Photoelectron angular distribution from the valence state of π-conjugated molecules on the surface has been studied to clarify the molecular orientation quantitatively with multiple scattering theory of photoelectron emission intensity. After developing the photoelectron emission techniques together with molecular thin-film preparations, a novel concept of photoemission tomography enables to investigate the electronic states in depth to understand the molecular functional properties in a sense of orbital. The article reviews the recent progress and development of the photoelectron momentum microscope related techniques for molecular science.

Three-dimensional momentum-resolved photoelectron spectroscopy with a µm-order field of view was achieved by combining Photoelectron Momentum Microscope with a soft X-ray synchrotron radiation source. We built a new experimental station dedicated for momentum-resolved photoelectron spectroscopy and microscopy at BL6U, an undulator-based soft X-ray beamline of the UVSOR synchrotron facility. This experimental station provides a means to characterize the electronic structures of surface atomic sites, thin films, molecular adsorbates, and bulk crystals.

In order to study electronic structures of solids including the spin information, the spin resolved 2 dimensional (2D) momentum microscopy (MM) has been developed very recently and now applicable to various materials. The principle of the 2D-MM is briefly explained and then the 2D mirror type spin-filter is explained. Very high detection efficiency of the SP-2D-MM facilitates the detection of the spin polarization P(E_B(k_x,k_y)) with the spatial resolution down to tens nm with high momentum and energy resolutions. Present status as well as the future prospect are briefly discussed.

We have investigated the spin structure of photoelectrons emitted from a spin-orbit coupled surface state of Bi₂Se₃(111). The spin polarization is observed only in the direction perpendicular to the mirror plane with p- and s-polarized lights and is inverted upon switching the light polarization from p to s. On the other hand, the photoexcitation with a circularly polarized light leads to the three-dimensional spin-vector rotation in the final state. This is produced by the spin interference in the photoemission process, resulting from simultaneous excitation of the spin-up and spin-down states coupled to the symmetric and antisymmetric parts of the wave functions respectively.