By combination of high-resolution photoemission data and linewidth analysis of low-energy electron diffraction spots we are able to derive a semiquantitative understanding of how disorder ͑induced by argon-ion bombardment and subsequent insufficient annealing͒ broadens photoemission peaks. At the example of surface states on Cu͑100͒ and Cu͑111͒ we demonstrate how the linewidth may be extrapolated to ''perfectly'' ordered surfaces in favorable cases. We also present experimental evidence that disorder-induced broadening is dominated by defect scattering of the photohole and is inversely proportional to the effective masses. Extrapolated ''intrinsic'' linewidths ͑full width at half maximum͒ are ⌫ i р(21Ϯ5) meV at ⌫ on Cu͑111͒, ⌫ i р(20Ϯ3) meV at M on Cu͑111͒, and ⌫ i р(13Ϯ4) meV at M on Cu͑100͒.
Teaching prismatic colours usually boils down to establishing the take-home message that white light consists of ‘differently refrangible’ coloured rays. This approach explains the classical spectrum of seven colours but has its limitations, e.g. in discussing spectra from setups with higher resolution or in understanding the well saturated colours of simple edge spectra. Besides, the connection of physical wavelength and colour remains obscure—after all, colour and wavelength are not equivalent. In this paper, we suggest that teachers demonstrate these impressive experiments in the classroom by using a video projector and a prism to disperse black-and-white slit images. We introduce experimental and diagrammatic methods for establishing the connection between the original slit image and the wavelength composition of the resulting spectrum. From this (or any other given) wavelength composition, students can systematically derive the colours with a simple RGB approach, thus gaining a more accurate picture of the relation between wavelength and colour.
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