, and microparticle manipulation. We introduce a property of light beams, manifested as a temporal OAM variation along a pulse: the self-torque of light. Although self-torque is found in diverse physical systems (i.e., electrodynamics and general relativity), it was not realized that light could possess such a property. We demonstrate that extreme-ultraviolet self-torqued beams arise in high-harmonic generation driven by time-delayed pulses with different OAM. We monitor the self-torque of extreme-ultraviolet beams through their azimuthal frequency chirp. This class of dynamic-OAM beams provides the ability for controlling magnetic, topological, and quantum excitations and for manipulating molecules and nanostructures on their natural time and length scales.

Probing Attosecond Electron Coherence in Molecular Charge Migration by Ultrafast X-Ray Photoelectron Imaging

Electron coherence is a fundamental quantum phenomenon in today’s ultrafast physics and chemistry research. Based on attosecond pump–probe schemes, ultrafast X-ray photoelectron imaging of molecules was used to monitor the coherent electron dynamics which is created by an XUV pulse. We performed simulations on the molecular ion H 2 + by numerically solving time-dependent Schrödinger equations. It was found that the X-ray photoelectron angular and momentum distributions depend on the time delay between the XUV pump and soft X-ray probe pulses. Varying the polarization and helicity of the soft X-ray probe pulse gave rise to a modulation of the time-resolved photoelectron distributions. The present results provide a new approach for exploring ultrafast coherent electron dynamics and charge migration in reactions of molecules on the attosecond time scale.

From Symmetry Breaking via Charge Migration to Symmetry Restoration in Electronic Ground and Excited States: Quantum Control on the Attosecond Time Scale

This article starts with an introductory survey of previous work on breaking and restoringthe electronic structure symmetry of atoms and molecules by means of two laser pulses. Accordingly,the first pulse breaks the symmetry of the system in its ground state with irreducible representationIRREPg by exciting it to a superposition of the ground state and an excited state with differentIRREPe. The superposition state is non-stationary, representing charge migration with period T inthe sub- to few femtosecond time domains. The second pulse stops charge migration and restoressymmetry by de-exciting the superposition state back to the ground state. Here, we present a newstrategy for symmetry restoration: The second laser pulse excites the superposition state to the excitedstate, which has the same symmetry as the ground state, but different IRREPe. The success dependson perfect time delay between the laser pulses, with precision of few attoseconds. The new strategyis demonstrated by quantum dynamics simulation for an oriented model system, benzene.