Nonreciprocal phase shift within zeptosecond temporal scale

We investigate the zeptosecond-timescale delayed ionization process induced by ultrafast laser propagation in different directions across the molecule. The experimental measurements by Grundmann et al. Science370 339 (2020) serve as a basis for our study, where they extract the birth time delay of photoelectron emission from two nuclei, amounting to a few hundred zeptoseconds. By comparing and analyzing the results, we observe that asymmetric systems, such as the 2pσ state of HeH²⁺, exhibit nonequivalent responses to forward and backward laser propagation, resulting in an asymmetric dependence of the interference structure in the photoelectron momentum spectra. This process is considered as an ultrafast nonreciprocal phase shift with zeptosecond resolution. Through computational simulations, we explore the relationship between this kind of ultrafast nonreciprocity effect and molecular orbital symmetry. This study broadens our understanding of nonreciprocal physical mechanisms in the field of strong-field ultrafast dynamics, and provides a theoretical basis for the experimental investigation of the nonreciprocal phase shift within the zeptosecond timescale in the response processes of matter under ultrafast laser irradiation.