Detecting chirality in molecules by imaging photoelectron circular dichroism

Abstract

In this Perspective we discuss photoelectron circular dichroism (PECD), a relatively novel technique that can detect chiral molecules with high sensitivity. PECD has an enantiomeric sensitivity of typically 1-10%, which is two to three orders of magnitude larger than that of the widely employed technique of circular dichroism (CD). In PECD a chiral molecule is photoionized with circular polarized light, and the photoelectron angular scattering distribution is detected using particle imaging techniques. We present the general physical principles of photoelectron circular dichroism and we address both single- and multiphoton excitation. PECD has been measured with synchrotron radiation in single-photon ionization as well as, very recently, with femtosecond laser radiation in multiphoton ionization. We discuss the experimental implementation of PECD, focusing on velocity map coincidence imaging where the momentum distribution of both the electron and the coincident ion is measured. The coincident detection of the mass and momentum of the ion adds very powerful mass-correlated information to the PECD measurement of the chiral molecule. We illustrate the capabilities and the potential of PECD with various experimental examples and introduce computational methods that are able to model quantitatively experimental PECD results. We conclude with an outlook on novel developments and (analytical) implementations of PECD that may further broaden the application of PECD for the sensitive detection of chirality in molecules.

Identifying and Understanding Strong Vibronic Interaction Effects Observed in the Asymmetry of Chiral Molecule Photoelectron Angular Distributions

“…Of course, one may anticipate that with technical developments in light sources, detectors and analyzers more such measurements will be forthcoming. The recent development of resonance enhanced multiphoton PECD (MP-PECD) [20,21,23] will, however, surely bring its own increased opportunities to explore vibrational effects, either directly by exploiting additional sensitivity and selectivity in laser excitation of a resonant intermediate to explore and control vibrational quantum states in the ion, or through the evolution of vibrational wavepackets prepared in time domain PECD measurements. Recent work has showed how selective excitation of specific vibronic bands in resonant ionization could be used to achieve conformer specific ionization in a two-photon absorption CD measurement, [27] something that would be of great interest for MP-PECD.…”

“…Such developments are now attracting considerable interest because of the possibilities for time-resolved chiral probing, and convenient lab-based analytical applications that are thereby opened up. [23][24][25] In particular, the use of resonantly enhanced multiphoton ionization (REMPI), as opposed to single VUV photon ionization, creates new opportunities for increased vibrational selectivity in PECD, as ionization then occurs out of a prepared vibrational level or wavepacket. [26] Besides permitting greater control of vibrational excitation and mode specificity, REMPI brings with it opportunities for selective isomer and/or conformer specific ionization.…”

The Interplay Between Conformation and Absolute Configuration in Chiral Electron Dynamics of Small Diols

“…variants of PECD that are emerging [19][20][21] offer a clear prospect of performing time-resolved pump-probe PECD measurements capable of following the interconversion of conformers, and any associated chiral switching, in real time. [6,7] …”

“…[4] Here we examine exhibited chirality using photoelectron circular dichroism (PECD). A particular advantage over other chiroptical measures is that PECD provides an orbitally selective probe of molecular chirality, [5][6][7] allowing specific focus to be given to responses of the HOMO frontier orbital. Specifically, PECD isolates and extracts the phenomenological coefficient, 1 {+1} , of an odd cos term in the photoelectron angular distribution (See Supporting Information and Refs [5,6] ) which can then directly express the chiral asymmetry observed in the forward-backward ( = 0, ) directions as a Kuhn asymmetry factor, = 2 1 {+1} .…”

Polarization Characterization of Soft X-Ray Radiation at FERMI FEL-2

“…Circular dichroism enables investigations into the chirality of systems and the determination of magnetic properties [1][2][3] or specific molecular characteristics [4,5]. The use of highly intense, ultrashort elliptically polarized X-ray pulses offers the advantage of selective tunning to resonances, as well as the ability to probe on a small spatial scale, and on a femtosecond time scale, materials with tightly bound electrons.…”