Multiphoton ionization by circularly polarized laser pulses is a powerful way to probe chiral molecules in the gas phase. The ionization process often involves going through intermediate states, in a resonance-enhanced multiphoton ionization scheme. The importance of these resonances in the photoelectron circular dichroism (PECD) remained unclear. To elucidate it, we decoupled the photoexcitation and photoionization steps, using two laser pulses with independent polarization states. We found that the ionizing photons had by far the greatest influence on the resulting PECD signal. Changing the helicity of the ionization laser pulse leads to a sign change of the PECD, while changing the helicity of the excitation laser pulse only slightly changes the magnitude of the PECD. We even observed PECD using a linearly polarized excitation laser pulse, as long as the ionization laser pulse was circularly polarized.

Scilight article on our work:

Multiphoton photoelectron circular dichroism of limonene with independent polarization state control of the bound-bound and bound-continuum transitions
S. Beaulieu, A. Comby, D. Descamps, S. Petit, F. Légaré, B. Fabre, V. Blanchet, and Y. Mairesse, The Journal of Chemical Physics 149, 134301 (2018);