Description
Chirality is a fundamental concept in many research fields, including physics, chemistry, and biology. A chiral object is not superimposable on its mirror image or, equivalently, lacks an improper rotational axis. Each chiral molecule exists in two structures with opposite handedness, called enantiomers. The difference in geometrical structure between enantiomers leads to specific reactivity with other chiral objects, such as circularly polarized light or other chiral molecules. As a result, chiral molecules facilitate molecular-level recognition, a property widely used by living organisms, which utilize only one enantiomer of a particular biomolecule. It is less commonly known that chiral molecules also offer superb control over electron spin transport due to their handedness[1].
Photoelectron circular dichroism (PECD) spectroscopy is a powerful technique that probes molecular chirality through forward–backward asymmetries in the photoelectron angular distribution induced by circularly polarized light. In this contribution, we will focus on the first time-resolved PECD measurements using circularly polarized harmonics, thereby introducing a general experimental approach for chiral femtochemistry. These experiments include several novel aspects: First, we have introduced a simple approach to generating circularly polarized VUV radiation based on low-order harmonic generation[2]. Second, we have performed a complete polarization measurement of the generated VUV harmonics[2]. Third, we have combined this light source with angle-resolved photoelectron detection to measure time-resolved PECD with sub-100 fs temporal resolution[3].
One of the structurally simplest chiral molecules, CHFBrI, will be discussed in terms of its molecular dynamics and time-dependent chirality associated with photo-induced C–I bond breaking. The experimental results are supported by high-level ab initio electron-molecule scattering calculations. The broad applicability of the time-resolved PECD measurement scheme is demonstrated with another chiral molecule, 2-iodobutane. Whereas CHFBrI displays a non-vanishing PECD at long pump-probe delays, the PECD decays to zero after the photo-dissociation of 2-iodobutane, reflecting the effective chirality of the product radicals on long timescales.
[1] Naaman, R., Waldeck, D.H.; JPCL, 2012, 3, 2178-2187
[2] Svoboda, V. et al.; Optics Express, 2022, 30, 14358-14367.
[3] Svoboda, V. et al.; Science Advances, 2022, 8, eabq2811.
