Quasiparticle interactions broaden spectral features at surfaces, which can be measured using the scanning tunneling microscope (STM). We report the first study of lifetime effects on Shockley surfacestate electrons using low-temperature STM spectroscopy. Data taken from Ag(111) are analyzed using detailed tunneling calculations and a simple model, and are found to correspond to a self-energy of S 4.9 6 0.6 meV. This is considerably below values determined by angle-resolved photoemission, but remains higher than theoretical predictions. [S0031-9007(98)07612-1]

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