Observation of Photoemission Line Widths Narrower than the Inverse Lifetime

Abstract
Quantum mechanics stipulates that experimental spectral lines cannot be narrower than the inverse lifetime. However, this limitation can be overcome in angle-resolved photoemission, and we have observed unusually narrow photoemission peaks derived from a bulk valence band of Ag. This observation is made possible by judiciously choosing a grazing emission geometry to reach a specific zone in the reciprocal space, where the final-state lifetime effect is suppressed, and simultaneously the initial-state lifetime contribution is kinematically compressed.