PHOTOFIELD EMISSION STUDY OF METAL SURFACES (ELECTRON YIELD IN PFE, LASER-INDUCED THERMAL EFFECTS)

YONGLI GAO, Purdue University

Abstract

In this thesis the efforts made to better understand the mech- anisms and to refine the apparatus of photofield emission are described. Photofield electron yield and photofield emission from surface electronic have been measured. Both demonstrate the surface sensitivity of this new technique. Analyses of laser-induced thermal effects are also reported which allows the simultaneous determination of both the temperature and workfunction of a laser illuminated surface as small as 50(ANGSTROM) in spatial extent. Theoretical calculations of photofield emission based on the relativistic elec- tronic band structure of tungsten are also carried out and the results are compared to experiment. Data from W(110) and W(111) for a number of different photon energies between 2.41eV and 3.54eV are reported. The measured yield is compared to the theoretical models of the photoexcitation process. Based on this comparison, it is concluded that the spatial variation of the vector potential (')A near the metal-vacuum interface is the dominant excitation mechanism in producing the photocurrent from a laser illuminated field emission tip. Photofield emission from surface electronic states is a unique probe to examine the radiation field near the vicinity of a metal sur- face since the surface states are localized within the first atomic layer of the surface. Studies of photofield emission from surface states on W(100), Mo(100) and Ir(111) are presented in the text. Data analysis shows that photofield emission from localized sur- face states depends predominantly on the normal component of the vector potential (')A. Laser-induced thermal effects on various surfaces of tungsten and molybdenum have been studied using field emitted electrons. Energy distributions measured by a dispersive electron energy ana- lyzer allow the temperature increase of a laser-illuminated surface to be determined to within (+OR-)10 K. In addition to estimating the tem- perature rise, small temperature-induced variations in the work func- tion (PHI) can also be determined. Fits to experimental data indicate that changes in (PHI) of a laser-illuminated surface can be estimated to within (+OR-)2 meV. Photofield emission energy distributions from W(110) and W(111) were measured from 2.41eV to 3.54cV. Results are compared with theoretical expectations based on a relativistic electronic band structure and on a free electron model. Data analysis shows that surface emission is strong in PFE and that effects of bulk direct tran- sitions, although small, are still observable. It can be concluded that photofield emission provides means for inspecting the electronic band structure of transition metals between the Fermi energy and the vacuum level. (Abstract shortened with permission of author.)

Degree

Ph.D.

Subject Area

Condensation

COinS