What defines the lifetime of electronic states in artificial quantum
structures? We measured the spectral widths of resonant eigenstates in a
circular, CO-based quantum corral on a Cu(111) surface and found that the
widths are related to the size of the corral and that the line shape is
essentially Gaussian. A model linking the energy dependence with the movement
of single surface electrons shows that the observed behavior is consistent with
lifetime limitations due to interaction with the corral walls