Validity of the independent-particle approximation in x-ray photoemission: The exception, not the rule

Abstract

A combined experimental and theoretical study of argon valence photoionization illustrates the discovery of the broad lack of validity of the independent-particle approximation ͑IPA͒ for x-ray photoemission. In addition to previously known breakdowns of the IPA, which are limited to high photon energies and regions very near threshold, the observed breakdown in photoionization at intermediate energies demonstrates generally that the IPA is valid only in very restricted domains. These restrictions are expected to be relevant throughout the periodic table, with consequences for a wide variety of applications. ͓S1050-2947͑99͒50810-X͔

Dramatic Nondipole Effects in Low-Energy Photoionization: Experimental and Theoretical Study of Xe5s

“…In addition, for dipole photoionization, interchannel coupling, which is simply configuration interaction in the continuum, has been shown to be important for most subshells of most atoms at most energies [15,16]; this work was contrary to the previous conventional wisdom that the independent-particle approximation (IPA) was generally valid away from thresholds. It had been suggested, however, that such interchannel coupling was not important in quadrupole photoionization channels [17], but recent theory has suggested that interchannel coupling can indeed be significant in quadrupole photoionization channels as well [14].…”

“…Experiments have shown the importance of nondipole effects in the 1-3 keV photon-energy region [8,9], in the hundreds-of-eV range [10], and, in one case, at 13 eV [11]. Concurrently, theory has predicted significant nondipole contributions to electron angular distributions for atomic valence shells down to threshold at a few tens of eV photon energy [12 -14].In addition, for dipole photoionization, interchannel coupling, which is simply configuration interaction in the continuum, has been shown to be important for most subshells of most atoms at most energies [15,16]; this work was contrary to the previous conventional wisdom that the independent-particle approximation (IPA) was generally valid away from thresholds. It had been suggested, however, that such interchannel coupling was not important in quadrupole photoionization channels [17], but recent theory has suggested that interchannel coupling can indeed be significant in quadrupole photoionization channels as well [14].…”

High-energy photoeffect

“…For example, the high-energy behavior of total cross sections for single ionization of a many electron atom (in its ground state) resulting in a vacancy in an nl subshell after absorption of a photon of energy o5m is (Dias et al, 1997;Hansen et al, 1999;Amusia et al, 2000; …”

Some recently found interchannel coupling effects in atomic photoionization processes

“…It was soon realized (Hansen et al, 1999;Chakraborty et al, 2001) from another combined theoretical and experimental study that there must be a broad lack of validity of the IPA in X-ray processes leading to the conclusion that it would be only in limited cases, if any, that the IPA would be valid, even at high energies. Estimates of the energy dependence of interchannel coupling effects have been made (Amusia et al, 2000), and it has been suggested that the high-energy limit of cross-section for photoionization of nl subshell would not go as E Àðlþðn=2ÞÞ as predicted by the IPA, but would rather be independent of the orbital angular momentum quantum number, falling at high energies as 9 2 power of the photon energy.…”