Simple molecules mimic double slits

Simple molecules mimic double slits. The double slit experiment, in which a coherent beam of particles diffracts through closely spaced slits to produce interference fringes, is perhaps the simplest and most famous demonstration of wave–particle duality. It has been shown to work with photons, electrons, and even the comparatively massive buckminsterfullerenes. According to a theory outlined by Howard Cohen and Ugo Fano nearly 50 years ago, interference fringes should also result from the photoionization of a diatomic molecule, provided electrons on each atom are ejected simultaneously and bestowed with enough kinetic energy that their wavelengths are similar to the interatomic distance, as illustrated here. Unfortunately, the direct approach—fixing molecules’ orientation in space, photoionizing them, and detecting fluctuations in the angular distribution of electron density—is, for the most part, impractical. Instead, an international team of scientists led by Fernando Martín (Autonomous University of Madrid) used an elaborate first-principles theory to predict the effect that interference should have on the relationship between a molecule’s photoionization cross section and its vibrational energy. Working at Lawrence Berkeley National Laboratory’s Advanced Light Source, they then used extreme UV light pulses to ionize hydrogen, nitrogen, and carbon monoxide gas. For H₂, the simplest molecule of the bunch, the theory almost perfectly described the vibrationally resolved ionization spectra. For N₂ and CO, the theory was less accurate, but still in qualitative agreement with the data. The results could help to resolve ambiguities surrounding previous attempts to demonstrate the phenomenon. (S. E. Canton et al., Proc. Natl. Acad. Sci. USA108, 7302, 2011.) —