Site-resolved Recoil-Induced Asymmetry of Nondipole Molecular-Frame Photoelectron Angular Distributions in Inner-Shell Photoionization of CO by 20 Kev Photons

Angular distributions of K-shell photoelectrons from CO molecules ionized by 20 keV photons are studied experimentally and theoretically in the molecular frame of reference. At this high photon energy, nondipole contributions to the light-matter interaction induce a strong forward-directed emission of the photoelectrons, which is only slightly modulated by scattering from the molecular potential. In addition, we observe a recently predicted asymmetry in the photoelectron emission distributions with respect to the polarization direction [Phys. Rev. Lett. 123, 243201 (2019)], which is caused by a rotational recoil of the nuclei imposed by the fast photoelectrons and high-energy photons. By addressing the C and O atoms separately in CO, we are able to resolve the molecular site that experiences this rotational kick, which was not possible in the previous study of N2 molecules. Our experimental results are explained by ab initio electronic structure calculations augmented by a semiclassical modeling of the rotational-recoil effect.