On the photoionization of the outer electrons in noble gas endohedral atoms

M. Ya. Amusia1, 2, A. S. Baltenkov 3 and L. V. Chernysheva2

1Racah Institute of Physics, the Hebrew University, Jerusalem 91904, Israel 2Ioffe Physical-Technical Institute, St.-Petersburg 194021, Russia 3Arifov Institute of Electronics, Tashkent, 700125, Uzbekistan

Abstract

We demonstrate the prominent modification of the outer shell photoionization cross-section in noble gas (NG) endohedral atoms F @ NG under the action of the fullerene F electron shell. This shell leads to two important effects, namely to strong enhancement of the cross-section due to fullerenes shell polarization under the action of the incoming electromagnetic wave and to prominent oscillation of this cross-section due to the reflection of the photoelectron from NG by the F shell. All but He noble gas atoms are considered. The polarization of the fullerene shell is expressed via the total photoabsorption cross-section of F. The reflection of the photoelectron is taken into account in the frame of the so-called bubble potential that is a spherical δ -type potential. It is assumed in the derivations that NG is centrally located in the fullerene. It is assumed also, in accord with the existing experimental data, that the fullerenes radius RF is much bigger than the atomic radius ar and the thickness of the fullerenes shell F ∆. These assumptions permit, as it was demonstrated recently, to present the F @ NG photoionization cross-section as a product of the NG cross-section and two well defined calculated factors.

PACS 31.25.-v, 32.80.-t, 32.80.Fb.

1. Introduction

In this paper we will consider the photoionization of outer shells of noble gas (NG) endohedral atoms, formed by a fullerene F inside of which a noble gas atom is staffed, NG@F. We will present data on all but He noble gases. In concrete calculations, as a fullerene F we will consider C60.
Recently, a great deal of attention was and still is concentrated on photoionization of endohedral atoms. It was demonstrated in a number of papers [1-9] that the C60 shell adds prominent resonance structure in the photoionization cross section of endohedral atoms. Although the experimental investigation of A@C60 photoionization seems to be at this moment very difficult, it will be inevitably intensively studied in the nearest future1. This justifies the current efforts of the theorists that are predicting rather non- trivial effects waiting for verification. The role of C60 in NG@C60 photoionization is manifold. C60 act as a spherical potential resonator that reflects the photoelectron wave coming from NG atom. This leads to interference of out-going and in-coming (reflected) waves and to confinement

1 As a first example of such a research, let us mention the tentative data on measurements of photoionization cross-section of Ce@C82 [10].

2 resonances or simply to oscillations in the frequency dependence of the photoionization cross sections [5]. The interference of the photoelectron spherical waves inside the resonator C60 affects significantly not only the total cross section but also the angular distribution of photoelectrons. This phenomenon was analysed in [6] where it was shown that the effects of confinement resonances are found also in the frequency dependencies of the dipole and nondipole parameters of the photoelectron angular distribution. The results of these studies give evidence that the reflection and refraction of the photoelectron waves by the potential resonator C60 is prominent up to 60 – 80 eV of the photoelectron energy. The C60 shell at some frequencies acts as a dynamical screen that is capable to suppress or enhance the incident electromagnetic radiation acting upon the doped atom A [11-13]. This effect is due to dynamical polarization of the collectivized electrons of the fullerene shell. Plasma excitations of these electrons generate an alternating dipole moment. This dipole moment causes the ionization of the electronic shells of the endohedral atom. The screening effects of the C60 shell are particularly strong for incident radiation frequency ω 2 of about that of the C60 Giant resonance, i.e. 20 – 22 eV, but is big enough in a much broader region, from ionization threshold up to 60 – 80 eV.
We will show in this paper that the dynamic polarization of C60 increases the outer shell photoionization cross-section at anyω , contrary to the statements in [11]. The maximal enhancement is in the region of C60 dipole polarizability maximum. Thus, the resonator and dynamic screen effects of the fullerene shell C60 manifest themselves as a prominent enhancement of the cross-section modulated by oscillating structure that appears due to reflection and refraction of the photoelectron wave by the fullerenes shell. We have studied before [12, 14] the effect of C60 upon the

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