Strong Nuclear Ring Currents and Magnetic Fields in Pseudorotating OsH4Molecules Induced by Circularly Polarized Laser Pulses

Spatio-Temporal Symmetries of Electronic Chirality Flips in Oriented RbCs Induced by two Coincident Laser Pulses with Circular ++, +-, -+, -- Polarizations

Recently it has been shown that two coincident well designed laser pulses with two different combinations of circular polarizations (+ + ${ + + }$ or- + ${ - + }$ ) can create chiral electronic densities in an oriented heteronuclear diatomic molecule. Subsequently, the chirality flips from the electronic Ra to Sa to Ra to Sa etc. enantiomers, with periods in the femtosecond (fs) and attosecond (as) time domains. The results were obtained by means of quantum dynamics simulations for oriented NaK. Here we investigate the electronic chirality flips in oriented RbCs induced by all possible (+ + ${ + + }$ ,- + ${ - + }$ ,+ - ${ + - }$ ,- - ${ - - }$ ) combinations of circular polarizations of two coincident well-designed laser pulses. Accordingly, the+ + ${ + + }$ and- - ${ - - }$ as well as the+ - ${ + - }$ and- + ${ - + }$ combinations generate opposite electronic enantiomers, e. g. Ra versus Sa, followed by opposite periodic chirality flips, e. g. from Ra to Sa to Ra to Sa etc. versus from Sa to Ra to Sa to Ra etc, with periods in the fs and as time domains, respectively. The laser induced spatio-temporal symmetries are derived from first principles and illustrated by quantum dynamics simulations.

Molecular Pseudorotation in Phthalocyanines as a Tool for Magnetic Field Control at the Nanoscale

Metal phthalocyanines, a highly versatile class of aromatic, planar, macrocyclic molecules with a chelated central metal ion, are topical objects of ongoing research and particularly interesting due to their magnetic properties. However, while the current focus lies almost exclusively on spin-Zeeman-related effects, the high symmetry of the molecule and its circular shape suggests the exploitation of light-induced excitation of 2-fold degenerate vibrational states in order to generate, switch, and manipulate magnetic fields at the nanoscale. The underlying mechanism is a molecular pseudorotation that can be triggered by infrared pulses and gives rise to a quantized, small, but controllable magnetic dipole moment. We investigate the optical stimulation of vibrationally induced molecular magnetism and estimate changes in the magnetic shielding constants for confirmation by future experiments.

On the Probability Density of the Nuclei in a Vibrationally Excited Molecule

For localized and oriented vibrationally excited molecules, the qualitative features of the one-body probability density of the nuclei (one-nucleus density) are investigated. Like the familiar and widely used one-electron density that represents the probability of finding an electron at a given location in space, the one-nucleus density represents the probability of finding a nucleus at a given position in space independent of the location of the other nuclei and independent of their type. In contrast to the electrons, however, the nuclei are comparably localized. Due to this localization of the individual nuclei, the one-nucleus density provides a quantum-mechanical representation of the "chemical picture" of the molecule as an object that can largely be understood in a three-dimensional space, even though its full nuclear probability density is defined on the high-dimensional configuration space of all the nuclei. We study how the nodal structure of the wavefunctions of vibrationally excited states translates to the one-nucleus density. It is found that nodes do not necessarily lead to visible changes in the one-nucleus density: Already for relatively small molecules, only certain vibrational excitations change the one-nucleus density qualitatively compared to the ground state. It turns out that there are simple rules for predicting the shape of the one-nucleus density from the normal mode coordinates. A Python module for the computation of the one-nucleus density is provided at https://gitlab.com/axelschild/mQNMc.

Quantum control of coherent π-electron ring currents in polycyclic aromatic hydrocarbons

We present results for quantum optimal control (QOC) of the coherent π electron ring currents in polycyclic aromatic hydrocarbons (PAHs). Since PAHs consist of a number of condensed benzene rings, in principle, there exist various coherent ring patterns. These include the ring current localized to a designated benzene ring, the perimeter ring current that flows along the edge of the PAH, and the middle ring current of PAHs having an odd number of benzene rings such as anthracene. In the present QOC treatment, the best target wavefunction for generation of the ring current through a designated path is determined by a Lagrange multiplier method. The target function is integrated into the ordinary QOC theory. To demonstrate the applicability of the QOC procedure, we took naphthalene and anthracene as the simplest examples of linear PAHs. The mechanisms of ring current generation were clarified by analyzing the temporal evolutions of the electronic excited states after coherent excitation by UV pulses or (UV+IR) pulses as well as those of electric fields of the optimal laser pulses. Time-dependent simulations of the perimeter ring current and middle ring current of anthracene, which are induced by analytical electric fields of UV pulsed lasers, were performed to reproduce the QOC results.

Induction of unidirectional π-electron rotations in low-symmetry aromatic ring molecules using two linearly polarized stationary lasers

A new laser-control scenario of unidirectional π-electron rotations in a low-symmetry aromatic ring molecule having no degenerate excited states is proposed. This scenario is based on dynamic Stark shifts of two relevant excited states using two linearly polarized stationary lasers. Each laser is set to selectively interact with one of the two electronic states, the lower and higher excited states are shifted up and down with the same rate, respectively, and the two excited states become degenerate at their midpoint. One of the four control parameters of the two lasers, i.e. two frequencies and two intensities, determines the values of all the other parameters. The direction of π-electron rotations, clockwise or counter-clockwise rotation, depends on the sign of the relative phase of the two lasers at the initial time. An analytical expression for the time-dependent expectation value of the rotational angular momentum operator is derived using the rotating wave approximation (RWA). The control scenario depends on the initial condition of the electronic states. The control scenario with the ground state as the initial condition was applied to toluene molecules. The derived time-dependent angular momentum consists of a train of unidirectional angular momentum pulses. The validity of the RWA was checked by numerically solving the time-dependent Schrödinger equation. The simulation results suggest an experimental realization of the induction of unidirectional π-electron rotations in low-symmetry aromatic ring molecules without using any intricate quantum-optimal control procedure. This may open up an effective generation method of ring currents and current-induced magnetic fields in biomolecules such as amino acids having aromatic ring molecules for searching their interactions.

Concerted Electronic and Nuclear Fluxes During Coherent Tunnelling in Asymmetric Double-Well Potentials

The quantum theory of concerted electronic and nuclear fluxes (CENFs) during coherent periodic tunnelling from reactants (R) to products (P) and back to R in molecules with asymmetric double-well potentials is developed. The results are deduced from the solution of the time-dependent Schrödinger equation as a coherent superposition of two eigenstates; here, these are the two states of the lowest tunnelling doublet. This allows the periodic time evolutions of the resulting electronic and nuclear probability densities (EPDs and NPDs) as well as the CENFs to be expressed in terms of simple sinusodial functions. These analytical results reveal various phenomena during coherent tunnelling in asymmetric double-well potentials, e.g., all EPDs and NPDs as well as all CENFs are synchronous. Distortion of the symmetric reference to a system with an asymmetric double-well potential breaks the spatial symmetry of the EPDs and NPDs, but, surprisingly, the symmetry of the CENFs is conserved. Exemplary application to the Cope rearrangement of semibullvalene shows that tunnelling of the ideal symmetric system can be suppressed by asymmetries induced by rather small external electric fields. The amplitude for the half tunnelling, half nontunnelling border is as low as 0.218 × 10(-8) V/cm. At the same time, the delocalized eigenstates of the symmetric reference, which can be regarded as Schrödinger's cat-type states representing R and P with equal probabilities, get localized at one or the other minima of the asymmetric double-well potential, representing either R or P.

Staircase patterns of nuclear fluxes during coherent tunneling in excited doublets of symmetric double well potentials

Step-by-step flux for one-by-one transfers of the lobes of the density, from the reactant (left) to the product (right) in the excited tunneling doublet.

Quantum theory of concerted electronic and nuclear fluxes associated with adiabatic intramolecular processes

Example of concerted electronic (right) and nuclear (left) fluxes: isomerization of B₄.

Optimal white light control of the negative to neutral to positive charge transition (NeNePo) in the electronic manifold of the silver trimer

Control over the electronic state of the Ag(3) cluster is approached via a progression of ultrafast photoinduced transitions within the full electronic manifold of the negative to the neutral and finally the cationic state of the system. High-bandwidth supercontinuum laser pulses ranging from 500 to 950 nm are employed for addressing the wide range of electronic resonance conditions associated with the ladder climbing process of a tandem photoelectron detachment and a resonance enhanced multiphoton ionization (REMPI). With the control of the phase over the full spectral envelope of the supercontinuum in a pulse shaper arrangement, pulse forms are generated with the aim of synchronizing ultrashort subpulse sequences to the characteristic dynamics of the system during charge reversal. Pulse forms ranging over several hundred femtoseconds in total duration and subpulse structures down to 15 fs duration with a variable spectral composition can be obtained for this purpose. A free optimization based on a closed-loop genetic algorithm is employed for ordering the subpulse sequences to match the structural evolution of the system. The effective control attainable in this scenario is evaluated in view of maintaining a defined sequence of electronic transitions within the complex dynamic response of the system during the photoexcitation. Further emphasis is made on analyzing the degree of control attainable in the nonlinear regime of multiphoton excitation at supercontinuum bandwidths.