Optimal photoelectron circular dichroism of a model chiral system.

We optimize the internuclear geometry and electronic structure of a model chiral system to achieve a maximal photoelectron circular dichroism (PECD) in its one-photon ionization by circularly polarized light. The electronic structure calculations are performed by the single center method, while the optimization is done using quantum alchemy employing a Taylor series expansion. Thereby, the effect of bond lengths and uncompensated charge distributions on the chiral response of the model is investigated theoretically in some detail. It is demonstrated that manipulating a chiral asymmetry of the ionic potential may enhance the dichroic parameter (i.e., the PECD) of the randomly oriented model system well beyond ß1 = 25%. Furthermore, we demonstrate that quantum alchemy is applicable to PECD despite the unusually strong coupling of spatial and electronic degrees of freedom and discuss the relative impact of the individual degrees of freedom in this model system. We define the necessary conditions for the computational design of PECD for real (non-model) chiral molecules using our approach.

Theoretical study of spin polarization in multiphoton ionization of Xe.

Spin polarization in the multiphoton above-threshold ionization of 5p3/2- and 5p1/2-electrons of Xe with intense 395nm, circularly polarized laser pulses, is investigated theoretically. For this purpose, we solve the time-dependent Schrödinger equation on the basis of spherical spinors. We, thus, simultaneously propagate the spin-up and spin-down single-active-electron wave packets, driven by the laser pulses in the ionic potential, which includes the spin-orbit interaction explicitly. The present theoretical results are in good agreement with the recent experimental results [D. Trabert et al., Phys. Rev. Lett. 120, 043202 (2018)].

Photoelectron circular dichroism of a model chiral anion.

Photoelectron circular dichroism (PECD) in the one-photon detachment of a model chiral anionic system is studied theoretically by the single center method. The computed chiral asymmetry, characterized by the dichroic parameter ß1 of up to about ±3%, is in good accord with the first experimental observations of the effect in photodetachment of amino acid anions [P. Krüger and K. M. Weitzel, Angew. Chem., Int. Ed. 60, 17861 (2021)]. Our findings confirm a general assumption that the magnitude of PECD is governed by the ability of an outgoing photoelectron wave packet to accumulate characteristic chiral asymmetry from the short-range part of the molecular potential.

Photoelectron circular dichroism in the multiphoton ionization by short laser pulses. III. Photoionization of fenchone in different regimes.

Photoelectron circular dichroism (PECD) in different regimes of multiphoton ionization of fenchone is studied theoretically using the time-dependent single center method. In particular, we investigate the chiral response to the one-color multiphoton or strong-field ionization by circularly polarized 400 nm and 814 nm optical laser pulses or 1850 nm infrared pulse. In addition, the broadband ionization by short coherent circularly polarized 413-1240 nm spanning pulse is considered. Finally, the two-color ionization by the phase-locked 400 nm and 800 nm pulses, which are linearly polarized in mutually orthogonal directions, is investigated. The present computational results on the one-color multiphoton ionization of fenchone are in agreement with the available experimental data. For the ionization of fenchone by broadband and bichromatic pulses, the present theoretical study predicts substantial multiphoton PECDs.

Controlling Dynamics of Postcollision Interaction.

A general scheme to get insight and to control postcollision interaction (PCI) by means of sequential double ionization with two high-frequency pulses is discussed. In particular, we propose to consider PCI of a slow photoelectron released by the pump pulse from a neutral atom with a fast photoelectron released by the time-delayed probe pulse from the created ion. This scheme is exemplified by the ab initio calculations performed for the prototypical helium atom. In order to visualize PCI effects in real time and real space, the corresponding time-dependent Schrödinger equation is solved by propagating two-electron wave packets in terms of essential stationary eigenstates of the unperturbed Hamiltonian. It is demonstrated that the exchange of energy between the slow and fast photoelectron wave packets in continuum, as well as the recapture of threshold photoelectrons owing to the PCI, can be controlled by the properties of the ionizing pulses and the time delay between them.

Photoelectron Circular Dichroism with Two Overlapping Laser Pulses of Carrier Frequencies ω and 2ω Linearly Polarized in Two Mutually Orthogonal Directions.

Using a model methanelike chiral system, we theoretically demonstrate a possibility to access photoelectron circular dichroism (PECD) by a single experiment with two overlapping laser pulses of carrier frequencies ω and 2ω, which are linearly polarized in two mutually orthogonal directions. Depending on the relative phase, the resulting electric field can be tailored to have two different rotational directions in the upper and lower hemispheres along the polarization of the ω pulse. We predict a strong forward-backward asymmetry in the emission of photoelectrons from randomly oriented samples, which has an opposite sign in the upper and lower hemispheres. The predicted PECD effect is phase and enantiomer sensitive, providing new insight in this fascinating fundamental phenomenon. The effect can be optimized by varying relative intensities of the pulses.

Photoelectron circular dichroism in the multiphoton ionization by short laser pulses. II. Three- and four-photon ionization of fenchone and camphor.

Angle-resolved multiphoton ionization of fenchone and camphor by short intense laser pulses is computed by the time-dependent single center method. Thereby, the photoelectron circular dichroism (PECD) in the three-photon resonance enhanced ionization and four-photon above-threshold ionization of these molecules is investigated in detail. The computational results are in satisfactory agreement with the available experimental data, measured for randomly oriented fenchone and camphor molecules at different wavelengths of the exciting pulses. We predict a significant enhancement of the multiphoton PECD for uniaxially oriented fenchone and camphor.

Circular Dichroism in Fluorescence Emission Following the C 1s→π* Excitation and Resonant Auger Decay of Carbon Monoxide.

Dichroism in angle-resolved spectra of circularly polarized fluorescence from freely-rotating CO molecules was studied experimentally and theoretically. For this purpose, carbon monoxide in the gas phase was exposed to circularly polarized soft X-ray synchrotron radiation. The photon energy was tuned across the C 1s→π* resonant excitation, which decayed via the participator Auger transition into the CO⁺ A ²Π state. The dichroic parameter ß1 of the subsequent CO⁺ (A ²Π → X ²Σ⁺) visible fluorescence was measured by photon-induced fluorescence spectroscopy. Present experimental results are explained with the ab initio electronic structure and dynamics calculations performed by the single center method. Our results confirm the possibility to perform partial wave analysis of the emitted photoelectrons in closed-shell molecules.

Photoelectron circular dichroism in the multiphoton ionization by short laser pulses. I. Propagation of single-active-electron wave packets in chiral pseudo-potentials.

A theoretical method to study the angle-resolved multiphoton ionization of polyatomic molecules is developed. It is based on the time-dependent formulation of the Single Center (TDSC) method and consists in the propagation of single-active-electron wave packets in the effective molecular potentials in the presence of intense laser pulses. For this purpose, the time-dependent Schrödinger equation for one electron, moving in a molecular field and interacting with an arbitrary laser pulse, is solved in spherical coordinates by an efficient numerical approach. As a test, the method is applied to the one- and two-photon ionizations of a model methane-like chiral system by circularly polarized short intense high-frequency laser pulses. Thereby, we analyze the photoelectron circular dichroism (PECD) in the momentum distribution. The considered model application illustrates the capability of the TDSC method to study multiphoton PECD in fixed-in-space and randomly oriented chiral molecules.

Electron Dynamics and Correlations During High-Order Harmonic Generation in Be.

We investigate theoretically the high-order harmonic generation in beryllium atom irradiated by a short 1850 nm linearly polarized laser pulse in the intermediate strong-field ionization regime with the Keldysh parameter of 0.85. To this end, the respective time-dependent Schrödinger equation is solved by the time-dependent restricted-active-space configuration-interaction (TD-RASCI) method. By systematically increasing the active space of included configurations, we demonstrate an individual effect of different physical processes evoked by the pulse, which, all together, significantly enrich and extend the computed high-order harmonic generation spectrum.