On the nature and origin of confinement resonances

Three different types of resonances arise in the photoionization spectra of atoms endohedrally confined within a fullerene cage: atomic resonances, confinement resonances and molecular resonances. In each case, a different mechanism is involved, and different theoretical models are necessary for their study. In this work, we exploit the flexibility of the spherical model-potential method to explore the properties of confinement resonances. Both repulsive and attractive shells are considered. It is demonstrated that the nature of confinement resonances (CRs) emerging in each case is not the same. For attractive shells it is found that CRs result from interference between three waves: the incident wave, and the waves reflected at each of the inner and outer cavity boundaries. We find significant sensitivity of near-threshold confinement resonances to the size and thickness of the shell, we demonstrate modulations and `beats' in the intensities of the resonances and we study them, both as a function of the parameters of the confining shell, and as a function of photoelectron energy. In the case of repulsive shells, the resonances can result from three-wave interference, as above, or can be due to quasi-discrete states appearing as a result of confinement.