Study decouples excitation and ionization steps in photoelectron circular dichroism

Because all living organisms are made of chiral building blocks of a well-defined enantiomeric form or handedness, chirality plays an important role in fields like biology, biochemistry, pharmaceuticals and the food industry. A recently developed technique called photoelectron circular dichroism (PECD) uses circularly polarized light to detect chiral molecules with high sensitivity.

In PECD, circularly polarized light ionizes gaseous chiral molecules, and an electron imaging detector records strong forward/backward asymmetry in the photoelectron distribution. In a new article, Beaulieu et al. used two superimposed laser pulses of different colors to ionize chiral molecules, to better understand the role of the different photons involved in laser-based PECD experiments.

The experimental setup included a 199-nanometer laser pulse to electronically excite molecules of the (+)-limonene enantiomer by single photon absorption, followed by a 402-nanometer laser pulse to ionize the excited molecules. In this way, the two steps were decoupled, which allowed for independent manipulation of each color’s polarization state.

The authors found that the ionizing photons had by far the greatest influence on the resulting PECD signal. Changing the helicity of the ionization laser pulse led to a sign change of the PECD, while changing the helicity of the excitation laser pulse only slightly changed the magnitude of the PECD. They even observed PECD using a linearly polarized excitation laser pulse, as long as the ionization laser pulse was circularly polarized.

In terms of future work, the group is currently using PECD to investigate the ultrafast dynamics of aligned chiral molecules. They will still use multiple ultrashort laser pulses of different colors but will scan the delay between them to reveal the ultrafast relaxation of the excited molecules.

Source: “Multiphoton photoelectron circular dichroism of limonene with independent polarization state control of the bound-bound and bound-continuum transitions,” by S. Beaulieu, A. Comby, D. Descamps, S. Petit, F. Légaré, B. Fabre, V. Blanchet, and Y. Mairesse, The Journal of Chemical Physics (2018). The article can be accessed at https://doi.org/10.1063/1.5042533 .