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

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.

Angular streaking in strong field ionization of chiral molecules

“…Elliptically polarized light leads to much richer chiral signals than circularly polarized light. This can be an asset if one aims for chiral recognition (compare Comby et al [3]). It allows one to combine attosecond angular streaking with an enantiosensitive signal.…”

“…Photoelectron circular dichroism (PECD) is a prominent example which is an enantiomer-dependent asymmetry of the electron emission probability with respect to the light propagation direction. PECD has proven its potential as a sensitive analysis method in chemistry and pharmacy [1][2][3]. PECD is also a versatile tool for studying photoionization dynamics [4].…”

Using photoelectron elliptical dichroism (PEELD) to determine real‐time variation of enantiomeric excess

“…Figure 4F clearly illustrates that c-PEELD measurements can detect fast evolutions. A precision of a few % can be reached as shown on the error bars in fig 5b from Comby et al 11…”

“…8,9 With a few hours of acquisition, the precision reached on this enantioselective analysis of VOC compounds was one decimal less accurate than that obtained by GC × GC-TOFMS, which is two-dimensional gas chromotography (GC × GC) using time-of-flight mass spectrometry detection (TOFMS). 10 In the work presented here, which is based on a recent publication, 11 instead of using mass spectrometry to identify each compound, we use the anisotropy of excitation induced by intermediate resonances in the multiphoton ionization regime. This anisotropy of excitation means that among all the randomly oriented molecules produced in the gas phase, the ones with their electronic transition dipole moments aligned with the laser electric field are preferentially excited.…”

“…Adapted with permission from Comby et al 11 shows a main component at 2 Ω, reflecting the fact that the photoionization efficiency maximizes every 90 , when the polarization is linear. Note that circular dichroism in the ionization rates is possible for a REMPI scheme 14 and would be evident as a Ω Fourier component of the ionization yield.…”