  Photoionization of atoms immersed in an environment such as an ultracold gas
is investigated. We show that the interference of two ionization pathways, one
passing directly to the continuum and one accounting for scattering processes
between the photoelectron and a neighboring atom, produces a fine structure in
the photoionization cross-section over an energy range less than 1 eV above
threshold. This fine structure includes all the details of the corresponding
three-body system, e.g. the interatomic distance or the scattering information
of the electron-atom subsystem; therefore, photoelectrons produced in a
multi-particle environment can be utilized as structural probes. As an
illustration, for experimentally relevant parameters, we propose a scheme based
on the photoionization of a Rydberg molecule where the low-energy electron-atom
phase shifts are extracted from the fine structure spectra using neural
networks.
