Thermionic, field, and threshold photoemission energy distributions from lanthanum-hexaboride
Abstract
Measurements of the energy distributions of electrons emitted from the various crystal facets of LaB$\sb6$ by thermionic, field, and threshold photoemission are reported. A standard free electron theory is used to analyze the data. For thermionic emission, this analysis reveals a sizable discrepancy ($\sim$25%) between the measured temperature of the LaB$\sb6$ (110) emitter and the temperature required by the best theoretical fit to the data. This discrepancy may be explained in part by modifying the potential barrier at the metal-vacuum interface, and thus modifying the electron transmission probability. Field emission energy distributions are used to measure the work function and electric field at the surface of the sample for various crystal facets. The results indicate that the shape of the emitter is dominated by the growth of crystal facets, and the composition of the emitter surface is rich in either lanthanum or boron. Field emission data obtained with sample temperatures greater than 1000 K strongly show the effects of the density of states of electrons at energies above E$\sb{\rm F}$. Threshold photoemission energy distributions are presented using photon energies in the range of 2.5 eV to 3.5 eV. The data show an excess number of electrons above what our model predicts photoemitted at an energy of $\sim$2.5 eV. This energy is just above the peak in the surface potential barrier, and the possibility these electrons may be the result of a complicated barrier is discussed. These excess electrons may also be related to the density of states in the crystal, or to the photoexcited population of 2.1 eV plasmons. Threshold photoemission measurements show the electric field dependence of the peak in the surface potential barrier is significantly different than that predicted by the standard model, suggesting the barrier at the surface of LaB$\sb6$ is complicated. The quantum yield of $\sim$1 $\times$ 10$\sp{-4}$ and photoemission current density of $\sim$0.2 A/cm$\sp2$ for $\hbar\omega \sim 3.5$ eV indicate that LaB$\sb6$ may find use as a high brightness laser-assisted electron source. In addition to our work with LaB$\sb6$, we have constructed a scanning retarding potential diode designed to measure the spatial variation of the work function on a flat surface. The equipment and its calibration are discussed, and recommendations for future improvements are also presented.
Degree
Ph.D.
Advisors
Reifenberger, Purdue University.
Subject Area
Condensation
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