Angle Dependent Ionization of Small Molecules

The ionization probability and both the direct and re-scattered photoelectron momentum spectrum are all sensitive to the angle of a molecule with respect to the laser field. We experimentally measure the sensitivity of the ionization probability to molecular alignment using H$_{2}$ the simplest molecule as well as N$_{2}$, O$_{2}$, and CO$_{2}$. Concentrating on O$_{2}$ and N$_{2}$ we then demonstrate the natural quantum interference that occurs when the electron tunnels from perpendicularly aligned O$_{2}$, contrasting it with N$_{2}$. We show that the direct electrons preserve the symmetry of the orbital from which they tunnel, filtered through the momentum filter of the tunnel. Finally we show that the re-scattered electrons are also sensitive to molecular alignment, writing the molecular structure onto their angular distribution.