Attosecond time delay in the photoionization of endohedral atoms   A@C$_{60}$: A new probe of confinement resonances
  The effects of confinement resonances on photoelectron group delay (Wigner time delay) following ionization of an atom encapsulated inside a C$_{60}$ cage have been studied theoretically using both relativistic and non-relativistic random phase approximations. The results indicate clearly the resonant character of the confinement oscillations in time delay of the $4d$ shell of Xe@C$_{60}$ and present a most direct manifestation of Wigner time delay. These oscillations were missed in a previous theoretical investigation of Ar@C$_{60}$ [PRL 111, 203003 (2013)] 
P. C. Deshmukh, A. Mandal, S. Saha, A. S. Kheifets, V. K. Dolmatov and   S T Manson
physics.atom-ph