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 this first step directly for the first time, observing how the molecule’s electron cloud balloons out before any of the atomic nuclei in the molecule respond.

While this response has been predicted theoretically and detected indirectly, this is the first time it’s been directly imaged with X-rays in a process known as molecular movie-making, whose ultimate goal is to observe how both electrons and nuclei act in real time when chemical bonds form or break.