Fundamental to our thinking about molecular physics is the Born–Oppenheimer approximation: the idea that because electrons and atomic nuclei differ so much in inertia, their dynamics can be mathematically separated. The electrons zip around in a Coulomb potential defined by the nuclear positions alone; the nuclear velocities are assumed to have no effect. It’s because of that simplification that we can talk about the rotational, vibrational, and electronic quantum states of a molecule as if they’re separate things.
But the approximation doesn’t always hold. Sometimes nuclei move too fast for the electrons to readjust. That’s especially true in molecules that include hydrogen, the lightest and most easily accelerated nucleus.
Now ETH Zürich’s Laura Cattaneo, her adviser Ursula Keller, and their colleagues have gotten the first direct look at how electronic and nuclear dynamics are entangled in an attosecond experiment. The system under study was the dissociative ionization of molecular hydrogen. Photoexciting H2 at energies of 25–40 eV quickly expels an electron and leaves the H2+ ion with enough residual energy to break up into H and H+ fragments, as shown in the figure.