Photoionization of atoms confined inside bucky-ball onions

We report on a theoretical study of photoionization of the atom $A$ placed at the center of nested fullerenes $C_{60}$, $C_{240}$, and $C_{540}$; the formation termed ``bucky-ball onion''. The fullerene cages are represented by spherical well potentials of adjusted inner radii, widths, and potential depths. The \textit{inner-shell} photoionization largely rules out the impact of both the change in the structure of the deepest inner-shell due to the confinement and polarizability of a bucky-ball on the photoionization process. In turn, the \textit{near-threshold} ionization eliminates the impact of the granular structure of the bucky-ball cage on the photoionization process. This makes our model workable, to a good approximation. Progressively complicating trends in the abundance of appearing confinement type resonances in the photoionization spectra with increasing number of nested bucky-balls is unraveled and the corresponding interpretation is given. The $1s$- and $2p$-near- threshold photoionization of $Ar$ from $Ar@C_{60}$, $Ar@C_{60}@C_{240}$, and $Ar@C_{60}@C_{240}@C_{540}$ is chosen as the case study.