Molecular-frame photoelectron angular distributions of strong-field tunneling from inner orbitals

Using photoelectron angular streaking by strong circularly polarized laser pulses, we measure angular-dependent tunneling rates from the inner orbitals of O-2 in the molecular frame, which correlate with the strong-field molecular dissociative ionization process. We show that the electronic density of molecular inner orbitals can be visualized by angular tunneling rates in the molecular frame. We demonstrate the orbital-by-orbital characterization of tunneling ionization from randomly oriented molecules. Compared with the single-active-electron approximation theories, i.e., the molecular strong-field approximation and the molecular Ammosov-Delone-Krainov model, the experiment shows that the tunneling from the excited states and the state coupling might play an important role in strong-field molecular tunneling ionization. Our results suggest major advances in quantum theory needed for molecular inner orbital tunneling and open intriguing perspectives to probe quantum dynamics related with molecular core electrons.