We report the first quantitative measurement of extended x-ray absorption fine structure (EXAFS) from hydrogen atoms. A single oscillation is observed from gaseous water consistent with the location of the covalently bonded hydrogen in H 2O. The experimental phase and amplitude of the oscillation are in excellent agreement with curved wave multiple scattering calculations for isolated water molecules. With this determination of the O-H scattering phase shift we have quantified the covalent hydrogen bond distance (0.95+/-0.03 A) in liquid water, thus demonstrating that hydrogen EXAFS can become a valuable complement to existing structural methods in chemistry and biology.
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