Nonlifetime effects in photoemission linewidths

The intrinsic linewidth in angle- and energy-resolved photoemission is generally assumed to be limited by the final-state lifetime. High-resolution measurements of the intrinsic linewidths for emission from bulk and surface states of Cu show that in some cases the linewidth is dominated by weakened $k_{\mathrm{}\left|\right|\mathrm{}}$ conservation. We tentatively attribute this effect to a finite mean free path for elastic scattering from the small number (≈ 0.01 monolayer) of impurities present at the surface. This suggests that as instrumental precision improves, sample preparation may be the limiting factor determining resolution.