Accurate modelling of optical phenomena is vital for understanding and discovering innovative photonics materials for photodetectors and optical communication applications. This is especially true regarding the development of photocathode materials used in x-ray free-electron laser (FEL) light sources. X-ray FELs are particle accelerators that use a beam of electrons to generate x-rays. These x-ray FEL light sources have recently enabled exciting new discoveries, such as monitoring bond formation in the active site of proteins, optically tuning the interlayer interactions in two-dimensional materials, and probing the formation of diamonds from laser-compressed hydrocarbons. However, current photoemission models cannot be applied to universally predict the emission properties of photocathode materials, which limits our ability to discover new and better photocathodes.

We have developed a density functional theory (DFT) based method to model the photoemission process. Most notably, this method expands on previous models by utilizing DFT-calculated density of states, rather than assuming constant density of states. Additionally, we incorporate the photoexcitation probabilities for all possible optical excitations and a more accurate electron transmission probability across the photocathode-vacuum interface. Increasingly more accurate physical representations may allow our model to be generalizable to a wider range of materials. Coupled with the relatively low cost of performing DFT calculations, the general nature of our model may enable the possibility of rapidly screening through thousands of novel photocathode materials.

*Workflow for calculating the intrinsic emittance for a given material. We first obtain the relaxed crystal structure of the bulk material. We then calculate the electronic structure of the material with KS-DFT. Finally, we determine the total escape probability (w _{i}) associated with all Kohn-Sham states and calculate the intrinsic emittance, ε_{int}.
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To test the ability of our model to generalize across a wide spectrum of materials, we validate our model on the most diverse range of photocathode materials to date. Across all materials, our model predicted the emittance of photocathode materials with an average mean absolute error (MAE) of 0.044 μm/mm and a maximum MAE of 0.058 μm/mm. For comparison, previously developed models achieved an average MAE of 0.088 μm/mm, with a maximum MAE of 0.270 μm/mm. While our model performs slightly better on average than the analytical expression, we note that our model is considerably more robust at handling a wider range of materials. In particular, our model predicts the emittance of PbTe with 5x lower error than previous models. This distinction becomes increasingly important when attempting to predict the photoemission properties of candidate photocathodes for which experimental data have not been obtained. Due to the general nature of this model, we expect our model will pave the way for future studies on a wide variety of photonic materials, providing a fast and accurate method to explore novel photocathode and photoemissive technologies.

**Current Research**: Now that we have developed our generalizable model for predicting the photoemission properties of photocathode materials, we are now utilizing our model to identify promising new photocathode materials.

**Contact**: Evan Antoniuk, eantoniuk_at_stanford.edu

Publication:

Antoniuk, E. R., Yue, Y., Zhou, Y., Schindler, P., Schroeder, W. A, Dunham, B., Pianetta, P., Vecchione, T., and Reed, E. J. Generalizable density functional theory based photoemission model for the accelerated development of photocathodes and other photoemissive devices. *Physical Review B* 101, 235447 (2020) doi:10.1103/PhysRevB.101.235447