First direct look at how light excites electrons to kick off a chemical reaction

The first step in many light-driven chemical reactions, like the ones that power photosynthesis and human vision, is a shift in the arrangement of a molecule’s electrons as they absorb the light’s energy. This subtle rearrangement paves the way for everything that follows and determines how the reaction proceeds. Now scientists have seen for the first time how the molecule’s electron cloud balloons out before any of its atomic nuclei respond.