Photoelectron angular distributions from single oriented molecules: Past, present and future

We provide a simple physical picture of the molecular frame photoelectron angular distributions (MFPAD). In the energy region near the ionization threshold, it has been demonstrated that the MFPAD can be understood by the interference between several partial waves. In the energy region higher than 100 eV, the MFPAD has been interpreted by the intra-molecule photoelectron diffraction. That is, it has been verified that multiple-scattering X-ray photoelectron diffraction (XPD) theory is a promising probe of the dynamical process. Therefore, we look into the future: a new methodology for molecular structure determination will be applied to determine ultrafast molecular structure changes due to ultrafast core-level XPD imaging from within, as demonstrated in our simulations for elemental processes of photo-induced unimolecular reactions.