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Compton scattering is the fundamental light-matter interaction process discussed in the textbooks as a billiard-type collision, in which a photon (as a particle) is deflected and transfers parts of its energy and momentum to an electron initially at rest. If electron is bound in an atom or molecule, its momentum distribution contributes to the balance, which is known as the impulse approximation [1]. As a consequence, the momentum distribution of direct Compton electrons is given by the Fourier transform of the initial orbital displaced by the photon momentum transfer. Recent experimental and theoretical studies [2,3] highlighted the need to go beyond the this approximation. In particular, Ref. [2] reported a backward scattering of the direct Compton electrons with respect to the photon momentum transfer and a simultaneous forward kick of the parent nucleus, while Ref. [3] demonstrated a scattering of the direct Compton electrons to all angles and a Coulomb focusing of the electrons by the ionization potential of the ion.
In the present work, we consider theoretically and experimentally the K-shell ionization of C and O atoms in carbon monoxide molecules by Compton scattering of 20 keV photons and report differential electron momentum distributions [4]. We observe diffraction patterns in the momentum distributions, which persist after integration over magnitudes and orientations of the photon momentum transfer in the frame of molecular reference. This phenomenon relies on the interference of the direct Compton electrons and those which are scattered on the parent and neighboring nuclei. The double-slit interference patterns in the electron momentum distribution can directly be related to the molecular orientation and the internuclear distance. The present results suggest that the imaging techniques, widely employed in the optical regime via laser-induced diffraction and soft x-ray domain via one-photon inner-shell photoionization, can be extended to the hard X-ray domain, where the photoionization is strongly suppressed, and the ionization by Compton scattering became dominant.
[1] J. W. M. Du Mond, Phys. Rev. 33, 643 (1929).
[2] M. Kircher et al., Nat. Phys. 16, 756 (2020).
[3] N. Melzer et al., Phys. Rev. Lett. 133, 183002 (2024).
[4] D. M. Haubenreißer et al, (2025) submitted
