Photoionization fine structure in dilute ultracold gases

The above-threshold photoionization of alkali atoms in the presence of a dilute environment, such as an ultracold gas, is investigated.

Our theoretical analysis shows that the photoionization spectra exhibit a fine structure in the energy range of less than 1 eV above threshold due to the presence of an environment.

More specifically, we show that the oscillatory fringes in the photoionization cross section result from the interference of the direct pathway of the photoelectron to the continuum with an indirect one.

The indirect pathway accounts for scattering processes between the photoelectron and the neighboring atoms in the gas.

This process is analogous to that responsible for the x-ray absorption fine structure in solids, however in our case applies to far lower energies and involves much larger length scales.

Therefore, we develop a theory taking into account the dominant Coulomb interactions as well as electron-atom collisions.

To illustrate the application of this theory, we propose a scheme to extract low-energy electron-atom scattering phase shifts from the photoionization cross section, utilizing a Rydberg molecule as an initial state.