  We measure molecular vibrations with femtometer precision using time-resolved
x-ray absorption spectroscopy. For a demonstration, a Raman process excites the
A$_{1g}$ mode in gas-phase SF$_6$ molecules with an amplitude of $\approx50$
fm, which is probed by a time-delayed soft x-ray pulse at the sulfur
$L_{2,3}$-edge. Mapping the extremely small measured energy shifts to
internuclear distances requires an understanding of the electronic
contributions provided by a many-body ab initio simulation. Our study
establishes core-level spectroscopy as a precision tool for time-dependent
molecular-structure metrology.
