arXiv subjects

Kenneth Schafer

Publications and source records attributed to Kenneth Schafer.

3 recordsLinked to original sources

arXiv2025

Polarization-Resolved Core Exciton Dynamics in LiF Using Attosecond Transient Absorption Spectroscopy

The ability to control absorption by modifying the polarization of light presents an exciting opportunity to experimentally determine the orbital alignment of absorption features. Here, attosecond extreme ultraviolet (XUV) transient absorption spectroscopy is used to investigate the polarization dependence of core exciton dynamics in LiF thin films at the Li+ K edge. XUV pulses excite electrons from the Li 1s core level into the conduction band, allowing for the formation of a p-orbital-like core exciton, aligned along the XUV light polarization axis. A sub-5 fs near-infrared (NIR) probe pulse then arrives at variable time delays, perturbing the XUV-excited states and allowing the coherence decay of the core exciton to be mapped. The coherence lifetimes are found to be ~2.4 +- 0.4 fs, which is attributed to a phonon-mediated dephasing mechanism as in previous core exciton studies. The differential absorption features are also shown to be sensitive to the relative polarization of the XUV and NIR fields. The parallel NIR probe induces couplings between the initial XUV-excited p-like bright exciton and s-like dark excitons. When crossed pump and probe polarizations are used, the coupling between the bright and dark states is no longer dipole-allowed, and the transient absorption signal associated with the coupling is suppressed by approximately 90%. This interpretation is supported by simulations of a few-level model system, as well as analysis of the calculated band structure. The results indicate that laser polarization can serve as a powerful experimental tool for exploring the orbital alignment of core excitonic states in solid-state materials.

cond-mat.mtrl-sci↗

arXiv2020

Charge Migration Manifests as Attosecond Solitons in Conjugated Organic Molecules

Charge migration is the electronic response that immediately follows localized ionization or excitation in a molecule, before the nuclei have time to move. It typically unfolds on sub-femtosecond time scales and most often corresponds to dynamics far from equilibrium, involving multi-electron interactions in a complex chemical environment. While charge migration has been documented experimentally and theoretically in multiple organic and inorganic compounds, the general mechanism that regulates it remains unsettled. In this work we use tools from nonlinear dynamics to analyze charge migration that takes place along the backbone of conjugated hydrocarbons, which we simulate using time-dependent density functional theory. In this electron-density framework we show that charge migration modes emerge as attosecond solitons and demonstrate the same type of solitary-wave dynamics in both simplified model systems and full three-dimensional molecular simulations. We show that these attosecond-soliton modes result from a balance between dispersion and nonlinear effects tied to time-dependent multi-electron interactions.%Our soliton-mode mechanism, and the nonlinear tools we use to analyze it, pave the way for understanding migration dynamics in a broad range of organic molecules.%For instance, we demonstrate the opportunities for chemically steering charge migration via molecular functionalization, which can alter both the initially localized electron perturbation and its subsequent time evolution.

physics.atm-clus↗

arXiv2019

Polarization-control of absorption of virtual dressed-states in helium

The extreme ultraviolet absorption spectrum of an atom is strongly modified in the presence of a synchronized intense infrared field. In this work we demonstrate control of the absorption properties of helium atoms dressed by an infrared pulse by changing the relative polarization of the infrared and extreme ultraviolet fields. Light-induced features associated with the dressed $1s2s$, $1s3s$ and $1s3d$ states, referred to as $2s^{+}$, $3s^{\pm}$ and $3d^{\pm}$ light induced states, are shown to be strongly modified or even eliminated when the relative polarization is rotated. The experimental results agree well with calculations based on the solution of the time-dependent Schrödinger equation using a restricted excitation model that allows efficient treatment of the three dimensional problem. We also present an analysis of the light induced states based on Floquet theory, which allows for a simple explanation of their properties. Our results open a new route to creating controllable superpositions of dipole allowed and non-dipole allowed states in atoms and molecules.

physics.optics↗