Real-time determination of enantiomeric and isomeric content using photoelectron elliptical dichroism

A Comby,S Rozen,V Blanchet,S Petit,E Bloch,J B Greenwood,C M M Bond,J Miles,Y Mairesse,D Descamps

doi:10.1038/s41467-018-07609-9

Abstract

The fast and accurate analysis of chiral chemical mixtures is crucial for many applications but remains challenging. Here we use elliptically-polarized femtosecond laser pulses at high repetition rates to photoionize chiral molecules. The 3D photoelectron angular distribution produced provides molecular fingerprints, showing a strong forward-backward asymmetry which depends sensitively on the molecular structure and degree of ellipticity. Continuously scanning the laser ellipticity and analyzing the evolution of the rich, multi-dimensional molecular signatures allows us to observe real-time changes in the chemical and chiral content present with unprecedented speed and accuracy. We measure the enantiomeric excess of a compound with an accuracy of 0.4% in 10 min acquisition time, and follow the evolution of a mixture with an accuracy of 5% with a temporal resolution of 3 s. This method is even able to distinguish isomers, which cannot be easily distinguished by mass-spectrometry.

Highlights

The fast and accurate analysis of chiral chemical mixtures is crucial for many applications but remains challenging
This effect, called vibrational circular dichroism (VCD), is used in commercial instruments to analyze chiral samples with high accuracy (~1%), and enables enantiomeric excesses to be monitored in real time[7,8]
We show that the resonance-enhanced multiphoton ionization (REMPI) of chiral molecules by elliptically polarized laser pulses produces strong forward–backward asymmetries in the three-dimensional (3D) photoelectron angular distributions which can evolve nonmonotonically with the ellipticity of the laser light

Summary

Introduction

The fast and accurate analysis of chiral chemical mixtures is crucial for many applications but remains challenging. It is necessary to use a physical process which is faster than the migration on a substrate This is naturally achieved in chiroptical measurements, which rely on the interaction of circularly polarized electromagnetic radiation with chiral molecules and whose response time is on the attosecond range[2,3]. Its interest was renewed in the 1970s when infrared CD revealed that transitions between vibrational states provided a unique chiral fingerprint, with unambigious determination of the absolute configuration when compared to ab initio calculations[4,5,6] This effect, called vibrational circular dichroism (VCD), is used in commercial instruments to analyze chiral samples with high accuracy (~1%), and enables enantiomeric excesses to be monitored in real time[7,8]. PECD was associated with coincidence electron-ion detection to analyze multi-component mixtures, distinguishing molecules with an accuracy of 20% in 18 h acquisition time[23]

Methods

Results

Conclusion