Abstract
The application of the ‘orbital tomography’ technique to obtain direct images of molecular orbitals of adsorbed molecules from angle-resolved photoemission data, first proposed by Puschnig et al (2009 Science 326 702), is an extremely attractive idea, but is based on the assumption that the photoemission can be described by a plane wave final state. It is well known that this neglect of the spherical-wave nature of the initial emission and of the role of final state scattering both within the molecule and from the substrate can lead to serious errors. Despite this, in the albeit simple systems studied so far the method appears to work reasonably well. Here we provide a detailed critique of this problem, highlight situations in which the orbital tomography approach is likely to lead to major errors, and propose test experiments that could provide clear information on the extent of these problems.
Highlights
Ultra-violet photoemission (UPS) has proved to be a valuable probe of the molecular orbitals of adsorbed molecules as first demonstrated in studies of the simple diatomic molecule, CO, on Ni surfaces by Eastman and Cashion [1]
In the early development of the theory of angle-resolved UPS (ARUPS) Gadzuk [17,18,19] proposed that the dominant effect was likely to be the initial-state distribution of the electrons and he investigated the photoelectron angular distribution assuming that the final state can be treated as a plane wave
A further implication of the apparently reasonable description provided by the plane wave final state approximation in the work of Puschnig et al is that, for the specific systems studied, the final state scattering by atoms outside the adsorbed molecules apparently leads to very little modification of the observed angular distribution of the photoemission
Summary
Ultra-violet photoemission (UPS) has proved to be a valuable probe of the molecular orbitals of adsorbed molecules as first demonstrated in studies of the simple diatomic molecule, CO, on Ni surfaces by Eastman and Cashion [1]. An important limitation of this method, is that the calculation of the angular distribution of the photoelectron current is based on the assumption that the final state can be approximated by a single plane wave.
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