Design Of An Efficient Monochromatic Electron Source For Inverse Photoemission Spectroscopy

In recent years, increasing attentions have been paid to the molecular films of organicsemiconductors, and the study on electronic structure of their surfaces and interfaces will help togain insight into their properties, which can further guide their thin film growth. For the present,Photoemission Spectroscopy (PES) is the most widely used technology to probe the electronicstructures, but it cannot study the unoccupied states directly. Inverse Photoemission Spectroscopy(IPES), on the other hand, can cover the dispersion of unoccupied states with sufficient accuracyin a wide energy range above the Fermi level. Therefore, with PES and IPES combined, the wholeelectronic structure can be obtained. However, IPES is now confronted with low energy resolutionand low counting rates, which can be solved partially by reducing the energy width and the sametime increasing the beam current of the electron source, one vital part of IPES apparatus. In thisthesis, an efficient monochromatic electron source for IPES apparatus is designed.The thesis first introduced the theory of IPES: electrons from an electron source are incidentonto a sample surface and radiative transitions are created from an unoccupied state to anotherunoccupied state above the Fermi level. Next, the cross section of IPES process is proved to bemuch lower than that of PES. Therefore, the Bremsstrahlung Isochromatic Spectroscopy (BIS)working mode of IPES will be adopted in this thesis due to its easy design and high counting rates.The electron source consists of an electron gun, a Hemisphere Deflection Monochromator(HDM) and a transfer electrostatic lens. All the designs have been investigated by electron-raytracing simulations using the SIMION program with two pass energies (P.E.) of10eV and5eV,respectively. The electron gun consists of a cathode and an electrostatic lens of four electrodes. Inthe electron gun design, a BaO cathode is chosen with its working temperature of nearly1100Kand an intrinsic energy spread of232.5meV. Using this design, a beam current of nearly128μAcan be obtained based on measured results. In the HDM design, rectangle structures are adopted atthe entrance and exit, which can achieve high beam current without energy resolution beingcompromised. To correct the fringing effects of the HDM, we came up with an original solutionusing two electrodes which is simpler and easier to adjust compared with the traditionalcorrections. A standard five-element true zoom lens is adopted as the transfer lens. Through thistransfer lens, electron energy ranging continuously from5eV to20eV can be obtained with thefocal point and linear magnification keeping unchanged. Finally, the energy resolution and beamcurrent are estimated to be98meV and49μAfor P.E.=10eV, and53meV and27μAfor P.E.=5eV, respectively.