H ₂ , the smallest and most abundant molecule in the universe, has a perfectly symmetric ground state. What does it take to break this symmetry? We found that the inversion symmetry can be broken by absorption of a linearly polarized photon, which itself has inversion symmetry. In particular, the emission of a photoelectron with subsequent dissociation of the remaining H ⁺ ₂ fragment shows no symmetry with respect to the ionic H ⁺ and neutral H atomic fragments. This lack of symmetry results from the entanglement between symmetric and antisymmetric H ⁺ ₂ states that is caused by autoionization. The mechanisms behind this symmetry breaking are general for all molecules.

We have measured momentum spectra and branching ratios of charged ionic fragments emitted in the double ionization of ${\mathrm{D}}_2$ (and ${\mathrm{H}}_2$) molecules by short intense laser pulses. We find high-energy coincident ${\mathrm{D}}^{+}$ (and ${\mathrm{H}}^{+}$) ion pairs with kinetic energy releases between 8 and 19 eV which appear for linearly polarized light but are absent for circularly polarized light. The dependence on the polarization, the energy distributions of the ions, and the dependence on laser intensity of yield ratios lead us to interpret these ion pairs as due to a rescattering mechanism for the double ionization. A quantitative model is presented which accounts for the major features of the data.

Diffraction of a low energy ($<4\mathrm{e}\mathrm{V}$) carbon-$K$-photoelectron wave that is created inside a CO molecule by absorption of a circularly polarized photon is investigated. The measurements resolve the vibrational states of the $K$-shell ionized $\mathrm{C}{\mathrm{O}}^{\mathrm{+}}$ molecule and display the photoelectron diffraction patterns in the molecular frame. These show significant variation for the different vibrational states. This effect is stronger than predicted by state-of-the-art theory. As this study is performed close to C-$K$-threshold and, therefore, far below the molecule’s $\sigma$-shape resonance, this surprisingly strong effect is not related to that resonance phenomenon.