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A practical example of the use of single-structure diffusion pumping groups in an experiment requiring hydrocarbon-free, ultrahigh vacuum is presented. With the use of proper vacuum procedures and careful foreline design and operation, but with no exotic hardware or liquid nitrogen, we have demonstrated that such a pump could provide vacuum that was "clean," i.e., which had no residual pump-oil contamination to a partial pressure below 10-8 mbar. Their use is thus compatible, at least in peripheral vacuum chambers, with standard "UHV-surface science" processes. We discuss our vacuum system in detail, and present a review of the literature on the production of clean vacua with diffusion pumps.