Resolving Vibrations in a Polyatomic Molecule with Femtometer Precision Via X-Ray Spectroscopy

We measure molecular vibrations with femtometer precision via resonance energy shifts using time-resolved x-ray absorption spectroscopy. For a demonstration, a Raman process excites the A(1g) mode in gas-phase SF6 molecules with an amplitude of approximately 50 fm, which is probed by a time-delayed soft-x-ray pulse at the sulfur L-2,L-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.