Thema der Dissertation:
Detection and control of chiral molecules Symmetry considerations and applications to photoelectron spectroscopy
Detection and control of chiral molecules Symmetry considerations and applications to photoelectron spectroscopy
Abstract: Chirality, the property of an object to exist in two non-superimposable mirror images, is central to chemistry, biology, and emerging technologies. The interaction between chiral objects is profoundly influenced by their handedness. On the molecular scale, this poses substantial challenges for enantioselective synthesis and drug design. From the perspective of fundamental science, chiral molecules are a promising playground to explore fundamental physical effects like parity violation and quantum tunneling. Both fields rely on efficient techniques for the detection and control of molecular chirality.
A particularly versatile and sensitive tool is photoelectron circular dichroism (PECD), i.e., the forward-backward asymmetry in the photoelectron angular distributions (PADs) of chiral molecules upon ionization with circularly polarized light. This thesis pursues two core objectives: (i) developing strategies for enhancing the efficacy of photoelectron spectroscopy in chiral-sensitive optical analysis, and (ii) exploring laser-based methods for generation and control of molecular chirality itself.
To achieve (i), we follow three approaches: Firstly, we present a combined theoretical and experimental implementation of a robust attosecond control scheme leveraging interferences in the photoelectron continuum driven by an extreme-ultraviolet pulse train and a delayed infrared pulse. By varying the pulse delay, the strength of chiral signatures in PADs and photoionization time delays of methyloxirane can almost be doubled. Secondly, we model how photoelectron recoil can introduce asymmetries in angular distributions of photofragments. A circular dichroism in the angular correlation of electron-ion coincidence spectra is reported already in the limit of negligible photoelectron recoil. Finally, we explore how rotational excitation can facilitate the detection of chirality in racemic mixtures. For this purpose we analyze PECD in anisotropic ensembles. We find that it is sensitive to inversion symmetry and to mirror-symmetry planes parallel or perpendicular to the polarization plane of the ionizing light; however, it does not exclude the possibility of a symmetry plane altogether, and thus is not a definitive indicator of molecular chirality in oriented molecular ensembles. We demonstrate that sequences of pulses polarized along three mutually orthogonal directions yield PECD in racemic mixtures. Whereas such chiral sequences of static, optical or THz fields yield PECD also for achiral polar asymmetric molecules, microwave three-wave-mixing can, in principle, unambiguously detect chirality in arbitrary molecular ensembles.
For (ii), we demonstrate theoretically that vibrational symmetry breaking can induce chirality in achiral molecules. Enantiomeric excess requires a field configuration generating the triple products of dipole moments and polarization vectors typical in chiroptical analysis. We also demonstrate that chiral pulse sequences are capable of inducing molecular chirality via angular momentum, opening a new route for laser based control of molecular chirality. Collectively, these advances offer new perspectives on chiral signatures in photoionization and provide tools for the controlled manipulation of molecular chirality relevant in both fundamental and applied contexts.
A particularly versatile and sensitive tool is photoelectron circular dichroism (PECD), i.e., the forward-backward asymmetry in the photoelectron angular distributions (PADs) of chiral molecules upon ionization with circularly polarized light. This thesis pursues two core objectives: (i) developing strategies for enhancing the efficacy of photoelectron spectroscopy in chiral-sensitive optical analysis, and (ii) exploring laser-based methods for generation and control of molecular chirality itself.
To achieve (i), we follow three approaches: Firstly, we present a combined theoretical and experimental implementation of a robust attosecond control scheme leveraging interferences in the photoelectron continuum driven by an extreme-ultraviolet pulse train and a delayed infrared pulse. By varying the pulse delay, the strength of chiral signatures in PADs and photoionization time delays of methyloxirane can almost be doubled. Secondly, we model how photoelectron recoil can introduce asymmetries in angular distributions of photofragments. A circular dichroism in the angular correlation of electron-ion coincidence spectra is reported already in the limit of negligible photoelectron recoil. Finally, we explore how rotational excitation can facilitate the detection of chirality in racemic mixtures. For this purpose we analyze PECD in anisotropic ensembles. We find that it is sensitive to inversion symmetry and to mirror-symmetry planes parallel or perpendicular to the polarization plane of the ionizing light; however, it does not exclude the possibility of a symmetry plane altogether, and thus is not a definitive indicator of molecular chirality in oriented molecular ensembles. We demonstrate that sequences of pulses polarized along three mutually orthogonal directions yield PECD in racemic mixtures. Whereas such chiral sequences of static, optical or THz fields yield PECD also for achiral polar asymmetric molecules, microwave three-wave-mixing can, in principle, unambiguously detect chirality in arbitrary molecular ensembles.
For (ii), we demonstrate theoretically that vibrational symmetry breaking can induce chirality in achiral molecules. Enantiomeric excess requires a field configuration generating the triple products of dipole moments and polarization vectors typical in chiroptical analysis. We also demonstrate that chiral pulse sequences are capable of inducing molecular chirality via angular momentum, opening a new route for laser based control of molecular chirality. Collectively, these advances offer new perspectives on chiral signatures in photoionization and provide tools for the controlled manipulation of molecular chirality relevant in both fundamental and applied contexts.
Zeit & Ort
08.09.2025 | 15:30
Hörsaal A (1.3.14)
(Fachbereich Physik, Arnimallee 14, 14195 Berlin)