Photoionization of atoms encapsulated in endohedral ions A@C 60 ±z V. K. Dolmatov 1, * and S. T. Manson 2,† 1 Department of Physics and Earth Science, University of North Alabama, Florence, Alabama 35632, USA 2 Department of Physics and Astronomy, Georgia State University, Atlanta, Georgia 30303, USA Received 12 October 2005; published 3 January 2006 Results of a theoretical study of photoionization cross sections and photoelectron angular asymmetry pa- rameters of atoms confined by positively C 60 +z or negatively C 60 -z charged fullerene shells are presented. For negatively charged C 60 , entirely new confinement resonances, termed Coulomb confinement resonances, that dominate the spectra of the encapsulated atoms are predicted. In addition, the effect of a negative C 60 shell is to move some of the oscillator strength of the encapsulated atom from the discrete excitation region into the continuum. For positively charged C 60 , the situation is much different; no Coulomb confinement resonances occur in the photoionization spectrum of the encapsulated atom, and charging the shell positively does nothing to the photoionization cross section as a function of photon energy except to increase the threshold energy. The findings result from model Hartree-Fock calculations of 1s photoionization of Ne confined by neutral, negative, and positive C 60 . DOI: 10.1103/PhysRevA.73.013201 PACS numbers: 36.40.Vz, 32.80.Dz, 32.80.Fb I. INTRODUCTION Endohedral fullerenes A@C 60 —the nanostructure forma- tions wherein a multielectron atom A is trapped inside a hol- low fullerene cage C 60 —are a subject of significant interest in recent years 1,2, because they exhibit properties that can lead to important applications in nanostructure science and technology. For example, it has been shown that C 60 confine- ment could have some unique advantages in isolating the atom from its environment, thereby providing a building block for the qubits of the quantum computer 3. A number of theoretical studies have explored the response of atoms A encapsulated in C 60 cages to ionizing electromagnetic radia- tion, and phenomenology has been uncovered such as reso- nances in photoionization cross sections and photoelectron angular distributions of encapsulated atoms 4–10 termed confinement resonances 7see also a recent review paper 11 on this subject. So far, however, the spectroscopy of confined atoms has been, perhaps, experimentally rather difficult to perform, and theoretical predictions have far outstripped experiment. It is likely that this situation will change, and that laboratory in- vestigations of the spectroscopy of confined atoms will be- come available to test the theoretical predictions. In the meantime, we turn our attention to filling in aspects of the problem that so far have been ignored, such as the photoion- ization spectrum of the atom encapsulated inside a charged C 60 ±z shell: the endohedral ion A@C 60 ±z . At this point in time, nothing is really known about these spectra, but their prop- erties will clearly be of importance to understand given that investigations of endohedral ions, e.g., N@C 60 -z 12–14and references therein have been under way for some time now. When spectroscopy of such confined atoms becomes experi- mentally developed, it will be a matter of significant impor- tance to the understanding to be capable of distinguishing which part of the photoionization spectrum of charged A@C 60 ±z is due to its confining C 60 ±z shell, and which to photo- ionization of the trapped atom A alone. It is precisely the aim of this paper to explore theoretically features of the photoionization spectra of atoms encapsulated inside both endohedral positive ions cations and negative ions anions, A@C 60 ±z . We focus on uncovering effects that can occur in the photoionization process rather than to make the most precise calculations possible. To accomplish this end, we choose a system that is easier for us to study— Ne@C 60 ±z —and we investigate 1s photoionization of the con- fined Ne. The choice is dictated by a number of reasons. To begin with, neutral endohedral fullerenes with encapsulated noble gas atoms do exist 15, and there is no reason that noble gas endohedral cations and anions should not exist as well. Secondly, the deepest 1s shell is clearly the least af- fected by the confining shell, and we use it to the advantage of our qualitative theory. Moreover, the choice of the deepest atomic shell with the ionization threshold of hundreds or more electron volts makes it possible to ignore completely the effect of dynamical screening in the photoionization of a confined atom by a fullerene shell 10; the effect is very strong at photon energies around 24 eV, but rapidly falls off to negligible at much higher energies. In our study, we perform calculations using Hartree-Fock HF wave functions to include charged confinement effects as a first step in understanding which aspects of the photo- ionization spectra are most interesting to study in greater depth. One important finding of the research is the observa- tion of strong confinement resonances, termed Coulomb con- finement resonances, which dominate the photoionization and photoelectron angular distribution spectra of encapsu- lated atoms inside a negatively charged C 60 -z cage. Another unexpected finding observes the redistribution of oscillator strengths of encapsulated atoms between the discrete and continuum spectrum depending on the magnitude and sign of the charge on a fullerene shell. *Electronic address: vkdolmatov@una.edu † Electronic address: smanson@gsu.edu PHYSICAL REVIEW A 73, 013201 2006 1050-2947/2006/731/0132016/$23.00 ©2006 The American Physical Society 013201-1