Chiral discrimination by recollision enhanced femtosecond laser mass spectrometry

Abstract

chiral molecules and their interactions are critical in a variety of chemical and biological processes. circular dichroism (cD) is the most widely used optical technique to study chirality, often performed in a solution phase. However, CD has low-efficiency on the order of 0.01-1%. therefore, there is a growing need to develop high-efficiency chiroptical techniques, especially in gas-phase, to gain background-free in-depth insight into chiral interactions. By using mass spectrometry and strongfield ionization of limonene with elliptically polarized light, we demonstrate an efficient chiral discrimination method that produces a chiral signal of one to two orders of magnitude higher than the conventional cD. the chiral response exhibits a strong dependence on wavelength in the range of 1,300-2,400 nm, where the relative abundance of the ion yields alternates between the two enantiomers. The origin of enhanced enantio-sensitivity in intense laser fields is attributed to two mechanisms that rely on the recollision dynamics in a chiral system: (1) the excited ionic state dynamics mediated either by the laser field or by the recollision process, and (2) non-dipole effects that alter the electron's trajectories. our results can serve as a benchmark for testing and developing theoretical tools involving non-dipole effects in strong-field ionization of molecules. Chiral molecules lack S n symmetry (improper rotation axis) due to the presence of a chiral center in which an atom is connected to four different groups of atoms 1. Consequently, there is a handedness to the molecule. The left-and right-handed molecules are non-superimposable mirror images of each other, called enantiomers. They have identical physical and chemical properties making it difficult to differentiate them. Chiral discrimination requires an interaction between two chiral systems-a chiral reagent with known handedness that can induce enantio-selectivity in the second chiral system. In nature, bio-molecules such as amino acids that make up life on earth are homochiral, where one enantiomer exists predominantly over the other due to asymmetric interactions 2. Homochirality plays a pivotal role in our daily life by serving as a chiral reagent for enantio-selectivity. For example, olfactory receptors in our nose, responsible for the sense of smell, interact differently with the two enantiomers of a chiral system. For instance, the right-handed enantiomer of limonene has an orange odour while the left-handed enantiomer has a lemon odour 3. In practice, enantio-selectivity can be achieved using circularly polarized light (CPL) as a chiral reagent. CD is one of the most widely used chiroptical techniques. It arises due to the coupling of the electric and magnetic dipole transitions, thereby resulting in a differential absorption of left-and right-circularly polarized light 1,4-8. CD has low-efficiency on the order of 0.01-1% because magnetic dipole transitions are involved 9-12. Moreover, CD measurements are mostly conducted in solution phase, making it diffi...

Recent Advances in Separation and Analysis of Chiral Compounds

“…The combination of circular dichroism with laser MS is a specific technique to discriminate enantiomers. In this technique, a polarized femtosecond laser is employed as a type of laser ionization source, and the circular dichroism in ion yield is determined based on MS analyzer to discriminate different conformers of a chiral species. , However, the measured anisotropies of enantiomers in this way are generally weak considering measurement errors. Witte et al further improved the circular dichroism-laser MS method by adding a common path optical setup to generate a pair of counter-rotating laser foci in the interaction region before the MS analyzer.…”

Nonlinear helical dichroism in chiral and achiral molecules

“…Chiral sensitivity of CD is poor, on the order of 0.01-1% 3 , because it involves coupling of electric and magnetic dipole transitions. CD can be enhanced by employing super chiral light 4 , plasmonic structures 5,6 , and strong field techniques using elliptically polarized light 7,8 . In PECD, photo-ionization of a chiral molecule results in an asymmetric photo-electron angular distribution that is identical but opposite in sign for the two enantiomers 9 .…”

Signatures of magnetic-field effects in nonsequential double ionization manifesting as backscattering for molecules versus forward scattering for atoms

“…Hence, for smaller frequencies ω of the field, i.e., midinfrared wavelengths, nondipole effects arise at smaller field strengths E 0 . Magnetic field effects give rise to striking features in multielectron ionization of systems driven by intense infrared and midinfrared laser fields [3][4][5][6][7].…”