Photoelectron Chiral Dichroism Induced by Lasers without Helicity via Excited Chiral Electronic Wave Packets

Photoelectron circular dichroism (PECD) is a method where randomly oriented chiral molecules are photoionized due to irradiation by circularly polarized lasers, yielding large chiral signals in the photoelectron momentum distribution. Recently, PECD was explored with polarization-tailored light, such as bichromatic and noncollinear drivers, which still produces significant chiral signals. Yet, all known PECD configurations to date exhibit nonzero time-local chirality. That is, they are driven by an intrinsically helical light source. Nonetheless, "chiral" light can also be nonhelical if its chirality manifests on longer timescales (e.g., an optical centrifuge). It remains unknown whether PECD can arise from nonhelical coherent light. Here, we predict that chiral dichroism indeed emerges from nonhelical light by employing a train of linearly polarized intense laser pulses with a rotating polarization axis, which are phase coherent and time delayed. We find strong forward backward asymmetry in the model chiral molecule CBrClFH under a wide parameter regime that can be optimized up to ∼8% by tuning delays between pulses, suggesting quantum interference. We directly show that the physical mechanism for this type of response differs from the standard PECD case, relying on a chiral excited electron attosecond wave packet evolving in the molecule, induced by the first linear pulse. Our Letter shows that multiple mechanisms can give rise to chiral dichroism on longer timescales and provides a novel approach for ultrafast chirality spectroscopy and coherent chiral wave-packet manipulation.