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J Chem Phys ; 153(14): 144705, 2020 Oct 14.
Artigo em Inglês | MEDLINE | ID: mdl-33086829


High-performance photocathodes for many prominent particle accelerator applications, such as x-ray free-electron lasers, cannot be grown in situ. These highly reactive materials must be grown and then transported to the electron gun in an ultrahigh-vacuum (UHV) suitcase, during which time monolayer-level oxidation is unavoidable. Thin film Cs3Sb photocathodes were grown on a variety of substrates. Their performance and chemical state were measured by x-ray photoelectron spectroscopy after transport in a UHV suitcase as well as after O2-induced oxidation. The unusual chemistry of cesium oxides enabled trace amounts of oxygen to drive structural reorganization at the photocathode surface. This reorganization pulled cesium from the bulk photocathode, leading to the development of a structurally complex and O2-exposure-dependent cesium oxide layer. This oxidation-induced phase segregation led to downward band bending of at least 0.36 eV as measured from shifts in the Cs 3d5/2 binding energy. At low O2 exposures, the surface developed a low work function cesium suboxide overlayer that had little effect on quantum efficiency (QE). At somewhat higher O2 exposures, the overlayer transformed to Cs2O; no antimony or antimony oxides were observed in the near-surface region. The development of this overlayer was accompanied by a 1000-fold decrease in QE, which effectively destroyed the photocathode via the formation of a tunnel barrier. The O2 exposures necessary for degradation were quantified. As little as 100 L of O2 irreversibly damaged the photocathode. These observations are discussed in the context of the rich chemistry of alkali oxides, along with potential material strategies for photocathode improvement.

Nano Lett ; 8(9): 2944-8, 2008 Sep.
Artigo em Inglês | MEDLINE | ID: mdl-18698833


We present a general model for computing an optical response function of a finite shell lattice of semiconducting or metallic nanoparticles. Within a second quantization formalism, a cylindrical shell of induced, coupled dipoles is considered in the presence of an external electric field. Numerical analysis of the eigenmodes and quantum mechanical response function allow us to identify resonator effects due to constructive interferometric interaction of the light to the dipole lattice. Adjusting the wavelength of the external electric field, a coherent resonance excitation is possible for a fixed parameter of the cylinder radius.