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

A combined experimental and theoretical study of argon valence photoionizat ion illustrates the discovery of the broad lack of validity of the independ ent-particle approximation (IPA) for x-ray photoemission. In addition to pr eviously known breakdowns of the IPA, which are limited to high photon ener gies and regions very near threshold, the observed breakdown in photoioniza tion at intermediate energies demonstrates generally that the IPA is valid only in very restricted domains. These restrictions are expected to be rele vant throughout the periodic table, with consequences for a wide variety of applications. [S1050-2947(99)50810-X].