Abstract
ABSTRACTDevelopment of a spectrograph for inverse photoemission spectroscopy and a study of the electronic structure of alkalis on Al(111).(電子光系統的建立及鹼金屬吸附在鋁(111)表面之電子組態研究)Ren-Yu Chu (朱仁佑)The last decade has seen the rapid development of inverse photoemission Spectroscopy ( IPES ). It is widely used to study the wave vector and energy dependence of unoccupied electronic states at the surface and in the bulk of metals and semiconductors, compounds and even polymers. Like photo-emission, inverse photoemission is surface sensitive, and an important application is the study of the unoccupied surface states. It can complement photoemission, which provides information on the occupied states.Alkali adsorption on metal surfaces are important adsorption systems that have received much attention for many years. Alkali induced changes of the electronic properties and of the reactivity of surfaces are of importance for negative electron affinity cathodes and for the promoter effect of alkalis in heterogeneous catalysis. Since the absorbate has a simple valence electronic structure ( one single s electron ), alkali adsorption on metal surfaces has been regarded as simplest possible adsorption system. However, recent discovery on the unusual substitution site in the reconstructed Al(111) surfaces when alkali metals adsorbed has sparked numerous experimental and theoretical investigations on the geometry as well as electronic structure of related system. We have carried out an extensive study on the electronic structures of Na and K on reconstructed and unreconstructed Al(111) surfaces combining low energy electron diffraction, photoemission, inverse photoemission, and soft x-ray emission. Three structures of are compared. They are K adsorbed at top site at low temperature (LT), K adsorbed at substitution site at room temperature (RT), and Na adsorbed at substitution site at RT.The peaks in density of states in -K-LT show large dispersion, as measured by both photoemission and inverse photoemission, indicating large overlap of K valence electrons. On the other hand, the dispersion in -K-RT minimal, suggesting localized states due to strong bonding with neighboring Al atoms. Comparing PE data from K and Na in the substitution phase at RT revealing that Na induced state is more localized, consistent with the smaller size of Na atoms.In comparison to the calculation of Na on Al(111), we find their RT result has better agreement with our PE and IPE data while their LT result shows much larger band width of adsorbate induced state than our data of K, in opposition to expectation.Finally, our LEED study of K adsorbation at LT shows that beyond coverage, the K atoms are not just simply compressed, as reported before as split ; they further exhibit rotation of with respect to the principle axes of structure, resulting in clear straight segments in LEED.I separate the thesis into two parts. The first part describes the inverse photoemission system I built from scratch. In the second part I describe the study of alkali adsorption by applying this newly developed system along with other surface techniques.