Obtaining the Highest Occupied Molecular Orbital Peak of Organic Matter from Photoelectron Yield Spectra

We propose a simple theory of photoelectron yield spectra (PYS) on the basis of a density-of-states approximation and universal transmission and loss functions. The theory reproduces the known threshold behavior of metals and that of semiconductors in the indirect-transition regime. We show that a Gaussian-shaped density of states leads to a PYS spectrum that increases very nearly as the third power of energy from threshold, indicating that the empirical cube law of PYS of organic matter reflects the Gaussian-broadened highest occupied molecular orbital peak. We show how the peak position and width can be extracted from the PYS spectrum. The results obtained for several organic compounds agree well with ultraviolet photoemission data. We further show that the density of occupied states is approximately proportional to the second derivative of the PYS spectrum in the near-threshold region.