PHOTOEMISSION LINEWIDTHS AND QUASI-PARTICLE LIFETIMES

Expressions are derived for the relationship between the measured line widths in angle-resolved valence-band photoemission spectra and the li fetimes of the photoelectron and photohole. The expressions are differ ent for the energy-distribution curve (EDC), constant initial-state sp ectrum (CIS), and constant final-state spectrum modes of data acquisit ion, and are significantly angle dependent for the EDC and CIS. The ci rcumstances in which the lifetime widths of the photoelectron and phot ohole can be separated from each other are discussed with special refe rence to a debate on high-temperature superconductors. Surface states offer the only case where the photohole lifetime can be rigorously iso lated. The nearly-free-electron approximation is elaborated and used t o explain some counterintuitive behavior in recent data on Ag(100).