The independent particle approximation is shown to break down for the photoionization of both inner and outer nᐉ ͑ᐉ . 0͒ electrons of all atoms, at high enough energy, owing to interchannel interactions with the nearby ns photoionization channels. The effect is illustrated for Ne 2p in the 1 keV photon energy range through a comparison of theory and experiment. The implications for x-ray photoelectron spectroscopy of molecules and condensed matter are discussed. [S0031-9007 (97)03382-6] PACS numbers: 32.80.Fb

The effect of a spin–orbit interaction activated interchannel coupling in 4d photoionization channels of Xe on the 5s photoionization amplitudes is studied and analysed. The dramatic effect of confinement of the Xe atom in a C60 cage on the 4d photoionization parameters and the consequent effect on 5s photoionization via an interchannel coupling are also discussed.

Angular distributions of valence photoelectrons showing effects due to highermultipole photon interactions have been measured for the first time. Neon 2s and 2p photoemission exhibits effects beyond the dipole approximation throughout the 250-1200 eV photon-energy range studied. The results suggest that any photoemission experiment, on any sample, can be affected at relatively low photon energies, pointing to a general need for caution in interpreting angle-resolved-photoemission measurements.

The effects of confinement resonances on photoelectron group delay (Wigner time delay) following ionization of an atom encapsulated inside a C60 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@C60 and present a most direct manifestation of Wigner time delay. These oscillations were missed in a previous theoretical investigation of Ar@C60 [PRL 111, 203003 (2013)]. PACS numbers: 32.80.Rm 32.80.Fb 42.50.Hz Unprecedented advances in experimental techniques in measuring time intervals at the attosecond level [1] have engendered the ability to scrutinize the time delay in photoionization of atomic systems in the laboratory [2][3][4], thereby allowing us to probe the fundamental process of photoionization in the time domain. Specifically, using attosecond pulses of electromagnetic radiation, the time difference between the emergence of photoelectrons from two neighboring atomic subshells has been measured both in Ne [3] and Ar [4, 5]. These experimental results have stimulated a host of theoretical calculations to explain and to further explore this phenomenon [6][7][8][9]. This is of great interest, not only as a new way to study a fundamental process of nature, but also as an outstanding, unique opportunity towards a deeper understanding of the most informative parameter of the process, the photoionization amplitude. This is because the time delay is related to the energy derivative of the phase of the amplitude driving the process [10]. Indeed, to date, the only method for getting the maximum experimental information on photoionization lies through a set of measurements of total and differential photoionization cross sections, but allows only the absolute values and relative phases of matrix elements to be deduced; this is known as a complete photoionization experiment [11]. Time delay investigations, however, go beyond the complete experiment strategy and yield the derivative of the phase with respect to the photoelectron energy. Time delay investigations, thus, provide a new avenue to discern the characteristics of the basic physical quantity -the photoionization amplitude -and, thus, of the photoionization phenomenon itself. It is the ultimate aim of this paper to promote the expansion of time delay studies towards situations where they have not yet been exploited and where novel effects might occur -to atoms under confinement.The theory of time delay in physics was developed some time ago [12] and was originally envisioned as a way to study resonances -the temporary trapping of one (or more) electrons in a quasi-bound state or a potential well. Indeed, the Breit-Wigner formula of resonant scattering τ = 2/Γ equates the time delay τ with the resonant width Γ at half maximum of the cross-section [13]. Resonances are ubiquitous in photoionization of atoms, and these resonances can be of different natures: inner-shell excitations, tw...