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.
To get a complete view of the process, the researchers combined two experimental techniques, each challenging in its own right. With reconstruction of attosecond beating by interference of two-photon transitions, or RABBIT (see Physics Today, January 2018, page 18), they measured how long the electron took to escape the molecule. With cold-target recoil-ion momentum spectroscopy, or COLTRIMS (see Physics Today, August 2003, page 19), they measured the momenta of the electron and the H+ ion in coincidence and calculated the kinetic energy released (KER) in the H2+ breakup.
According to the Born–Oppenheimer approximation, the photoionization delay and KER should be independent: The electron should be long gone by the time the H2+ ion finally dissociates, so neither should have any effect on the other. But as the researchers found, that’s not the case. Plotted as a function of KER and the electron energy, the photoionization delay exhibits a complex dependence on both.
Cattaneo and her colleagues interpret their results to mean that after the electron is liberated and as the H2+ ion is about to break up, the charged particles continue to exert significant forces on one another. That interaction, which depends on KER, can speed up or slow down the escaping electron and thus have a measurable effect on the photoionization delay. (L. Cattaneo et al., Nat. Phys., in press, doi:10.1038/s41567-018-0103-2.)
