ECAMP 15

Investigations aiming at the determination of molecular chirality by chiroptical techniques continue to attract a significant amount of interest in chemistry, physics biology and pharmacology. Some focus has been put on the measurement and analysis of the photo ion circular dichroism (PICD) – a total ion yield effect – and the Photoelectron Circular Dichroism (PECD) – a molecular frame angular distribution effect. Most efforts have put immense work into establishing “perfect” circular polarization.
In recent years it has been demonstrated, that Photoelectron Elliptical Dichroism (PEELD) – which considers a systematic variation of the elliptical polarization of light – is also a powerful approach for studying chirality. Basically, PEELD is the extension of PECD to a variation of elliptical polarization. So far, PEELD has been reported for a number of neutral analytes using multi photon ionization, demonstrating advantages over plain PECD experiments.[1,2,3] PEELD measurements with single photon ionization have not been reported yet, due to the comparably high ionization energy of neutral analytes.
In this work we demonstrate the measurement of PEELD in single photon photodetachment of electrons from electro-sprayed anions. Photodetachment of electrons from anions has recently been developed as a new technique for chirality analysis.[4,5,6,7] When combined with electro-spraying the anions of interest, the technique, ESI-PECD, allows the study of proteins containing many amino acids.[5]
As analytes we chose, amino acids, for which the quantification of enantiomeric excess has already been demonstrated by ESI-PECD in our group, i.e. phenylalanine and tryptophan.[7] In this work, we show that the PEELD of the phenylalanine-anion scales linearly with the STOKES parameter S3 for small values of S3. There is a distinct extremum of the PEELD in the region of [S3 / So ] = 0.95 with very good symmetry between D- and L-phenylalanine. We note, that there are literature reports indicating non-linear variation of the PEELD with S3, however, in the case of multi photon ionization.[1,2,3]

The study of PEELD in photodetachment is expected to shed new light on the fundamental question of asymmetry in the photoelectron angular distribution comparing multiphoton and single photon processes. It is argued that a well-defined elliptic polarization has to be preferred over an ill-defined circular polarization.

[1] A. Comby, D. Descamps, S. Petit, E. Valzer, M. Wloch, L. Pouységu,
S. Quideau, J. Bocková, C. Meinert, V. Blanchet, B. Fabre, Y. Mairesse,
Phys. Chem. Chem. Phys. 2023, 25, 16246 – 16263.
[2] A. Comby, E. Bloch, C. M. M. Bond, D. Descamps, J. Miles, S. Petit, S. Rozen, J. B. Greenwood, V. Blanchet, B. Fabre, Y. Mairesse, Nat. Commun. 2018, 9, 5212.
[3] J. B. Greenwood, I. D. Williams, Phys. Chem. Chem. Phys. 2023, 25, 16238 – 16245.
[4] P. Krüger, K.-M. Weitzel, Angew. Chem. Int. Ed., 2021, 60, 17861 – 17865.
[5] P. Krüger, J. H. Both, U. Linne, F. Chirot, K.-M. Weitzel, J. Phys. Chem. Lett., 2022, 13, 6110 – 6116.
[6] J. Triptow, A. Fielicke, G. Meijer, M. Green, Angew. Chem. Int. Ed., 2022, 62, e202212020.
[7] J. H. Both, A. Beliakouskaya, K.-M. Weitzel, Analytical Chemistry, 2025, in press, (https://doi.org/10.1021/acs.analchem.4c05964).