Hard X-ray helical dichroism of disordered molecular media

TL;DR

This study demonstrates for the first time that hard X-ray helical dichroism can be used to probe molecular chirality in disordered samples, revealing new insights into chiral interactions with light's orbital angular momentum.

Contribution

The paper introduces a novel application of hard X-ray helical dichroism to disordered molecular media, overcoming previous limitations of optical methods and demonstrating measurable dichroism signals.

Findings

HD spectra show expected sign inversions between enantiomers.

Asymmetry ratios range from 1% to 5% for different OAM charges.

First demonstration of HD in disordered molecular powders at the Fe K-edge.

Abstract

Chirality is a structural property of molecules lacking mirror symmetry that has strong implications in diverse fields, ranging from life to materials sciences. Established spectroscopic methods that are sensitive to chirality, such as circular dichroism (CD), exhibit weak signal contributions on an achiral background. Helical dichroism (HD), which is based on the orbital angular momentum (OAM) of light, offers a new approach to probe molecular chirality, but it has never been demonstrated on disordered samples. Furthermore, in the optical domain the challenge lies in the need to transfer the OAM of the photon to an electron that is localized on an {\AA}-size orbital. Here, we overcome this challenge using hard X-rays with spiral Fresnel zone, which can induce an OAM. We present the first HD spectra of a disordered powder sample of enantiopure molecular complexes of [Fe(4,4'-diMebpy)3]2+ at the iron K-edge (7.1 keV) with OAM-carrying beams. The HD spectra exhibit the expected inversions of signs switching from a left to a right helical wave front or from an enantiomer to the other. The asymmetry ratios for the HD spectra are within one to five percent for OAM beams with topological charges of one and three. These results open a new window into the studies of molecular chirality and its interaction with the orbital angular momentum of light.