Energy Dependence of Fatigue-Enhanced Photoemission

This is a progress report on the subject of fatigue-enhanced photoemission. In our studies on fatigueenhanced photoemission, the ultimate goal is to see whether or not the photoyield can be used as a tool to uniquely define the early and later stages of fatigue on structural materials. This yield results in an electron current off the specimen. Disciplines Materials Science and Engineering | Structures and Materials This 9. emissions for nde applications is available at Iowa State University Digital Repository: http://lib.dr.iastate.edu/ cnde_yellowjackets_1975/33 ENERGY DEPENDENCE OF FATIGUE-ENHANCED PHOTOEMISSION Otto Buck Science Center, Rockwell International Thousand Oaks, California This is a progress report on the subject of fatigue-enhanced photoemission. In our studies on fatigue-enhanced photoemission, the ultimate goal is to see whether or not the photoyield can be used as a tool to uniquely define the early and later stages of fatigue on structural materials. This yield results in an electron current off the specimen. It seems to be customary now in the literature to call this current 11 exo-electrons 11 , alth~ugh this term was used earlier for non-stimulated electron current only . We should clearly distinguish between nonstimulated and photo-stimulated "exo-electrons". The intensity of the non-stimulated effect is much smaller than it is for the stimulated one. All our work is done using the photo stimulation in which an outside light source and a monochromator for energy selection are used. A variety of mechanisms for stimulated emission have been proposed, such as: 1) Release of stored energy 2) Change of the work function of the metal or metal oxide 3) State of the internal stress 4) Formation of cracks 5) Exposing fresh metal surface 6) Change of surface roughness At the time we began with our ~ork, we became aware of the surface roughness studies by Endriz and Spicer2,3 at Stanford. They looked at the photostimulated yield on relatively smooth aluminum specimens with the rms roughness in the range of a few Angstroms to about 25 A. The yield changed -markedly over the roughness range they had available. We thought that this was a very attractive idea and thus included this possibility in our list of mechanisms. Since we concentrated mostly on this roughness effect, let me briefly outline the Endriz and Spicer ideas. It is well known that oscillations of surface charges on the metals or alloys do exist. It is further possible to couple incident photons to the oscillations of surface charges if a grating exists on this surface. These oscillations can decay by transferring their energy to a single electron, ejecting it from the metal and contributing to the photoyield. This model may be described by the dispersion curves, shown in Fig. 1. The straight line in this figure represents the ~k curve for the incident radiation. The dispersion curve for the surface charge oscillations