  Photoemission orbital tomography provides direct access from laboratory
measurements to the real-space molecular orbitals of well-ordered organic
semiconductor layers. Specifically, the application of phase retrieval
algorithms to photon-energy- and angle-resolved photoemission data enables the
direct reconstruction of full 3D molecular orbitals without the need for
simulations using density functional theory or the like. A major limitation for
the direct approach has been the need for densely-sampled, well-calibrated 3D
photoemission patterns. Here, we present an iterative projection algorithm that
completely eliminates this challenge: for the benchmark case of the pentacene
frontier orbitals, we demonstrate the reconstruction of the full orbital based
on a dataset containing only four simulated photoemission momentum
measurements. We discuss the algorithm performance, sampling requirements with
respect to the photon energy, optimal measurement strategies, and the accuracy
of orbital images that can be achieved.
