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, co nfinement resonances and molecular resonances. In each case, a different me chanism is involved, and different theoretical models are necessary for the ir study. In this work, we exploit the flexibility of the spherical model-p otential method to explore the properties of confinement resonances. Both r epulsive and attractive shells are considered. It is demonstrated that the nature of confinement resonances (CRs) emerging in each case is not the sam e. For attractive shells it is found that CRs result from interference betw een 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 a nd we study them, both as a function of the parameters of the confining she ll, and as a function of photoelectron energy. In the case of repulsive she lls, the resonances can result from three-wave interference, as above, or c an be due to quasi-discrete states appearing as a result of confinement.